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.

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.