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.