Submerged River Channels.—One of the most interesting features in connection with the continental shelf bordering North America is that its generally plane surface is trenched in several places by cañon-like depressions similar to the narrow steep-sided valleys which streams sometimes cut in the surfaces of plateaus. This suggestion that the surface of the continental shelf is crossed by stream-cut channels is supported by the fact that several such depressions, leading seaward from the present mouths of large rivers, have been discovered by the sounding-line. The best known example occurs off the mouth of the Hudson and has been traced from New York Bay about 120 miles seaward to the edge of the continental shelf. It is deepest and best defined on the outer portion of the submerged plateau, where for a distance of 23 miles, beginning 97 miles from Sandy Hook, it has an average width of 3 miles and a maximum depth of about 2,500 feet below the surface of the bordering submarine plain, which has 20 fathoms of water over it. This cañon opens out in the seaward face of the plateau and forms a deep notch in the generally uniform crest-line of that escarpment. Farther "up-stream," so to speak, the channel narrows to a mile and a quarter, with some irregularities in depth, and near Sandy Hook it is not apparent, owing to the amount of débris, largely sand, swept
into it by shore currents. This evidence, strengthened by the fact that the true rock-cut valley of the Hudson as far as Troy is filled with clay and sand to a considerable but unknown depth, is abundant proof that the land was formerly higher than at present by at least 3,000 feet, and that the now submerged continental shelf off Long Island was then a plain above water, across which the ancient Hudson was extended. The river flowed across this plain for a sufficient length of time to excavate a cañon over 2,500 feet deep and 3 miles wide from crest to crest of its walls in its seaward portion. This submerged channel has the characteristics of a young, stream-cut valley and suggests that the plain across which it flowed to the eastward of Long Island was a submerged continental shelf previous to being upraised so as to be trenched by the Hudson.
The evidence as to changes in the elevation of the Atlantic coast furnished by the submerged valley of the Hudson does not stand alone. Similar but less well-defined channels have been discovered by soundings off the mouths of the Delaware and the Susquehanna, while the most remarkable instance of all is furnished by the submerged valley of the St. Lawrence, which has been traced through the Gulf of St. Lawrence and out to the brink of the submerged continental escarpment some 200 miles eastward of Nova Scotia. The tide now rises and falls in the St. Lawrence to within a few miles of Montreal; that is, the "Greater St. Lawrence" has lost about 1,000 miles of its length owing to a downward movement of the land.
Evidence of the nature just considered is lacking, or, more correctly, surveys and soundings which would perhaps reveal the presence of submerged river channels have not been made about the shores of the more northerly portion of the continent, but instructive results in this connection are to be expected when that region is thoroughly studied.
On the Pacific coast several transverse channels in the continental shelf, similar to the submerged valleys of the Hudson, have been discovered by soundings, some of
which are thought to be true stream-cut valleys; others, however, start from the coast where there are no rivers entering, and may be due to other and as yet unknown causes.
An exceptional feature in the geography of the Pacific coast to the south of the United States-Canadian boundary is furnished by the islands off the southern portion of California. These islands, of which Santa Rosa, Santa Cruz, Santa Catalina, and San Clemente are the most important, rise from water that is 300 or more fathoms deep to a height of from a few hundred to about 2,000 feet above the present sea-level. These islands are the summit portions of mountains similar to those which give a characteristic relief to southern California. One feature concerning the islands referred to which is of interest in connection with the study of the continental shelf is the presence on them of numerous terraces. These occur on a series of level, step-like areas, which sweep about the slopes of the islands, in a general way parallel to the present shore-line, and are records of an upward movement of the land. The highest well-defined terrace on San Clemente occurs at an elevation of 1,320 feet, but there are less distinct beach-lines up to 1,500 feet above the present sea-level. The evidence of movements of the land along the Pacific border of the continent might be multiplied, but enough has been stated to show that the western border of North America, like its eastern portion, is subject to fluctuation in reference to the level of the sea. The line which marks the passage of the solid earth beneath the waters of the sea is ever changing, owing to movements in the earth's crust. It is ever changing, also, owing to the action of waves and currents on the ocean's shores and other causes.
Life on the Continental Shelf.—No attempt need be made here to give an accurate description of the plants and animals which find a congenial home on the continental shelf. One needs, however, to have in mind a general idea of the wonderful abundance and variety of organic forms in the shallow waters adjacent to the continent to fully appreciate the changes in the relief of the ocean-floor, in which they
play a leading part, and also the vast economic importance of these marine harvest-fields and pastures, as they may be termed.
The continental shelf fringing North America reaches from within 500 miles of the equator to probably a less distance of the north pole. Its length following its broader curves is in the neighbourhood of 25,000 miles, and its average width is not far from 50 miles; its area is therefore something like 1,000,000 square miles. On account of the vast extent of this submarine plain and its great range in latitude, the conditions influencing the lives of the plants and animals inhabiting its surface or living in the waters covering it vary from place to place between wide extremes. The waters resting on it have a mean annual surface temperature of from 70° to 80° F. at the south, and about 32° F. at the north. In places great rivers and the turbid waters from glaciers bring in sediments and form muddy deposits; at other localities the currents, as in the path of the Gulf Stream off the Carolina coast, sweep the bottom clear of all light débris; and again bare rocks of limited extent are exposed. The depth of the water resting on the shelf varies through all gradations down to 100 fathoms. At the south the hours of light and darkness are approximately equal each day, but at the north there are six months of sunlight and six months of darkness each year. Still other variations, as of strength of currents, salinity, etc., exert an influence in this realm and lead to great diversity in its living organisms. Throughout its entire extent, however, the continental shelf abounds in both plant and animal life.