Physiography

Abstract

The simplest geophysical measurement in the ocean is the depth of water—soundings that permit the construction of topographic charts from which physiography, structure, and stratigraphy can be inferred. Navigational value of lead-line soundings and of the traces of bottom sediment adhering to the lead weight long ago caused soundings to be obtained routinely in shallow water, as indicated in 450 B.C. by Herodotus (Carter, 1958, book II, chap. 5). Even deep-water soundings were obtained by the ancients according to Posidonius, who reported 1000-fathom depths off Sardinia in about 85 B.C. (see Strabo—Hamilton and Falconer, 1854, book I, chap. 3). Although the practice of sounding in shallow depths continued (see also Paul’s description of his voyage from Caesarea to Rome and wreck at Malta—Acts 27:28), sounding in great depths appears to have ceased for about 1500 years. The next recorded attempt was by Ferdinand Magellan, who in 1521 tried to sound the bottom in the Tuamotu Island group of the Pacific Ocean with six ordinary sounding lines tied together. Finding no bottom at an estimated depth of 750 m (Pettersson, 1954, p. 26), Magellan supposed that he had reached the deepest part of the ocean. The first truly deep-ocean sounding was made by Captain James C. Ross of H.M.S. Erebus in 1840 at Lat. 27°26’S, Long. 17°29’W (west flank of southern Mid-Atlantic Ridge). His measurement was 2425 fathoms (4435 m), about 15 per cent too deep as a result of inaccurate determination of the depth at which the line began to pay out more slowly or inaccurate ship’s position (Dietz and Knebel, 1968).