Our knowledge of the oceans a hundred years ago was confined to the two-dimensional
shape of the sea surface and the hazards of navigation presented by the irregularities in
depth of the shallow water close to the land. The open sea was deep and mysterious, and
anyone who gave more than a passing thought to the bottom confines of the oceans
probably assumed that the sea bad was flat. Sir James Clark Ross had obtained a sounding
of over 2,400 fathoms in 1839, but it was not until of deep soundings was obtained in the
Atlantic and the first samples were collected by dredging the bottom. Shortly after this the
famous H. M. S. Challenger expedition established the study of the sea-floor as a subject
worthy of the most qualified physicists and geologists. A burst of activity associated with
the laying of submarine cables soon confirmed the challenger's observation that many parts
of the ocean were two to there miles deep, and the existence of underwater features of
considerable magnitude.
Today, enough soundings are available to enable a relief map of the Atlantic to be drawn
and we know something of the great variety of the sea bed's topography. Since the sea
covers the greater part of the earth's surface, it is quite reasonable to regard the sea floor
as the basic form of the crust of the earth, with, superimposed upon, it the continents,
together with the islands and other features of the oceans. The continents form rugged
tablelands which stand nearly three miles above the floor of the open ocean. From the
shore line, out a distance which may be anywhere from a few miles to a few hundred miles,
runs the gentle slope of the continental shelf, geologically part of the continents. The real
dividing line between continents and oceans occurs at the foot a steeper slope.
This continental slope usually starts at a place somewhere near the 100-fatheom mark and
in the course of a few hundred miles reaches the true ocean floor at 2,500-3,500 fathoms.
The slope averages about 1 in 30. but contains steep, probably vertical, cliffs, and gentle
sediment-covered terraces, and near its lower reaches there is a long tailing-off which is
almost certainly the result of material transported out to deep water after being eroded
from the continental masses.