The superficial covering which gives the continental shelf its smooth contours is composed largely of sediments such as rivers bring from the land. This material is coarsest and in greatest abundance near shore and decreases both in the size of the particles composing it and in abundance towards the seaward borders of the shelf. The wash from the land is mostly deposited within a few miles of the coast-line and, as has been shown by dredging, is seldom carried, even under the most favourable conditions, more than about 100 miles seaward. Supplementing the fragmental material derived from the land, and increasing in thickness towards the seaward margin of the continental shelf—coincident with the increase in depth of the water—is a deposit of light-coloured calcareous mud or ooze, formed of the hard parts of animals and plants which live in the waters of the sea. The organisms which supply this material are in the main microscopic and live especially in the warmer seas in countless myriads. Their dead shells or cases fall to the sea-floor in a constant shower, much as the snow falls from the air, but continuously year after year and century after century. This descent of the hard parts of organisms, both calcareous and siliceous, from the waters of the sea has led to the accumulation of a sheet of slimy sediment over almost the entire sea-bottom. How thick this layer is we have no means of knowing, but it is probably many hundreds of feet.
The organic débris falling on the continental shelf descends through only a few hundred feet of water and is but little affected by its solvent action. The great number of organisms, such as the Foraminifera which secrete calcareous tests or "shells" causes the slime on the continental
shelves to be calcareous and in the condition to form limestone if cemented or subjected to sufficient pressure. In the deep sea, where the hard parts of dead organisms fall through many thousands of feet of water, their more soluble portions are removed and the bottom is covered throughout vast areas with a pinkish clay composed of the more insoluble residue of the calcareous shells and the cases of silica-secreting animals and plants.
The continental shelves are, in general, within the influences of ocean currents, and fine débris, as we seem justified in concluding, is removed from their surfaces, carried beyond their margins, and deposited on their seaward slopes. The shelves are thus built outward and are largely constructional topographic forms. Their outer slopes, where best defined, represent about the "angle of repose" in water of the fine material of which they are composed. These slopes are in several regions so precipitous that they probably would not retain their present forms, but descend in landslides, should the restraining pressure of the sea-water be removed.
In certain favoured regions, as about the southern extremity of Florida, over an extensive area in the West Indies, and on both sides of Central America, the conditions favour the growth of reef-building coral-polyps, and portions of the continental shelf in that region are covered with an irregular layer of living coral and dead coral rock. The importance of this resistant superficial layer on the minor features of the relief of the submarine banks, etc., needs to be considered in studying the secondary topographic features of many portions of the floor of shallow tropical seas.
In addition to the débris from the land and the rain of the hard parts of organisms from the water covering the continental shelf there is in northern regions a third but less important source of material furnished by floating ice. About the northern shores of America sea ice forms in winter, some of which is frozen fast to boulders and stones in shallow water, and when this ice-foot, as it is termed, is adjacent to steep cliffs, rock débris falls upon it. When
the ice becomes broken into cakes in the spring-time or during storms, it floats away, under the influence of the winds and currents, and as it melts drops its freight on the floor of the sea. This shore ice seldom travels far, and is probably not an important factor in the building of continental shelves. Of greater interest are the bergs derived from glaciers, especially in Greenland, many of which contain hundreds of thousands and even millions of cubic feet of ice and travel hundreds of miles before melting. In some instances these bergs carry with them rock masses, mud, etc., derived from the land over which their parent glaciers flowed, and as they melt, distribute this material over the sea-floor. The greater portion of this ice-carried freight derived from Greenland is dropped on the continental shelf, and not infrequently reaches the latitude of Halifax, and even journeys farther south. This berg-carried débris is mainly deposited on the continental shelf, for the reason that the cold currents which bring the bergs southward follow the coast in a general way, and are bordered on their seaward margins by warmer currents flowing northward. To the north of Nova Scotia the additions of material to the continental shelf through the agency of bergs is considerable in the aggregate, and as the process has been in operation for thousands of years, the banks or shoals in the sea off the Newfoundland coast are due in part to this cause.
Ice-carried débris forms an important source of material for the building of the continental shelf from New England northward and westward about the shores of North America, including Greenland, to Bering Sea, and to a less extent on the south coast of Alaska, where many comparatively small bergs are set afloat by glaciers which reach tide-water. Supplementing the distribution of débris over the continental shelf by shore ice and bergs, is the similar work carried on by the ice discharged into the sea by northern rivers, such as the St. Lawrence, Mackenzie, and the Yukon.
During the glacial epoch great ice-sheets like those now discharging bergs along the Greenland coast, but vastly
larger, entered the Atlantic all the way from New York to the Arctic Ocean, and along the Pacific coast from the Aleutian Islands to the State of Washington. During certain periods of this time of intense glaciation great additions of ice-borne débris must have been made to the continental shelf. The banks to the east of Newfoundland and other similar shoals as far south as Nantucket are probably due in large part to the débris deposited by the glaciers which formerly entered the sea in that region. It is of interest in this connection to note that the glaciers, even at the time of their greatest expansion, could not have extended beyond the seaward margin of the continental shelf, for the reason that on passing that boundary and entering deep water they must have broken off and given origin to bergs.