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The Geological Society of America
Special Paper 65
THE FLOORS OF
THE OCEANS
I. The North Atlantic
Text to Accompany the Physiographic Diagram of the North Atlantic

BY
Bruce C. Heezen, Marie Tharp, and Maurice Ewing

Lamont Geological Observatory (Columbia University) Palisades, New York

April 11, 1959

Made in the United States of America

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"Could the waters of the Atlantic be drawn off, so as to expose to view this great sea-gash, which separates continents, and extends from the Arctic to the Antarctic, it would present a scene the most rugged, grand, and imposing. The very ribs of the solid earth, with the foundations of the sea, would be brought to light, and we should have presented to us at one view the empty cradle of the ocean...." (M. F. Maury, The Physical Geography of the Sea, 1855.)

FOREWORD

The diagrammatic portrayal of the relief of continental land areas of the world has been completed by both the late A. K. Lobeck and Erwin Raisz, whose magnificent diagrams are familiar to all geologists and geographers. The authors of the present sheet are preparing a similar series of marine physiographic diagrams.

The Physiographic Diagram: Atlantic Ocean; Sheet 1 is the first of this projected series. The Atlantic Ocean diagram will consist of five sheets at a scale of about 1:5 million. A diagram of the South Atlantic Ocean at a scale of about 1:11 million now nearly completed will form the first of a general series planned to portray the world oceans.

In addition, diagrams of selected areas well covered by sounding profiles will be prepared at scales of about 1:2 million.

Each sheet or series of sheets will be accompanied by descriptive notes treating the nomenclature, morphological, geological, and geophysical characteristics of each of the physiographic provinces.

Lamont Geological Observatory Contribution 308

NOTE

Physiographic Diagram: Atlantic Ocean, Sheet 1 (Plate 1)

Copies of the Physiographic Diagram: Atlantic Ocean, Sheet 1 are available unfolded so that each will be suitable for wall mounting. The diagram is therefore not physically inserted or attached to this volume although it forms the basic part of the paper.

ACKNOWLEDGEMENTS

The studies of submarine topography at the Lamont Geological Observatory have been supported by the United States Navy Bureau of Ships under Contract NObsr 64547. The expeditions which obtained topographic data were supported by the Office of Naval Research under Contracts N6 onr 27124 and N6 onr 27113 and the Bureau of Ships under Contract NObsr 64547. Three cruises were supported by the National Geographic Society, the Woods Hole Oceanographic Institution, and Columbia University. Financial support has been received from The Geological Society of America (Grant 635-54). The preparation of this paper was supported in part by the Bell Telephone Laboratories.

The studies that led to the present paper began at the Woods Hole Oceanographic Institution just after World War II. With the founding of the Lamont Geological Observatory in 1949 the work was transferred to that observatory at Palisades, New York. Topographic data from Woods Hole cruises were incorporated with Lamont data until 1953, when a separate program was established at Woods Hole by J. B. Hersey, and Columbia University acquired the Vema as its own research vessel. Although Woods Hole data obtained between 1953 and 1957 have not been used in preparing sheet 1, arrangements recently concluded provide for the incorporation of past and future Woods Hole data in subsequent sheets of this series.

The soundings were read, compiled, and plotted by Morris Wirshup, the late Andrew Nelson, Ivan Tolstoy, G. Leonard Johnson, III, the authors, and several others. The profiles were plotted by M. Wirshup, Hester Haring, the authors, and several others.

The soundings were taken primarily on board the Research Vessels Vema and Atlantis (Woods Hole Oceanographic Institution), but important sounding lines were obtained by the R/V Albatross (Woods Hole Oceanographic Institution), M/V Theta, and R/V Caryn (Woods Hole Oceanographic Institution) and in the eastern Atlantic by the R.R.S. Discovery (National Institute of Oceanography).

The following officers made outstanding contributions to the navigational plotting: A. K. Lane, the late A. Karlson, J. Pike, R/V Atlantis; D. Gould, the late F. S. Usher, D. Smith, V. Sinclair, H. Kohler, and K. Simonson, R/V Vema; and the late A. Nelson, R/V Vema and M/V Theta.

The echo sounders have been installed, maintained, and improved by B. Luskin, H. R. Johnson, A. Roberts, M. Landisman, C. Hubbard, H. Van Santford, M. Langseth, G. Sutton, and many others.

The entire scientific party of each of the more than 50 expeditions represented in the data of this paper took turns marking and adjusting the echo sounder, and all navigational officers on these expeditions took the fixes and kept the logs. To these sea-going scientists and mariners too numerous to list the authors are extremely grateful.

Soundings in the northeast Atlantic compiled by the British Admiralty Hydrographic Department were kindly provided by Cmdr. J. S. N. Pryor of that organization. Dr. M. N. Hill of Cambridge and Dr. G. E. R. Deacon of the (British) National Institute of Oceanography were instrumental in obtaining many of these valuable deep-sea soundings. Original sounding sheets of many areas were provided by the Coast and Geodetic Survey through the courtesy of Admiral A. Karo and Mr. G. F. Jordan.

B. Luskin's development of the Precision Depth Recorder and his continued research and development in echo-sounding equipment made it possible to obtain many of the detailed data of this paper.

The following expedition chief scientists conducted sounding surveys which have been incorporated in this paper: M. Ewing, J. L. Worzel, J. E. Nafe, I. Tolstoy, R. S. Edwards, G. R. Hamilton, C. L. Drake, B. Luskin, W. C. Beckmann, F. Press, J. Northrop, J. Hirshman, M. J. Davidson, R. J. Menzies, F. C. Fuglister, E. T. Miller, and B. C. Heezen.

The writers are grateful to the great number of scientists who encouraged them in this work and especially to those who offered suggestions and discussed the data and conclusions. We are particularly indebted to W. H. Bucher for discussions relative to tectonics, to David B. Ericson for problems relating to sediment distribution and analysis, and to C. O'D. Iselin and C. H. Elmendorf for general encouragement and support during the several years of this study.

CONTENTS

ILLUSTRATIONS

ABSTRACT

The Physiographic Diagram: Atlantic Ocean, Sheet 1, which portrays the North Atlantic between 17° and 50° North Latitude, is the first of a projected series of diagrams. The diagram is based on continuous echo-sounding traverses made by research vessels. The relief shown on the profiles was sketched in perspective using the technique introduced by Lobeck. Between sounding profiles the relief is speculative, based on extrapolation of trends noted in the profiles.

The area of the diagram is divided into three major physiographic regions which are in turn subdivided into the following categories of provinces.

• CONTINENTAL MARGIN
• Category I
• Continental Shelf
• Epicontinental Seas
• Marginal Plateaus
• Category II
• Continental Slope
• Marginal Escarpments
• Landward Slopes of Trenches
• Category III
• Continental Rise
• Marginal Trench-Outer Ridge Complex
• Marginal Basin-Outer Ridge Complex
• OCEAN BASIN FLOOR
• Abyssal Floor
• Abyssal Plains
• Abyssal Hills
• Abyssal Gaps and Mid-Ocean Canyons
• Oceanic Rises
• Seamount Groups
• MID-OCEANIC RIDGE
• Crest Provinces
• Rift Valley
• Rift Mountains
• High Fractured Plateau
• Flank Provinces
• Upper Step
• Middle Step
• Lower Step

Each province is defined, briefly described, and illustrated with profiles and photographs of echo-sounding records.

The boundaries of the physiographic provinces, defined solely by bottom topography, show good correlation with variations in crustal structure as determined by seismic-refraction measurements and with anomalies of the gravity and magnetic fields. In addition, the province boundaries correlate well with distribution patterns of bottom sediments. The physiographic provinces are thus really morpho-tectonic provinces. The precise correlation of topographic provinces and structure observed in specific sections can thus be extrapolated along province boundaries to deduce the geology in large areas where no geophysical work has been done. The tectonic map of the Atlantic prepared in this manner will be presented in a subsequent publication.

[PART 1. PREPARATION OF THE PHYSIOGRAPHIC DIAGRAM]

Several steps are involved in the preparation of a marine physiographic diagram. The raw data consist of continuously recorded echograms and lists of positions of the research ship. Echograms are profiles of ocean depth, automatically plotted against time (Luskin et al., 1954). The first step is to read and tabulate the depth at each peak, trough, or change of slope. These readings are plotted on a chart (1:1,000,000) as a series of closely spaced soundings. Depth profiles are plotted against distance at a standard vertical exaggeration of 40:1. The sounding lines are also plotted on a chart of small scale (1:5,000,000) which is at the same scale as the final physiographic diagram. The subsequent steps in the preparation of the diagram are illustrated by Figures 1a-d. The exaggerated profiles (1b) along the tracks (1a) show a succession of peaks and valleys. These features are sketched in along the tracks (1c). After all the tracks in a large area are sketched in this way, the major trends are estimated, and the diagram is completed by interpolation and extrapolation (Fig. 1d; Pl. 1). The vertical scale of the diagram is 1 inch = 5000 fathoms which is an effective vertical exaggeration of 20 to 1. The final diagram as printed is at a scale of 1:5 million at 40° N. on a Mercator projection.

There is a fundamental difference between the preparation of a terrestrial and a marine physiographic diagram. In the former the major problem is to select from more-detailed maps the features to be represented. Except in unexplored, inaccessible areas, the shape of all land features is a matter of recorded fact; the problem is to abstract and artfully draw the features in question. In contrast, the preparation of a marine physiographic diagram requires the author to postulate the patterns and trends of the relief on the basis of cross sections and then to portray this interpretation in the diagram.

PHYSIOGRAPHIC PROVINCE CHART: A study of the exaggerated profiles plotted during the preparation of the physiographic diagram revealed the existence of morphological features and morphological provinces not previously delineated. The limits of areas of contrasting morphology were noted on the profiles, and these points were plotted on a chart of small scale (also about 1:5 million at 40° N.) (Pl. 20).

CONTROL: Almost all the echo-sounding profiles used in the preparation of the physiographic diagram (Pl. 1) and the physiographic province chart (Pl. 20) were obtained by expeditions of the Lamont Geological Observatory and the Woods Hole Oceanographic Institution (Pl. 21). Some soundings were provided by the Hydrographic Department, British Admiralty (Pl. 21) and the International Hydrographic Bureau (Monaco).

Figure 1.—Method of preparation of physiographic diagram

(a) Positions of sounding lines (A, B) are plotted on chart; (b) Soundings are plotted as profiles (A, B) at 40:1 vertical exaggeration; (c) Features shown on profiles (A, B) are sketched on chart along tracks; (d) After all available sounding profiles are sketched the remaining unsounded areas are filled in by extrapolating and interpolating trends observed in a succession of profiles.

The echo soundings made by research vessels fall into three classes: (1) precision soundings (accuracy better than 1 fathom in 3000); (2) nonprecision soundings obtained by research vessels using commercial echo sounders with control or close check on time standard; (3) poor to bad soundings made with commercial echo sounders without timing control or adequate checks. Most of the soundings used in this paper fall into the first two categories. In Figure 2 the Precision Depth Recorder (PDR) sounding tracks are shown. In Figure 3 the good but nonprecision tracks are shown. The soundings of the third class are not shown. All tracks used in the preparation of the diagram are shown in Plate 21. Most of the sounding lines were located by standard dead-reckoning procedures from astronomical fixes. Errors of a few miles are probably common. Position errors do not seriously affect the work described here since we are dealing largely with texture read from profiles and plotted on a small-scale sheet.

In addition to the sounding tracks shown in the control chart, spot depths shown on U. S. Hydrographic Office charts HO 0955a, 0955b, 0956a, 0956b, and 5487 and on feuille A-1 of the Carte Générale Bathymétrique des Océans (1935) were used where profiles were lacking. Along the east coast of the United States the Coast and Geodetic Survey soundings published by Veatch and Smith (1939) were used for the continental shelf and slope. Other important sources of published soundings include Hill (1956), De Andrade (1937), Dietrich (1939), Wüst (1940a), Emery (1950), and Tolstoy (1951).

The land areas of the diagram were sketched to the same rigid vertical scale as that used for the deep sea. Elevations for the United States were taken from United States Geological Survey and Army Map Service quadrangle maps; elevations for Europe and Africa are from Bartholomew maps; and elevations for the islands from United States Navy Hydrographic Office charts.

EXAGGERATED PROFILES: The profiles plotted at 40:1 vertical exaggeration are the basis for the topography sketched on the physiographic diagram. A selection of these profiles is reproduced in Plates 22, 24, 25, and 27, and in Figure 45. All profiles from precision soundings were originally plotted at a vertical scale of 2 inches equals 1000 fathoms and a horizontal scale of 2 inches equals 40 miles. Nonprecision soundings were plotted at scales of 1 inch equals 1000 fathoms and 1 inch equals 40 miles. In a typical area 40 to 60 soundings were plotted for each 60 miles of profile. The points were connected and then qualitatively checked against the original echogram. Although all the larger features are represented on these profiles, features of less than a mile in width may be missed. The small scale of the physiographic diagram excluded the possibility of portraying most of the features less than 3-6 miles in width and less than 20 fathoms in height.

Detailed study of the small-scale features less than 2 or 3 miles in width is best accomplished by a study of the original echograms. The PDR records are ideal for this purpose.

Figure 2.—Precision depth recorder (PDR) sounding lines obtained by research vessels

Most of soundings shown were obtained by the Lamont Geological Observatory's R. V. Vema, 1953-1957.

Figure 3.—Good, but nonprecision sounding lines obtained by research vessels

Most soundings obtained by the Woods Hole Oceanographic Institution's R. V. Atlantis, 1946-1953.

NORTH ATLANTIC SOUNDINGS: The study of the North Atlantic deep-sea bathymetry began a little more than a century ago with the taking of the first deep-sea soundings by lead line. By 1860, largely because of the great public interest in the proposed trans-Atlantic cables and the enthusiastic encouragement of Matthew F. Maury (1855), several hundred soundings had been taken in the North Atlantic in depths greater than 1000 fathoms. Meanwhile, on either side of the Atlantic surveys of coasts, harbors, offshore banks, and the continental shelf were being made for navigational use. The Hudson Submarine Channel and the head of the Hudson Canyon were discovered by the United States Coast Survey during this period. By 1912 more than 1800 deep-sea soundings had been taken in the North Atlantic by the laborious method of using a lead lowered at first by hemp line and later by wire. Between 1900 and 1920 Fessenden in the United States, Behm in Germany, and Langevin and Florisson in France established that acoustic echo sounding was possible and built machines to take echo soundings. In 1922, echo sounding became a practical operation. Although many of the early echo sounders were fitted with automatic recorders, they were in general suitable only for use in shallow water (less than 500 fathoms). Deep-sea echo soundings were obtained by listening on earphones for the returning echo and timing the interval by eye with a suitable clock. The improvement of sounding gear continued, and by the mid 1930's automatic recording deep-sea echo sounders were manufactured and put into limited use, although, by and large, all pre-World War II deep-sea (> 1000 fathoms) echo soundings were discrete observations by the "ear and eye" method. A good review of pre-World War II echo-sounding apparatus is given in a publication of the International Hydrographic Bureau (Anon., 1939). During the war the NMC[1] echo sounder was developed and installed on many U. S. ships. It was adequate for deep-sea sounding if in perfect condition; but the designers, being cautious, had arranged for recording only in the depth range of 0-2000 fathoms. The NMC sounder on Atlantis was modified to record in greater depths in 1945, and many thousands of miles of tracks were obtained of the deep sea with this apparatus. The NMC had a small record chart (6¼ inches = 2000 fathoms; ½ inch = about 3 miles). The precision was low since the apparatus depended on a ship's regular AC power supply for its time standard. A new sounder, the UQN-1B, was developed in the United States following World War II. The instrument as manufactured recorded on an extremely small chart (8 inches = 6000 fathoms) but could be modified for multiple 600-fathom scale recording (8 inches = 600 fathoms). The timing function was usually accomplished by poorly regulated ship's AC power supply, and errors were consequently large (Dietz, 1954; Heezen, 1954). In addition, the stylus arrangement required constant adjustment. After only a few thousand miles were obtained by the Lamont Observatory expeditions it became obvious that a new recorder incorporating precision timing and large recorder presentation was necessary for an adequate knowledge of topography.

[1] U. S. Navy designation.

Bernard Luskin of the Lamont Geological Observatory, in co-operation with the Times Facsimile Company, adapted the Times Facsimile receiver to do the timing and recording function of the echo sounder, using a standard UQN receiver and transmitter (without recorder). More than 200,000 miles of PDR soundings have now been obtained by expeditions of the Lamont Geological Observatory. The apparatus originally described by Luskin et al. (1954) has been extensively improved (Luskin and Israel, 1956). The Times Facsimile-Lamont PDR performs the timing and recording functions with an accuracy of better than 1 fathom in 3000. This was a considerable improvement over older apparatus. The PDR generally uses multiple 400-fathom scales in which 400 fathoms is represented by 18¾ inches of record; the paper is carried through the machines at 24 inches an hour. Other vertical scales (i. e., 200, 800, 1200) can easily be provided, and the paper transport can be changed by steps from 12 to 96 inches per hour. The laminated recording paper consists of two layers of light gray and a center layer of black. The record is made by burning the upper gray layer and thus exposing the underlying dark layer. The facsimile recording paper differs from the conventional echo-sounder record paper in that a greater range of shades can be reproduced. Several PDR records are shown in the following text (Pls. 3, 4, 5, 6, 8, 9, 10, 12, 13, 14, 16, 17, 18). Effective study of the physiography of the deep-sea floor was made possible by introduction of the PDR. Echo soundings obtained by the English in the area southwest of England have been used in the present study. The accuracy of their equipment has not been adequately treated in the literature, but it appears by comparison that most soundings are accurate to within at least 1 per cent.

PART 2. PHYSIOGRAPHIC PROVINCES

INTRODUCTION

Descriptions of physical features of the earth's surface are found in the earliest-known writings. However, the systematic classification of land forms is relatively recent and followed the development of the science of physical geology. The natural topographic divisions of the continents have been classified into physiographic provinces according to several similar systems (Lobeck, 1939; Fenneman, 1938; Atwood, 1940; and others). These systems take into account form and age of the relief, as well as the structure of the underlying rocks. Descriptions are usually given in terms of age, process, and structure, with the ultimate aim the understanding of the origin and history of topography. Detailed topographic maps at 1:50,000 or larger are available and are used in conjunction with direct field observations. More recently aerial photographs have greatly aided geomorphic studies.

The oceans, in contrast, have been subdivided by oceanographers merely into basins separated by ridges and swells. This was done on the basis of widely spaced discrete soundings shown on charts rarely of larger scale than 1:10 million. The basins were delimited by arbitrarily chosen and often crudely controlled isobaths. The development and installation of continuously recording deep-sea echo sounders and their extensive use in the deep sea provide for the first time detailed topographic information on the deep-sea floor and thus a new basis for description and classification.

It is perhaps presumptuous at this time to refer to the topographic divisions of the sea floor as physiographic provinces when we have only scant information concerning the structure of each province, the age, the physical processes, and, in fact, the details of topography. Therefore, the classification described in the following pages is presented as a first attempt, with the full knowledge that it will be modified and expanded by subsequent exploration.

We are only beginning to understand the structural significance of deep-sea physiographic provinces. We now think that the correlation of topography and structure will be better under the sea than on land because of less vigorous erosion at depth in the sea. If this is true, deep-sea structural patterns may eventually be quite simple to map.

NOMENCLATURE AND CLASSIFICATION OF DEEP-SEA RELIEF

Before the advent of continuously recorded echo-sounding profiles, and their revelation of the texture of the sea-floor relief, classification and nomenclature of submarine topography were based on broad closed isobaths. We can characterize the older system as the bathymetric system of nomenclature in contrast to that employed in this paper, which we can call a geomorphic or textural system.

The terms "basin" and "deep" used in the older literature are usually defined by closed 3000-, 4000-, or 5000-meter contours as represented on the Carte Générale Bathymétrique des Océans (International Hydrographic Bureau). For many purposes this terminology is useful, particularly in describing the habitat of a deep-sea fish or the locale of a water mass. Consequently some such system should be retained even though in many areas basin boundaries are difficult to define, and regardless of the fact that many boundaries cut arbitrarily through physiographic provinces without regard for local province boundaries. The Atlantic has been subdivided by Wüst (1940b) (Fig. 4) whose system is now in general use.

Figure 4.—Major basins of the North Atlantic, after Wüst (1940)

Heavy solid lines indicate boundary formed by axis of Mid-Atlantic Ridge. Light solid lines indicate boundary formed by shelf breaks and submarine ridges. Dashed lines indicate arbitrary boundaries.

The nomenclature of deep-sea topography has been considered by several committees during the past half century. The most recent recommendations published by Wiseman and Ovey (1953; 1955) are followed wherever applicable. The older systems of nomenclature, however, are not rigidly employed since we are dealing with textural provinces based on profiles obtained with continuously recording echo sounders rather than bathymetric provinces defined by closed isobaths.

UNITS OF DEPTH AND SLOPE

On the profiles and echograms the vertical scale is in units of echo-sounding time rather than in units of true depth. In other words, all depths are calculated under the assumption that the vertical sound velocity is 800 fathoms per second. Considering that the sound travels to the bottom and back, the calculation is based on 400 fathoms per second of lapsed time.

Since the average vertical velocity is, within the area covered, always slightly less than 800 fathoms per second, the true depth is always slightly greater than the "echo-time depth" as expressed in "nominal fathoms". Figure 5 shows the range of corrections which must be applied in various parts of the area. The spot depths indicated on the physiographic diagram are in units of true depth corrected according to Matthews' tables (1939) for regional variations in the average vertical sounding velocity.

Figure 5.—Sound-velocity corrections for echo soundings

Add correction to uncorrected echo sounding to obtain true depth. Curves I and II are representative of North Atlantic 17°-50°N. exclusive of the Grand Banks region. Curves III and IV are representative of the deep-water areas near the Grand Banks. Curves are based on Matthews (1939) and are for use only where assumed sounding velocity is 800 fathoms/second. All soundings mentioned in the text are uncorrected for sound velocity.

Figure 6.—Conversion diagram for degrees, per cent grade, feet per statute mile, and gradient

(a) Values of gradient from 1:10-1:1500; (b) Values of gradient from 1:100-1:5000

The inclination of the bottom is given as the tangent of the angle between the sloping plane and the horizontal expressed as a ratio of integers. These ratios are referred to as gradients. In Figure 6 slope values expressed in degrees, per cent grade, feet per statute mile, and gradient are compared. With a few exceptions all profiles are represented with a 40:1 vertical exaggeration. To facilitate judging the magnitude of slopes on these profiles, Figure 7 shows various gradient ratios at a 40:1 vertical exaggeration. Slope corrections have not been made to the soundings. Except in special cases such corrections would make insignificant changes in the 40:1 profile.

All distances are given in nautical or geographical miles (1 nautical mile = 6080 feet).

Figure 7.—Gradients from 1:5 to 1:8000 shown at a 40:1 vertical exaggeration

Most profiles reproduced in this paper are at 40:1 vertical exaggeration. This template is provided to aid the reader in judging slopes on these exaggerated profiles.

Figure 8.—Outline of submarine topography

Line 1, first-order features of the crust; line 2, major topographic features of the ocean; line 3, categories of provinces and super-provinces; line 4, provinces; line 5, sub-provinces and other important features.

CONTINENT AND OCEAN

The two first-order morphologic divisions of the earth's crust are continent and ocean. The oceans can be divided into a few major divisions which are in turn subdivided into categories of physiographic provinces and then into individual provinces. The area of the present study is composed of the three major divisions shown in Figure 9: continental margin, ocean-basin floor, and mid-oceanic ridge. The discussion and description of the physiographic provinces of the North Atlantic will follow the schematic outline shown in Figure 8.

Figure 9.—Major morphologic divisions: North Atlantic Ocean

The profile is a representative profile from New England to the Sahara Coast.

CONTINENTAL MARGIN

DEFINITION AND GENERAL CATEGORIES

The continental margin includes those provinces of the continents and of the oceans which are associated with the boundary between these two first-order features of the earth.

General categories.—In most areas three parallel categories of provinces can be distinguished in the continental margin (Fig. 10). The relatively flat portions of the submerged continental platform constitute category I. These provinces are: continental shelf, epicontinental marginal seas (e. g., Gulf of Maine), and continental-margin plateaus (e. g., Blake Plateau). The provinces of category II include the continental slope, marginal escarpments (e. g., Blake Escarpment), and the landward slopes of trenches. These provinces mark the edge of the continental block. Category III includes the continental rise, marginal trench-outer ridge, and marginal basin-outer ridge complexes.

Figure 10.—Three categories of continental-margin provinces

Category I provinces lie on the continental block, Category II provinces form the side of the continental block, and Category III provinces are the upturned or depressed margins of the oceanic depression.

The most common type of continental margin is made up of continental shelf (I), continental slope (II), and continental rise (III). (Fig. 10, Sahara and New York). In areas where the continental rise is well developed it is composed of two parts, the upper and the lower continental rise (Fig. 15). In some areas the lower continental rise is replaced by an outer ridge, and the upper continental rise is replaced by a marginal basin or marginal trench. These two latter types are illustrated in Figure 10 by profiles marked Blake Plateau and Puerto Rico respectively. Seamounts and islands occur in all the continental-margin provinces.

Category I provinces.—The ocean overflows its basin onto the edge of the continents. The principal physiographic province of this submerged portion of the continental platform is the continental shelf which is present off all the lands of the world. The continental shelf is a smooth area with very low relief and is nearly everywhere limited to depths less than 250 fathoms. The continental blocks are also flooded by epicontinental seas. Some of these have rough bottoms, as the Gulf of Maine; others are relatively smooth-floored. Marginal plateaus, where present, lie in depths of 500-1200 fathoms, and many are nearly as smooth as the continental shelves.

CONTINENTAL SHELF: The continental shelf is a shallow (averaging less than 100 fathoms), gently sloping (less than 1:1000) surface of low local relief (less than 10 fathoms) which extends from the shore line to the shelf break where the seaward gradient sharply increases to greater than 1:40. Its width ranges from a few miles to more than 200 miles.

Continental shelves border all land areas. Because of their proximity to shore, their shallow depth, and their importance in navigation the continental shelves are now the best-known part of the oceans (Veatch and Smith, 1939; Shepard, 1948).

The transition from the continental shelf to the continental slope is generally abrupt and is known as the shelf break. This feature ranges in depth from 20 to more than 100 fathoms and in form from a sharp edge to a rounded shoulder. The change in the gradient is from less than 1:1000 to greater than 1:40.

EPICONTINENTAL MARGINAL SEAS: The epicontinental marginal seas are those shallow seas (less than 1500 fathoms) which lie on the continental block and can be distinguished from the continental shelves by their greater depth (usually > 100 fathoms) and possibly greater topographic irregularity. Most of them are enclosed by shallow banks (< 50 fathoms) of the continental shelf and by land. The Gulf of Maine, the Gulf of St. Lawrence, and the channels of the Bahamas belong to this class.

MARGINAL PLATEAUS: A marginal plateau is a shelflike feature which lies at greater depths than the continental shelf and is separated from the continental shelf by an incipient continental slope. These features generally lie at depths greater than 100 fathoms and less than 1200 fathoms. They can be distinguished from epicontinental marginal seas by their lack of a seaward barrier or sill. The surface of a marginal plateau is generally quite similar to the continental shelf in slope and in the frequency and magnitude of minor relief features. The Blake Plateau is the only well-expressed representative of this morphologic type in the area of the diagram. Well-developed marginal plateaus are also found off the coast of southern Argentina and east of New Zealand.

Category II provinces.—The steep slopes which border the continental block are grouped into category II provinces. Loosely speaking, we are referring to the continental slope but, because of the complications imposed by such features as marginal plateaus and marginal trenches, we distinguish three province types.

CONTINENTAL SLOPE: The continental slope is that relatively steep (3°-6°) portion of the sea floor which lies at the seaward border of the continental shelf. It typically drops from depths of 50-100 fathoms to depths of 750-1750 fathoms. The top of the slope is usually well marked by a relatively sharp shelf break. The base of the slope, although less definite, is generally abrupt. As a basis of classification in those few areas where no abrupt change is noted, we have set the gradient of 1:40 as the lowest gradient of the continental slope. The setting up of a lower limit for the gradient marks a departure from the older usage in which the continental slope was defined as "the slopes leading from the outer edge of the continental shelves down to the great depths of the ocean" (Shepard, 1948). This older definition included the continental rise, marginal plateaus, and marginal escarpments. The continental slopes are a world-wide phenomenon. The details of their regional distribution in the North Atlantic are covered in a later section.

MARGINAL ESCARPMENTS: A marginal escarpment is a precipitous escarpment which forms the seaward slope of a marginal plateau. Such escarpments begin in depths of 500-1500 fathoms and are 1000 to 2000 fathoms high. The base of the escarpment is well marked by an abrupt change in slope. Gradients of marginal escarpments exceed 1:10. The Blake Escarpment is a marginal escarpment. Similar features are found in the Gulf of Mexico and off the southeast coast of Argentina.

LANDWARD SLOPES OF TRENCHES: This term was introduced to set apart the landward slopes of marginal trenches from the usual continental slopes found off trench-less coasts. These steep slopes (> 1:40) drop from depths of a few hundred fathoms near a continent to a depth of a few thousand fathoms in a marginal trench. In the North Atlantic the one example is north of Puerto Rico. A large part of the circumference of the Pacific is bounded by such features.

Category III provinces.—At the base of the continental slope a gentle gradient continues to the local level of the abyssal floor. This seaward gradient ranges from 1:100 to 1:700 and extends over a strip from a few miles to a few hundred miles in width. On many topographic profiles made at right angles to the slope of the continental margin three major breaks are visible: the shelf break, the base of the continental slope, and the point where the near-by level of the ocean-basin floor intersects the more steeply sloping continental margin. Since we have limited the continental slope to gradients greater than 1:40, we split off this lower portion of the continental margin into a separate province, the continental rise. In the older "bathymetric" classification of relief the ocean basin—continental slope boundary was along the 2000- or 2500-fathom contour, an arbitrary division which cut across the then-unrecognized continental rise. At the base of the Blake Escarpment lies an enclosed basin, and north of Puerto Rico the sea floor drops almost directly to the floor of a marginal trench. These seemingly diverse provinces of continental rise, marginal escarpments, enclosed marginal basins, marginal trenches, and outer ridges are placed in category III because of their similarity in position with respect to the continents and ocean floor and because of other similarities shown in the deeper structure of the continental rise.

CONTINENTAL RISE: The continental rise, where present, lies at the base of the continental slope. Gradients on the continental rise generally range from 1:100 to 1:700, while the width ranges from a few score to a few hundred miles. However, gradients as high as 1:50 are observed in segment 3 of the upper continental rise, and gradients as low as 1:2500 are locally present in segment 2 of the lower continental rise (Tables 1 and 2). The seaward limit of the continental rise is generally abrupt, and at this point regional gradients decrease to less than 1:1000. The depth on the continental rise ranges from 750 to 2800 fathoms. Local relief is moderate to low, and, except for infrequent seamounts and fairly frequent submarine canyons, the local relief of the continental rise rarely exceeds 20 fathoms.

The continental rise is well developed over most of the area covered by the physiographic diagram. The major exceptions are north of the Iberian Peninsula where the rise is present, but extremely narrow, and in the southwestern third of the map in the area south of Cape Hatteras, where it is not present. In this latter area the geographical position usually filled by the continental rise is occupied by the low, broad outer ridge and the enclosed marginal basin and marginal trench.

OUTER RIDGE: An outer ridge is a broad ridge generally more than 100 miles wide that rises from less than 100 fathoms to about 1000 fathoms above the adjacent floor. It lies parallel to the continental margin and may enclose a basin or trench on the landward side. The local relief of an outer ridge is generally a little greater than that of the continental rise but much more subdued than that on the oceanic rises and mid-oceanic ridges.

MARGINAL BASIN: A marginal basin, where present, lies at the foot of the continental slope or at the base of a marginal escarpment. It is slightly shallower than the general level of the ocean basins and is bounded on the seaward side by an outer ridge. Part of its floor is generally occupied by an abyssal plain.

MARGINAL TRENCH: A marginal trench is a narrow, steep-sided feature running closely parallel to the trend of the continental margin; it is generally at least 1000 fathoms below the general level of the adjacent ocean floor. It is separated from the ocean floor by a low outer ridge which rises 100-500 fathoms above the level of the adjacent ocean floor. The bottom of a trench is generally rugged except near the deepest spots where it is flat.

The combination of marginal basin and outer ridge replaces the continental rise east of the Blake Plateau. North of Puerto Rico this combination is replaced by a marginal trench-outer ridge complex. The reason for this grouping is discussed in a later section.

Submarine canyons cut across all the continental-margin provinces except isolated portions of the outer ridge. Submarine canyons range from less than a mile to more than 10 miles in width and from less than 10 to nearly 1000 fathoms in depth. Canyons are most abundant on the continental slope. However, a smaller number persist across the continental rise. They are also found on the marginal escarpments and on the landward slopes of trenches. Shepard (1948), Kuenen (1950), Veatch and Smith (1939), De Andrade (1937), Johnson (1939), and others have discussed the continental-slope canyons at great length. Canyons in the continental rise of the North Atlantic were discovered and mapped by Ericson, Ewing, and Heezen (1951).

Heezen et al., PL. 2

[PRELIMINARY CHART OF HUDSON SUBMARINE CANYON
Based on nonprecision coundings taken 1949-1050]

REGIONAL DESCRIPTION OF CONTINENTAL MARGIN

This discussion is based on continuously recorded echo-sounding traverses made by Lamont Geological Observatory expeditions. Profiles approximately perpendicular to the continental margin are reproduced in Plates 24 and 25. None is precisely perpendicular, and thus slight distortions of slopes and widths of the features are unavoidable.

EASTERN NORTH AMERICA: Thirty-four profiles of the continental margin of eastern North America are presented in Plate 24. The positions of the profiles are indicated on the index chart in Plate 23. All profiles show the three categories of continental-margin provinces. Profiles W-1 to W-21 Plate 24 show the more general succession of shelf, slope, and rise, while profiles W-22 to W-34 show the outer ridge-marginal basin and outer ridge-marginal trench complexes. Each of the 34 profiles exhibits a continental shelf although it may range from 20 to 300 miles in width. On each a shelf break is present at depths of 20-150 fathoms. Each profile shows a continental slope, the base of which may be from 300 to 1900 fathoms deep.

Northern Grand Banks Sector.—On profiles W-1 to W-6 (Pl. 24), across the Grand Banks of Newfoundland, the shelf ranges from 120 to 285 miles in width. Exceptionally strong local relief of 50-100 fathoms is found on the shelf in profile W-1 northeastward from Newfoundland. The shelf break, which occurs at 150 fathoms, is abnormally deep—more than twice the depths found off New England. The continental slope has a typical gradient of 1:20 but is unusually short as the continental rise is reached at 725 fathoms. From this depth the continental rise descends to the 1700-fathom curve at a gradient of 1:140. This gentle slope is interrupted by a group of exceptionally rugged lower continental rise hills which rise to 1250 fathoms. Northeast of the hills the 2200-fathom line marks the rather abrupt beginning of the abyssal plain which slopes seaward at a gradient of 1:1100.

Flemish Cap.—Profile W-2 crosses the Grand Banks, along its widest east-west axis, and also the semidetached bank called Flemish Cap. The shelf is much smoother than in profile W-1, except for a small deep of about 100 fathoms immediately east of Newfoundland. The shelf break at 150 fathoms is followed by a continental slope 150 to 500 fathoms deep which has a gradient of 1:20. The Flemish Cap is a difficult feature to classify. It is too large to be a seamount and too shallow to be a marginal plateau. We must treat it as a part of the continental shelf, semidetached from the rest by a 650-fathom-deep channel. The eastern flank of the Flemish Cap slopes off at gradients of 1:100 and 1:60 until at a depth of 650 fathoms the bottom drops precipitously to 1750 fathoms at a gradient of 1:10. Seaward of this point an 85-mile-wide continental rise has a gradient of 1:65 and 1:250 down to the Newfoundland Abyssal Plain which is at a depth of 2400 fathoms. Twenty miles east of the continental rise this profile crosses the Northwest Atlantic Mid-Ocean Canyon.

On profile W-3 the shelf is quite smooth, and the shelf break is reached at 60 fathoms. The profile runs slightly oblique to the continental slope and reveals a series of submarine canyons. The base of the slope is at 1700 fathoms where the gradient drops to less than 1:200.

Figure 11.—Two east-west profiles of Southeast Newfoundland Ridge

Positions of profiles are indexed on Plate 23. Both profiles plotted from nonprecision soundings (NMC).

Southeast Newfoundland Ridge.—From the southern tip of the Grand Banks a broad ridge runs southeasterly toward the Mid-Atlantic Ridge and forms a natural barrier between the Newfoundland Basin and the North America Basin [to the south]. Since it is almost impossible to define a boundary between the continental rise and the ridge, we consider the Southeast Newfoundland Ridge an extension of the continental rise. The ridge is 60-100 miles wide, and its crest plunges southeastward from depths of 1500 fathoms near 50° W. to depths of 2200 fathoms near 45° W. Profiles N-1 and N-2 (Fig. 11) cross the Southeast Newfoundland Ridge at about 41.5° N. and 39.5° N. respectively. Profile N-1 is of poor quality, which probably accounts for the lack of fine-textured relief. The Mid-Ocean Canyon is again seen at the eastern end of Profile N-2. Profile W-5 (Pl. 24) crosses the Southeast Newfoundland Ridge from north to south. The similarity of profiles W-5 and W-23 suggests that the Southeast Newfoundland Ridge is an outer ridge of the same kind as the one east of the Blake-Bahama region. The northern one is not so long, and it does not totally enclose a basin. Otherwise, it is quite similar to the outer ridge east of the Bahamas in relative position, size, and surface features. The term Southeast Newfoundland Ridge was proposed by Wüst (1940b; 1943) and the feature has been shown on bathymetric charts (Tolstoy, 1951) and profiles (Emery, 1950). This ridge will be discussed again in connection with the Mid-Ocean Canyon and the ocean-basin floor.

Southern Grand Banks Sector.—Profiles W-4 and W-5 cross the southern tip of the Grand Banks. The shelf break is at 50 fathoms on both profiles. On Profile W-4 an apparent gradient of 1:25 extends from 200 to about 1000 fathoms where, after some irregularities probably associated with submarine canyons, the gradient drops to 1:40. This lower gradient extends to 1750 fathoms. Profile W-5 is quite similar to W-4 except that a steep initial slope of 1:5, from about 200 fathoms to 650 fathoms, is followed by a gradient of 1:80 which continues to 1000 fathoms. This same terracelike feature is also seen on W-3, W-4, W-5, W-6, W-7, and W-8. Below 1000 fathoms a gradient of about 1:50 is found on profiles W-4, W-5, W-6, W-7, and W-8. Profile W-6 runs south of the Grand Banks through the epicenter of the 1929 Grand Banks earthquake and then south through the area passed over by the 1929 Grand Banks turbidity current (Heezen and Ewing, 1952). The depression marked by the 1150-fathom sounding on the continental slope in Profile W-6 is a canyon running south from the Laurentian Channel. The continental rise is 250 miles wide and has an average gradient of 1:400 over its deepest third. At a depth of 2750 fathoms the gradient abruptly drops to 1:2000, and this marks the northern edge of the Sohm Abyssal Plain.

Figure 12.—Laurentian Channel

Profile replotted from NMC echogram

Figure 13.—Eastern Channel, Gulf of Maine

Profile replotted from NMC echogram

Laurentian Channel.—Between Nova Scotia and Newfoundland a 60-mile-wide, steep-sided, flat-floored channel cuts across the continental shelf connecting the Gulf of St. Lawrence and the open ocean. The nearly flat, smooth floor of this channel lies at about 230 fathoms depth. Figure 12 shows a cross-section of the Laurentian Channel near its seaward end. The origin and physiography of the channel has been treated by Shepard (1931; 1948); its structure has been reported by Press and Beckmann (1954). The Laurentian Channel continues as a steep-sided, box-shaped feature for more than 500 miles into the Gulf of St. Lawrence.

Scotian Shelf Sector.—The term Scotian Shelf was introduced by Canadian oceanographers and refers to the continental shelf southeast of Nova Scotia from the Laurentian Channel to the Gulf of Maine. This region is illustrated by Profiles W-7, W-8, W-9, W-10, and W-11 which run at slightly different directions, all starting in the vicinity of Halifax, Nova Scotia. Along the entire Scotian Shelf a series of 120-fathom depressions are located 10 to 80 miles off shore. A nearly continuous bank 20-60 fathoms deep and 10-25 miles wide lies along the seaward edge of the Scotian Shelf. From northeast to southwest this feature is divided by low saddles into Banquereau Bank (20-40 fathoms), Sable Island Bank (0-20 fathoms), Emerald Bank (40-60 fathoms), Lahave Bank (50-60 fathoms), and Browns Bank (20-60 fathoms). These shelf-edge banks culminate in the low, sandy Sable Island which stretches for about 25 miles along the outer edge of the shelf. In profile W-7 the break from the nearly flat shelf to a gradient of 1:50 occurs at 50 fathoms; a second break occurs at 80 fathoms. A gradient of 1:10 is reached at the 150-fathom curve. Profile W-11 is somewhat similar to W-7 in the form of the shelf break. Profiles W-8, W-9, and W-10 show shelf breaks at 50, 60, and 70 fathoms respectively.

The gradient of the continental slope off the Scotian Shelf ranges from 1:10 to 1:25 along the profiles. In profiles W-7, W-8, W-10, and W-11 the 1:25 gradient abruptly decreases to 1:70 at about 700 fathoms; in W-9 the 1:25 gradient continues to almost 2000 fathoms. It is difficult to decide whether to include the 1:40 to 1:60 segments with the continental slope or with the continental rise. However, since we have picked the gradient of 1:40 as the minimum gradient for true continental slopes, these segments fall within the continental rise. The continental rise thus defined averages 160 miles in width off the Scotian Shelf. Gradients are generally greater here than in the continental rise farther south. The "Gully", a large submarine canyon shown on navigational charts, lies about 25 miles east of Sable Island. The submarine canyons of the Scotian Shelf have not been accurately mapped, but the existence of many canyons in this area has been shown by several fathograms obtained in this vicinity. Figure 14 illustrates one sounding profile nearly parallel to the shelf near the "Gully". Several small canyons about 100 fathoms deep occur between 100 and 700 fathoms. Several larger canyons 300-500 fathoms deep and 7-10 miles wide are crossed on the lower continental slope and upper continental rise.

Figure 14.—Submarine canyons off the Scotian Shelf

Profile replotted from NMC echogram runs nearly parallel to trend of continental slope near Sable Island. On navigational charts largest canyon is known as the "Gully".

Gulf of Maine Entrance.—Southwest of the Scotian Shelf there is a narrow gap in the continental shelf similar to the Laurentian Channel. This feature, called either the Northeast Trough (Shepard, 1948) or Eastern Channel of the Gulf of Maine, is 15 miles wide and about 150 fathoms deep; it provides a deep-water entrance to the Gulf of Maine (Fig. 13). The Gulf of Maine is enclosed by Georges Bank off the New England shelf, Cape Cod, and southern Nova Scotia. This entrance has recently been described by Torphy and Zeigler (1957).

Gulf of Maine Interior.—Much of the interior of the Gulf of Maine has been surveyed in exceptional detail by the Coast and Geodetic Survey. The reader is referred to Murray's paper (1947) for a thorough description of the floor of the Gulf of Maine. In general the floor is extremely irregular with several 20- to 40-fathom "hills" per mile. The floor is covered by sediment which transmits sound so readily that the area is noted for exceptionally pronounced sub-bottom reflections from the rock layers beneath the sediment.

Northeastern United States Sector.—From the northeast tip of Georges Bank to Cape Hatteras the continental margin is remarkably uniform in morphologic detail. Profiles W-12 to W-19 differ very little from the type profile off northeastern United States (Fig. 15). The continental shelf and slope in this area are better surveyed than in any other area in the Atlantic. The surveys of the Coast and Geodetic Survey were contoured and described by Veatch and Smith (1939). The sediment studies of Stetson (1936; 1938; 1949) and the seismic studies of Ewing and others (1937 et seq.) make this geologically the best-known shelf and slope in the world. Many large and well-mapped canyons cut the continental slope from Georges Bank to Cape Hatteras. The large submarine canyons off Georges Bank have attracted great interest because of their remoteness from rivers and associated discharges of river sediments.

The continental shelf is 50 to 100 miles wide in this sector. Toward Cape Hatteras the coastal plain widens as the shelf narrows. The combined features are called the "emerged and submerged coastal plain." The gradient of the continental slope ranges from 1:8 to 1:15 and the base of the slope with one exception is at 1150 ± 100 fathoms. The shelf break is at about 50 fathoms on all profiles. On profiles W-12, W-13, W-14, W-15, W-18, and Figure 1 of Plate 4 there is a second break at 75-100 fathoms.

The break between the continental slope and the upper continental rise is abrupt at some places and occupies a distance of 5 to 10 miles in other places (Fig. 16). In each case the gradient of the next lower 30- to 50-mile segment is 1:100.

All profiles from Georges Bank to Cape Hatteras, a span of more than 500 miles, show both an upper and a lower continental rise (Profiles W-13, W-19, and Fig. 15). The uniformity in the continental slope gradient carries over into the continental rise. Both the upper continental rise and the lower continental rise are divided into three segments. The width, gradient, and depths of each of the slope segments are remarkably similar. Representative values based on profiles W-13 to W-19 and Figure 15 are shown in Table 1.

Figure 15.—Continental margin provinces: Type profile off northeastern United States.

Profile plotted from PDR records. This profile is representative of the sector from Georges Bank to Cape Hatteras.

The upper continental rise and the lower continental rise are essentially terrace or shelflike features. Each has a relatively steep (1:50-1:200) outer face (segment 3) and a relatively gentle (1:250-1:2000) shelflike surface (segment 2). In each case a slope of intermediate gradient (1:80-1:300) (segment 1) connects the upper shelflike surface with the next higher face. In the case of the upper continental rise the next higher face is the continental slope. Other smaller-scale terracelike features may eventually be correlated along the strike when more data are available. The local relief exceeds 20 fathoms in the deeper parts of segment 3 of the lower continental rise. A range of hills extends for a few hundred miles along the base of the continental rise as indicated on the physiographic province chart (Pl. 20). These hills, known as the lower continental rise hills, are 30-100 fathoms high and each is 1 to 3 miles wide. An echogram (Pl. 3, fig. 4) shows three continental rise hills. The only other part of the continental rise where relief of more than 20 fathoms is generally encountered is in segment 1 of the upper continental rise. The irregularity in this case is probably related to the extensions of numerous continental-slope canyons onto the continental rise. Relief of 5-10 fathoms is almost universal in segments 1 and 2 of the upper continental rise. The echogram reproduced in Figure 1 of Plate 3 shows typical minor-relief features of the upper continental rise. An oblique crossing of a submarine canyon on the upper continental rise is shown in Figure 2 of Plate 3. The smooth topography typical of most of the remainder of the continental rise is well illustrated by the echogram shown in Figure 3 of Plate 3.

The Hudson Submarine Canyon cuts across the continental rise in this sector. A chart contoured by Ivan Tolstoy and the authors from surveys made in 1949 is shown in Plate 2 (Ericson, Ewing, and Heezen, 1951). A series of 30 cross profiles is shown in Figure 17.

Figure 16.—Tracings of PDR records of continental and insular slopes

• (a). Insular slope of Madeira southwest of Funchal.
• (b). Continental slope of Europe.
• (c). Continental slope off northeastern United States.

The Hudson Canyon, which is more than 500 fathoms deep and 5 miles wide in the continental slope (Upper Gorge), narrows to less than 2 miles and shallows to 50 fathoms at the base of the continental slope. As it cuts across segment 2 of the upper continental rise the canyon gradually deepens. When it cuts into the upper part of segment 3 the canyon deepens to 300 + fathoms, widens to 3 + miles, and forms the Lower Gorge. The canyon gradually narrows and shallows as it cuts across the lower continental rise. It ends near Caryn Peak where sediment cores indicate an extensive delta or submarine alluvial cone. The upper continental rise and the lower continental rise can be tentatively traced northeastward through the Scotian Shelf and Grand Banks sectors. The irregular bench at 2250-2450 fathoms on W-6 and the bench at 2300 fathoms on W-8 and W-11 can probably be referred to segment 2 of the lower continental rise.

Near Cape Hatteras the entire character of the continental margin changes. Benches which were barely discernible farther north widen to form a series of broad steps which resemble a giant staircase descending to the depths of the Atlantic. These benches appear to merge with the benches of the Blake Plateau and Escarpment farther south. However, insufficient profiles exist to permit a firm correlation.

Figure 17.—Cross sections of Hudson Submarine Canyon

Blake Plateau Sector.—This sector is divided into two parts, the northern part from Cape Hatteras to 29° N. (essentially a transition zone) and the southern or main Blake Plateau between 29° N. and the northern edge of the Bahamas at 26° N.

The shelf break lies parallel to the coast, about 60 miles offshore, from just south of Cape Hatteras to Cape Canaveral. The continental slope extends (at a gradient of 1:40) only to depths of 300-400 fathoms where the lower gradients (ca. 1:1000) of the Blake Plateau are found.

The main or southern Blake Plateau is 170 miles wide (east-west) and extends from the latitude of Grand Bahama Island to 30° N. From this point to Cape Hatteras the Blake Plateau narrows and disappears. The Blake Escarpment forms a precipitous drop to abyssal depths along the eastern edge of the plateau. The top of the Blake Escarpment lies at about 550 fathoms, and its base at about 2600 fathoms. The Escarpment is typically formed by two or three distinct slope segments.

An echogram obtained along a track running southeast from Charleston, South Carolina, is reproduced in Figure 1 of Plate 7. The continental shelf extends from the shore at an extremely low gradient to the 25-fathom isobath where a small definite notch marks an increase in gradient to 1:1000. This gradient continues to the 50-fathom isobath where it changes to 1:40. At the 90-fathom curve the gradient increases to 1:120 and continues to 160 fathoms where it finally increases to 1:40. This continental slope drops from 160 fathoms to 280 fathoms where the gradient flattens, and the surface changes from smooth to rough, with hills 10 to 20 fathoms high and half a mile to 1-½ miles wide. These hills, which extend for 4-6 miles along the profile, directly underlie the Gulf Stream.

For 50 miles seaward of these hills the ocean floor is irregular between 230 and 300 fathoms. At 90 miles from shore five eastward-facing scarps 10-20 fathoms high form a striking contrast to the generally smooth, gently rolling topography. At 300 fathoms the gradient increases to 1:200, and the sea floor drops for the next 24 miles to 400 fathoms where a few small hills are associated with a drop in the gradient to 1:1000. Southeast of this point the bottom is smooth until at a depth of 430 fathoms a steep scarp drops abruptly 30 fathoms to form a mile-wide depression 20-30 fathoms deep. The southeast side of this feature rises to 445 fathoms, and southeastward of a few 5-fathom scarps the surface of the Blake Plateau becomes smooth.

• Figure 1. Upper Continental Rise
• Figure 2. Oblique Crossing of Submarine Canyon
• Figure 3. Smooth Bottom of Lower Continental Rise
• Figure 4. Lower Continental Hills

REPRESENTATIVE PDR RECORDS FROM CONTINENTAL RISE OF NORTHEASTERN UNITED STATES

• Figure 1. Shelf Break and Continental Slope off New York
• Figure 2. Small Hills on Continental Slope off Daytona Beach, Florida
• Figure 3. Small Hills in the Straits of Florida
• Figure 4. Small Abrupt Depressions on the Inner Part of the Blake Plateau

PDR RECORDS FROM CONTINENTAL SLOPE AND BLAKE PLATEAU

The positions of all echo-sounding records are shown in Plate 30. Depth in fathoms.

• Figure 1. West of the Crest of the Outer Ridge
• Figure 2. Western Side of Outer Ridge near Abyssal Plain
• Figure 3. Blake-Bahama Abyssal Plain

PDR RECORDS FROM OUTER RIDGE EAST OF THE BLAKE PLATEAU AND FROM THE BLAKE-BAHAMA ABYSSAL PLAIN

Depth in fathoms: The outgoing "pings" as well as "scattering layers" are recorded in the 0-400 fathom scale range; the bottom topography may lie within the range of any multiple of 400 fathoms. The depth scales indicated on the plates refer to the profiles of bottom topography. For example—Figure 1 of Plate 5 shows a 300 fathom deep "scattering layer" over which the bottom profile has been superimposed.

PDR RECORD OF OUTER RIDGE SHOWING SUB-BOTTOM HORIZON

One 3 millisecond ping was transmitted and received once each second. Depth in fathoms.

The same general succession of topographic features is shown in a echogram (Pl. 7, fig. 2) taken along a southeast-northwest line east of Daytona Beach, Florida. The small definite notch at 26 fathoms is present, but a significant difference between the two echograms is seen between 90 and 300 fathoms. On the Charleston profile a steep 1:40 gradient slope marks this depth range, while, on the Daytona Beach profile, the gradient is relatively gentle (1:180); small but probably significant benches are found at 225, 270, 290, 375, and 385 fathoms. Both profiles have the same characteristic rugged 5- to 15-fathom hills at 400 fathoms at a point underlying the Gulf Stream. On the Charleston profile a broad, gently fractured arch separates the continental slope from the smooth outer part of the Blake Plateau. The small, sharp-crested hills noted on the Daytona Beach and Charleston profiles are also found at the north end of the Straits of Florida (Pl. 4, fig. 2).

Blake Escarpment.—Profiles W-23, W-24, and W-25 and Figure 18 illustrate the form of the Blake Escarpment. The outer edge of the Blake Plateau abruptly breaks off at about 600 fathoms. Here gradients increase to 1:30. This segment continues with a few minor breaks to a depth of 1200 to 1500 fathoms where a narrow bench or at least a major break in slope occurs. Below this bench the escarpment drops so steeply that only a few side echoes are recorded. The gradient here exceeds 1:2 in several profiles. At 2400 fathoms there is in places another narrow bench, but on other profiles the abyssal plain of the floor of the Blake-Bahama Basin lies directly at the foot of the steepest segment. A peculiar fact is that along many east-west cross sections the deepest point in the basin lies directly at the foot of the escarpment. A similar deepening adjacent to the Campeche and West Florida escarpments in the Gulf of Mexico has been reported (Ewing, Ericson, Heezen, 1958).

Antilles Outer Ridge.—South of Cape Hatteras a ridge ranging in width from 60 to 200 miles lies about 100 miles east of the Blake Escarpment and the Bahama Banks. The ridge has two parallel crests 100 miles apart which both plunge to the south. At 30°N. the crests average about 1600 fathoms in depth, but at 25°N. they are 2750 fathoms, a drop of nearly 1000 fathoms in 300 miles. The smooth rolling topography of the ridge between Cape Hatteras and 24°N. resembles the continental rise off New York or, in some areas, the somewhat stronger relief of the Bermuda Plateau of the central Bermuda Rise (Pl. 5, figs. 1, 2).

South of 24°N. and in the vicinity of Hispaniola the ridge is poorly known and difficult to study because of its low relief and the large errors in most nonprecision soundings taken in such a great depth of water. North of Puerto Rico the outer ridge appears as a clearly defined feature between the Puerto Rico Trench on the south and the Nares Abyssal Plain on the north. Again there are two parallel crests 60 miles apart marked by low relief of 20 to 100 fathoms at a depth of 2750 fathoms.

The Antilles Outer Ridge, continuing to the east, merges with the Lower Step of the Mid-Atlantic Ridge. But more probably it skirts the Lesser Antilles to join the continental rise off South America. The crest zone is covered by Globigerina ooze in the north and red clay in the south; it is isolated from the silty clays of the continental slope except on its northwest flank. Sub-bottom echoes appear on fathograms taken across the outer ridge. Data suggest that a prominent 8- to 10-fathom sub-bottom interface extends over the outer ridge between San Salvador and 30°N. (Pl. 6).

Bahamas Sector.—The Bahamas sector can be divided into two parts: (1) the broad (200 miles wide) northern area dominated by broad, shallow banks broken by relatively narrow, deep (ca. 1000 fathoms) tongues or channels; (2) the narrow southeastern part where the banks decrease in area and the tongues deepen (to 2200 fathoms) and widen. This southeastern part tapers to the east in the direction of the Puerto Rico Trench. The southeast tip of this area is formed by Navidad Bank, whose eastern slopes drop to the floor of the Puerto Rico Trench.

Figure 18.—Tracing of PDR record of Blake Escarpment

No soundings were recorded between 1600 and 2400 fathoms, a common difficulty on this precipitous escarpment. Note how Blake-Bahama Abyssal Plain slopes toward the base of the escarpment.

The Bahama Banks appear to consist of a slab superimposed on the same surface which forms the Blake Plateau. The Blake Escarpment merges with the lower part of the eastern slope of the Bahamas. The slopes of the Bahamas are precipitous; gradients are of the order of 1:4 to 1:8. Vertical cliffs, which lie just below the 50-fathom curve, have been reported by lead soundings (Armstrong, 1953). The Tongue of the Ocean and the Northeast and Northwest Providence channels form a network of submarine canyons (Hess, 1933). The floor of this canyon system has a continuous down-slope gradient to the floor of the Blake-Bahama Basin. Sediment cores from the floor of the Blake-Bahama Basin (2525 fathoms) (Ericson, Ewing, and Heezen, 1952) contained thick (1-3 m) beds of graded calcareous sand. The steep slopes of the Bahamas are generally rocky, and cores here reveal a variety of Tertiary and Cretaceous sediments. Exuma Sound also is linked by submarine canyons to the Blake-Bahama Basin. The graded calcareous sands of the Blake-Bahama Basin were probably carried through this submarine canyon system by turbidity currents. The topographic benches of Exuma Sound have been described by Lee (1951).

The southeastern Bahamas from Great Inagua to Navidad Bank consist of more numerous isolated banks and greater expanses of ocean floor in the depth range of 1700-2400 fathoms. The basins behind the southern Bahamas lie below the sill depth between the line of banks. Thus an abyssal plain lies entrapped in the Hispaniola-Caicos Channel and the southeastern portion of the Old Bahama Channel. Profile W-29 (Pl. 24) shows much irregular relief between 1000 and 1500 fathoms.

Puerto Rico Trench Sector.—With the disappearance of the Bahama Banks at the eastern edge of Navidad Bank, the continental margin assumes its third mode of expression: the marginal trench-outer ridge complex. The outer ridge, which nearly disappeared in the southeastern Bahama sector, again becomes a prominent feature. The last traces of the marginal plateau merge with the continental slope of Hispaniola and Puerto Rico. The Puerto Rico Trench develops rapidly east of Navidad Bank; it lies between the outer ridge and the continental slope or landward trench slope of the Greater Antilles. The relief of the outer ridge is somewhat greater than that observed on the outer ridge farther northwest near the Blake Plateau. The floor of the Puerto Rico Trench is divided into two parts by a longitudinal ridge. The deepest parts of both are floored by nearly level trench plains. The deeper one on the outer or northern side maintains a nearly constant depth at 4358 fathoms (4585 fathoms corrected) for 150 miles (Ewing and Heezen, 1955). The southern or inner trench is more variable in depth, and the trench plain lies intermittently along its length. Its depth ranges from 3600 to 4300 fathoms (uncorrected). The walls of the Puerto Rico Trench are formed by a series of extremely steep segments which show a remarkable persistence along the trench. Profiles W-32 and W-33 illustrate the typical trench profile north of Puerto Rico. Breaks in slope are observed at 700 fathoms and at 1500 fathoms; at 2000 fathoms the gradient steepens to > 1:6. In this region all soundings are side echoes. The outer or seaward wall of the trench is also characterized by a succession of laterally persistent slope segments. A bench at 3800 fathoms at the top of a scarp which drops to the bottom of the trench is characteristic of several profiles.

Anegada Passage.—The Virgin Islands Bank extends 30 miles east of Puerto Rico along the south side of the Puerto Rico Trench. Between the Virgin Islands Bank and St. Croix a deep passage cuts through from the Atlantic to the Caribbean. This deep passage is 130 miles long and runs along an e-ne-w-sw line. Its walls are extremely steep (9°-43°) (Frassetto and Northrop, 1957). The structure of this feature has been studied by Shurbet and Worzel (1957) and by J. Ewing et al. (1957).

Heezen et al., PL. 7

[TRACINGS OF PDR RECORDS ACROSS CONTINENTAL SLOPE AND PART OF THE BLAKE PLATEAU]

SOUTHWESTERN EUROPE AND NORTHWEST AFRICA: The continental margin of Europe and Africa is illustrated in Plate 25 by only 23 profiles, and therefore our description of this area cannot be as detailed as that for North America. For purposes of description we have broken the area into four sectors of contrasting type: (1) the Anglo-French sector: (2) the Iberian sector; (3) the Gibraltar sector; and (4) the North African sector. In all but the three profiles in the Gibraltar sector there is a well-defined continental shelf. The continental slope is everywhere present but ranges widely in height and gradient. The continental rise is extremely well developed off Africa but virtually absent in the Bay of Biscay. Abyssal plains are shown on almost every profile, but their depth ranges from 2550 to 3075 fathoms, and their width from 50 to 250 miles.

Anglo French Sector.—Profiles E-1, E-2, and E-3 are representative of the Anglo-French sector which extends from 45° N. to 60° N. Only the southern part of this sector is shown on the physiographic diagram. The continental slope is broken by a prominent bench or marginal plateau at 1000-1200 fathoms. The northwest corner of the physiographic diagram south to 42° N. is included in Hill's (1956) contour chart. According to this chart the prominent 1200-fathom bench extends for more than 900 miles along the continental margin from 45° to 60° North Latitude.

The continental slope from the shelf break to the prominent bench exhibits smaller benches and changes in slope, many of which probably will be correlatable when more profiles are obtained in this region. The general gradient of this portion of the slope ranges from 1:10 to 1:30. On the bench individual slope segments range from 1:40 to 1:80. Below the bench the sea floor drops from 1500 to 2100 fathoms at gradients of 1:15 to 1:30.

In profiles E-1 and E-3 a narrow continental rise about 70 miles wide with gradients of 1:250 to 1:800 lies at the foot of the continental slope. This narrow continental rise (Pl. 8, fig. 1) gives away to a 60-mile-wide abyssal plain at about 2500 fathoms depth (Pl. 8, fig. 3). Abyssal-plain gradients are about 1:2000 in this region. Toward the southeast corner of the Bay of Biscay the 1000- to 1200-fathom bench disappears (Fig. 16b); the continental rise and continental shelf narrow to 30 miles. We have only two profiles off the north coast of Iberia, but Hill's (1956) chart suggests that the slope is relatively steep with some prominent benches, and that the continental shelf and continental rise narrow to 10 to 15 miles in width. The prominent Cape Breton Submarine Canyon lies at the southeast corner of the Bay of Biscay (Bourcart, 1949).

Iberian Sector.—This sector was described by De Andrade (1937) on the basis of a large number of discrete soundings by the Portuguese Hydrographic Service. Relatively few echo-sounding profiles are available for the area, and little more can be added to De Andrade's description. The shelf in most places is less than 20 miles in width. The few echo-sounding profiles available indicate several prominent benches on the continental slope. Exceptionally large submarine canyons occur off Cape St. Vincent, Setúbal, Lisbon (2), and Nazaré. Preliminary investigations by the Lamont Geological Observatory indicate that Tertiary sediments outcrop on the walls of Lisbon and Setúbal canyons (Sutton et al., 1957). Tertiary sediments have also been obtained by dredging and cable grappling along the continental slope of northwest Iberia (Wiseman and Ovey, 1950).

Gibraltar Sector.—The continental margin in this sector is unique. The straits cut through at about 200 fathoms so that profiles through the straits show no continental shelf. A typical abrupt continental slope is also absent since only locally do slope segments have gradients exceeding 1:45. A series of prominent benches is seen in the sector from profile E-7 to E-10.

The dominant bench levels in this sector are 300, 600, 850, 1300, 1700, and 2100 fathoms. Insufficient profiles and dredgings are available in this area to permit the correlation and dating of these benches. Photographs in the area show sandy and rocky bottom, and thus dredging in this area might yield rich rewards in ancient sediments. The benches are so prominent that a detailed study of the topography should be equally rewarding. Of particular interest is the manner in which the broad benches in the Gibraltar area merge with the smaller benches and breaks in slope of the steep continental slopes of Portugal and Algeria. The topography in this sector most closely resembles that of the northern Blake Plateau (profiles W-21, W-22) and the northern part of the Anglo-French sector. In all these areas many benches are developed, and steep slopes are only locally developed.

Northwest African Sector.—The continental margin from northwest Morocco to Dakar is remarkably uniform and rather closely resembles the northeastern United States sector. The continental shelf and slope are well developed (Fig. 19). The shelf is 15 to 70 miles in width and thus is somewhat narrower than either the North American shelf or the Anglo-French shelf. The shelf break ranges from 50 to 80 fathoms. The continental-slope gradients range from 1:15 to 1:40 and are thus somewhat less steep than in the American sectors. Prominent benches are common at 300, 600, 850, 1200, and 1600 fathoms. The continental rise is well developed and is compound. The main contrast between the North African and American sectors is the greater width of the African continental rise. Off northeastern United States a line of isolated volcanic peaks cuts across the continental rise and abyssal plain. In the North African continental margin volcanic peaks are larger, more numerous, and lie in coalescing lines or along ridges. The Cape Verde and Canary groups lie in the continental rise near the outer edge of the upper continental rise. All provinces except the continental shelf widen from Gibraltar southward toward Cape Verde.

On profile E-11 off Casablanca the distance from the shelf break to the lower continental rise is only 50 miles as compared with a similar measurement of 500 miles at Cape Verde. Off Casablanca the continental slope extends to 1400 fathoms where the gradient drops to less than 1:40 from 1:10-1:20 on the continental slope. The upper continental rise which widens to more than 100 miles farther south is only poorly developed off Morocco. No other deep-sea echo-sounding profiles are available for the Moroccan continental margin. Surveys of the continental slope made by the French Hydrographic Service during the past few years will, when published in full, undoubtedly provide much valuable information on the topographic benches in this important area (Grousson, 1957).

• Figure 1. Continental Rise West of St. Nazaire, France
• Figure 2. Biscay Abyssal Plain. Note Small Mid-ocean Canyon
• Figure 3. Biscay Abyssal Plain

PDR RECORDS EUROPEAN CONTINENTAL RISE AND BISCAY ABYSSAL PLAIN

Depth in fathoms.

• Figure 1. Small-Scale Roughness, Upper Continental Rise
• Figure 2. Rolling Topography, Upper Continental Rise
• Figure 3. Cape Verde Abyssal Plain
• Figure 4. Abyssal Hills

PDR RECORD OF ABYSSAL HILLS, SOUTHEAST OF BERMUDA RISE

Note sub-bottom echos from beneath intermontane basin floor. Depth in fathoms.

Area of each photograph is about 6 by 8 feet.

Plate 11.—OCEAN-BOTTOM PHOTOGRAPHS ON THE CONTINENTAL MARGIN AND OCEAN-BASIN FLOOR

Figure 1. (Station T1-3, photo 27) Depth 260 fathoms, location 47° 42´N., 07° 34´W., just below shelf break west of St. Nazaire, France. Note small ripples which appear to be superimposed on larger ripples.

Figure 2. (Station T1-3, photo 7) Depth 285 fathoms, location several hundred feet from photograph in Figure 1. Note holothurians and solitary coral attached to rocks. Ripple marks are less prominent than in Figure 1.

Figure 3. (Stations T1-16, photo 28) Depth 2600 fathoms, location 46° 50´N., 11° 25´W., northern part of Biscay Abyssal Plain. Note tracks of bottom crawlers, and the prominent conical mounds, each with a central hole.

Figure 4. (Station T1-18, photo 29) Depth 2650 fathoms, location 43° 56´N., 11° 12´W., southern part of Biscay Abyssal Plain. Note meandering ridge made by subsurface burrower. Note also starfish, upper left, and quantity of fecal pellets and holes in the bottom.

Figure 5. (Station T1-20, photo 53) Depth 2850 fathoms, location 42° 18´N., 14° 47´W., northeastern part of Iberia Abyssal Plain. Note holes with converging tracks and large mound in upper right.

Figure 6. (Station T1-58, photo 14) Depth 3072 fathoms, location 29° 17´N., 57° 23´W., Abyssal Hills southeast of Bermuda Rise. The round objects are manganese nodules. Note shark's tooth in lower right. Note also small holes indicating bottom dwellers, and meandering raised ridge of sub-bottom burrower. Of particular interest are the small moats surrounding many of the nodules; they are probably scour marks caused by bottom currents. These currents must be very gentle since none of the nodules seems to show evidence of recent rolling.

Positions of stations shown on Plate 30.

Profile E-12 passes from the African coast between Fuerteventura and Gran Canary toward the northeast. Here the continental slope extends only to 1000 fathoms. The Canary Islands rise abruptly from the continental rise. Except for gradients of the order of 1:15 on the steep slopes of these volcanic islands the gradients of the continental rise are 1:300-1:1000.

Profiles E-13 and E-14 end on the east near Gran Canary and thus do not show most of the upper continental rise. They do show the remarkably wide and nearly level lower continental rise which reaches a width of more than 500 miles.

Profiles E-15 and E-16 lie off Spanish Sahara. In both profiles the gradient is 1:10 to 1:20 between the 50-fathom shelf break and a bench at 300-500 fathoms. In both profiles the gradient drops below 1:40 at about 1200 fathoms. Both profiles show numerous prominent benches on the continental slope. The upper continental rise with gradients of 1:350-1:1200 extends to the western limit of the profiles.

In profiles E-17 and E-18 the continental slope becomes gentler, and only in the upper 500 fathoms of E-18 does the gradient exceed 1:25. The upper continental rise is about 60 miles wide with depths predominantly about 1600 fathoms, and the lower continental rise lies at about 2100 fathoms and is very smooth.

Figure 19.—Continental-margin provinces: Type profiles off Northwest Africa

Profiles E-19, E-20, and E-21 cross the continental margin off Dakar and the Cape Verde Plateau which rises from the lower continental rise. The Cape Verde Plateau consists largely of the coalescing bases of the volcanic Cape Verde Islands. The lower continental rise and the abyssal plain reach their maximum width at this latitude. The width of the ocean, the width of the Mid-Atlantic Ridge, the width of the abyssal hills, and the depth at the axis of maximum depth all reach their maximum values for the North Atlantic at this point. The characteristics of the continental rise in this sector are listed in Table 2. The reliability of these figures is much poorer than those given for northeastern United States, owing to the smaller number of profiles in this sector.

A famous submarine canyon, the Fosse de Cayar, lies just north of Cape Verde. Other submarine canyons are certainly present in the sector since any profile parallel to the strike of the topography reveals large irregularities probably related to canyons. Echograms (Pls. 9, 13) taken in the continental rise in this sector show distinct contrasts in the topographic detail of the sea floor. The rugged topography of the abyssal hills (Pl. 9, fig. 4) contrasts sharply with the nearly flat, extremely smooth abyssal plain. (Pl. 9, fig. 3). The continental rise is nowhere so smooth nor so flat as the abyssal plain. The continental rise ranges from 10- to 20-fathom rolling hills 5-10 miles in width to 2- to 5-fathom hills a few hundred feet across (Fig. 19). At the seaward edge of the abyssal plain the echo sounder penetrates the bottom to reveal interfaces 5-20 fathoms below (Pl. 13, Fig. 4). Sub-bottom penetration of 5-15 fathoms is occasionally encountered on the continental rise on local topographic highs. Lower continental-rise hills of the type observed off eastern United States have not been observed off Africa.

BENCHES AND TERRACES OF THE CONTINENTAL MARGIN

The topography of the continental margin provinces is divided into a series of benches or terraces. The largest are the continental shelf and slope (continental terrace) and the upper and lower continental rise. Superimposed on each of these major features is a series of smaller benches and terraces which range from features a few miles wide to simple breaks in the gradient of the continental slope.

Many of these features can be traced for hundreds of miles (Heezen et al., in press); some are intermittent, others change in depth with distance along the shelf; still others are only locally developed. We can propose at least four possible origins for terraces or benches: (1) ancient shore features; (2) structural (or rock) benching; (3) block faulting; and (4) landslide or slump scars.

The submerged terraces within a few hundred feet below present sea level can probably best be explained as ancient beaches formed during the lower sea levels of the Pleistocene. The fact that the same levels are found along coasts of diverse geology and tectonic development supports the eustatic origin of terraces between sea level and 70-100 fathoms. The gradients of the continental shelf are so low and the benches are so persistent that block faulting and slump scars are excluded as general explanations. The benches of the continental slope extend to depths of 1500 fathoms and vary in depth from point to point along the continental slope. These cannot be Pleistocene eustatic levels unless we consider that they were formed prior to recent large crustal deformations. Again the persistence of the benches for many miles argues against a fault-scarp or slump-scar hypothesis. Thus, while the benches of the continental shelf are probably ancient beaches, particularly those traced at the same depth for thousands of miles, the benches of the continental slope are probably rock benches, while some may represent step faulting.

SUBMERGED BEACHES ON THE CONTINENTAL SHELF: In Table 3 the depths of terraces or persistent levels of the continental shelf are listed for selected points in the North Atlantic. There is a remarkable uniformity in these data; the same levels are found near Newfoundland, in Florida, and on oceanic islands far from the glaciated areas.

On the basis of data obtained in the North Atlantic it is not possible to date the different terraces, but probably most were formed in the period between 12,000 and 5,000 years B.P. when the sea rose in consequence of the melting of the Wisconsin glaciers. Coring and dredging on these submerged ancient beaches could probably produce material datable by the radiocarbon method.

Continental Margin Benches: On each profile across the continental margin is a series of benches and changes in gradient which range from the shelf break to slight changes in gradient on the continental slope.

If a field geologist enters a new area of sedimentary rocks where road cuts do not exist he invariably goes to the stream valleys, and here he gets his first and best view of the geologic section. The stream's gradient is adjusted to the resistance of the rocks over which it cuts, and the form of the valley-side slopes reveals the nature of the underlying rocks even if they are grassed over.

This obvious field method had never been fully applied to the continental margin. Stetson (1936) dredged in the canyons of Georges Bank, and his hauls included Cretaceous sandstones and Tertiary marls and green sands. He concluded that the canyons had been cut deep into the continental margin to expose the underlying Cretaceous rocks, but he considered the continental slope the product of depositional processes.

However Upham (1894) had suggested that the continental slope formed a continuous outcrop of Tertiary and Cretaceous sediments from Newfoundland to Florida, a suggestion the writers consider quite probable. That is to say, an analogy can be made between the continental slope and one face of the Grand Canyon or to an erosional escarpment bounding a high mesa or plateau like the Book Cliffs of Utah and Colorado.

Only a few areas of the world are sufficiently well sounded to provide data for a study of structural benches. One cannot expect to see identical structural benches in each profile even across a slope composed of a laterally uniform series of horizontal beds of contrasting lithologies. The exact mode of erosion, the local system of jointing, and chance variations in a number of other variables make it necessary to have a large number of closely spaced, accurately located profiles. We are fortunate that the Coast and Geodetic Survey has surveyed virtually the entire continental slope from Georges Bank to Norfolk, Virginia. Almost all these sounding lines are run at right angles to the strike of the topography and are thus suitable for analysis of structural benches. In this same area the dredgings of Stetson (1936) on Georges Bank and the Esso Hatteras Light test provide us with information on the stratigraphy of the sediments which form the continental shelf and slope. The seismic work of Ewing and collaborators (1937 et seq.) provides us with further information on the dips and on the depths of a number of sedimentary rock series of contrasting lithology.

Fishermen began finding fossiliferous rocks on Georges Bank well over a century ago. They were not particularly pleased to obtain rocks instead of fish and generally threw the accursed rocks back into the sea. Some curious fishermen brought a few of the rocks to shore, however, and in time some of these were received by the museums (Upham, 1894; Dahl, 1925). These rocks contain Tertiary and Cretaceous fossils. The depths and positions of recovery of the rocks were generally unknown to the museums, and no clear idea could be gained of the exact occurrence of this material. Stetson (1936; 1949) conducted a series of scientific dredging operations in the Georges Bank area. His aim was to recover more of these older rocks from known depth ranges and positions.