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 1_a-d_. The exaggerated profiles (1_b_) along
the tracks (1_a_) show a succession of peaks and valleys. These
features are sketched in along the tracks (1_c_). 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. 1_d_; 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).
[Illustration: 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.
[Illustration: 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.]
[Illustration: 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.