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 bed was flat. Sir James Clark Ross had obtained a sounding of over 2,400
fathoms in 1839, but it was not until 1869, when H.M.S. Porcupine was put at
the disposal of the Royal Society for several cruises, that a series 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 three 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 of 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.&