Southeast of Newfoundland the continental shelf has an irregular surface, marked by shoals and depressions, and furnishes the most valuable fishing-banks in the world. The 100-fathom curve is there over 500 miles from the coast. This is the broadest portion of the continental shelf now known on the Atlantic border of the continent. Northward of Newfoundland the Atlantic basin extends far into Davis Strait and Baffin Bay, and then its border swings outward about Greenland, but its true margin is there but imperfectly known.
To the north of the arctic coast of North America, as is suggested in part by the soundings made by Nansen, the submerged margin of the continent is probably broad and presents a steep escarpment to the arctic basin, but the outline of the true continent, as in the case of the present land extension in that direction, is unknown.
Soundings to the north of Cape Lisburne, on the northwest coast of Alaska, show that the 100-fathom curve is there over 200 miles from land. The exceptionally shallow sea covering this portion of the shelf continues westward to the coast of Asia, and southward through Bering Strait, so as to embrace the eastern portion of Bering Sea. The continental mass of North America is thus directly connected with the continental mass of Asia. A rise of the bottom of less than 200 feet in Bering Strait would bring about a land connection between the Old and the New World. This, as will appear later, is a most significant fact to students of geography and geology.
On the Pacific coast of North America the continental shelf is throughout much narrower than its average breadth on the Atlantic side of the continent, and is also more deeply submerged on its seaward border. The broad platform beneath the northern and eastern portions of Bering Sea—from which rise the low islands, St. Lawrence, St. Matthew, Nunivak, and the Pribilof group, now separated by water from 25 to 35 fathoms deep—extends to the south of the more easterly of the Aleutian Islands, and is prolonged eastward along the south border of Alaska, where the 100-fathom curve is from 10 to 20 miles from the coast-line, and approaches still nearer the land in the neighbourhood of the islands of southeastern Alaska and British Columbia. The shelf is narrow but well defined along the coasts of Washington and Oregon. Adjacent to California, Mexico, and Central America, its outer margin is barely 10 miles from land. Throughout the entire distance from the Aleutian Islands to Panama the outer border of the shelf is in general well defined, and its seaward escarpment descends abruptly to the floor of the vast Pacific
basin, where the sounding-line shows depths of from 2,000 to 3,000 fathoms.
Could the waters of the sea be removed and North America viewed from a distance, in the manner we are enabled to examine the surface features of the moon through a powerful telescope, an observer would behold a great plateau, having the present well-known triangular shape of the continent, rising boldly between the Atlantic and the Pacific basins. The surface of the plateau would be rough, in comparison with the generally smooth contours of the adjacent troughs, but even the highest mountains would be less in elevation above its general surface than the crests of its bordering escarpments above the adjacent depressions. The mountain-peaks when illuminated by the sun would appear as points of light with long, tapering morning and evening shadows, and the east and west plateau-borders would be strongly drawn bands of light or shadow, according to the time of day, 6,000 or 8,000 miles in length. The Bermuda, Hawaiian, and other islands now rising above the surface of the deep sea would stand on its desiccated floor as isolated, gigantic mountains—"Bermuda mountain" with an elevation of 15,000 feet, and the Hawaiian group of peaks with a culminating point of light 30,000 feet above the surrounding plain. The bordering slopes of the "North American plateau" and its slightly bevelled margin forming the present continental shelf would be lacking in details, and appear as a vast, smooth, curving belt of light or shadow, in striking contrast to the roughened surface now above water.
The North American continent is not exceptional in being partially submerged at the present time. Similar conditions occur about the margins of other continents which, as is well known, are fringed with broad submarine terraces built in part of their own débris. In fact, every large land mass on the earth under existing climatic conditions and present distribution of life, if it remained moderately stable for a sufficient length of time, would have a submarine shelf built about its borders.
Of what is the Continental Shelf Composed?—The rocks forming the present land surface of North America extend seaward from the existing shores and constitute the basal portions of the continental shelf, thus suggesting that the submerged platform is due, in part at least, to shore erosion—the waves having eaten into the land so as to make a terrace. That this is not the true explanation, however, may be shown in several ways.