From its position at the foot of the Appalachian Mountains this moderately elevated plain is termed the Piedmont plateau. The same plateau extends northeastward, however, where it is known as the New England plateau, and is without any definite boundary to separate it from the similar region in the maritime provinces of Canada. While local divisions of this great extent of moderately elevated plateau country are recognised, yet in a general view of the continent it is evident that the Piedmont plateau, the New England plateau, and the similar region, mostly of crystalline and igneous rocks, extending from Maine to Hudson Strait and beyond, in reality forms a single great geographical unit in which the geological structure and geographical features are much the same. The general history of this great Atlantic plateau, as it may, perhaps, be termed, shows that it consists mainly of metamorphic rocks, such as mica schist, gneiss, slates, etc., together with granite and other igneous rocks, and, to a minor extent, of sandstones, shales, and limestones, mostly of Jura-Trias and Carboniferous age. These rocks were upraised probably in part into lofty mountains, and then worn down by erosion nearly to sea-level, thus forming what is termed a peneplain, or a plain of subaerial denudation. It is not intended by this statement to imply that all of the
Atlantic plateau was ever a single great peneplain, but the same general history seems to apply to the entire region. The upheaval of the plains produced by erosion gave the streams greater energy, and they have begun the task of again reducing the land to sea-level, but have not as yet broadened their valleys so as to greatly modify the general plateau character of the region they traverse. The softer or more easily soluble rocks have been eroded away, leaving broad valleys, as in the several instances where sandstones and shales of what is known as the Newark system (Jura-Trias) occur in detached areas from South Carolina to Nova Scotia. Then, too, from northern New Jersey northward to Labrador and beyond, great glaciers have crossed the plateau or developed upon its broad north portion and have ground down its surface or left widely extended hills and ridges of morainal material upon it.
Where the process, just referred to, of planing down a tract of country nearly to sea-level is incomplete and remnants of former uplands still remain as isolated hills or groups of hills, such inheritances from the pre-peneplain stage may still exist when the region is elevated into a plateau and give diversity to its surface. An example of such a residual hill is furnished by Mount Monadnock, in southern New Hampshire, and, as proposed by W. M. Davis, the name of this old landmark is adopted as a technical term by which to designate all similar remnants of old uplands left standing on a peneplain. On the Atlantic plateau there are many monadnocks. They range in size from well-characterized hills to mountain-like forms, and may be isolated or occur in groups. When a monadnock stands alone its history may be easily read, but groups of such eminences, especially when of large size, become ranges of hills or even mountains, and may preserve so much of their former characteristics that they outrank the adjacent peneplain and become the dominant geographic feature of the region to which they give diversity. Such a passage from monadnocks to mountains seems to be furnished by the numerous isolated hills on the Atlantic plateau and the mountains of New England and of eastern Canada.
The most characteristic portions of what has just been termed provisionally the Atlantic plateau are the Piedmont plateau, which skirts the east base of the Appalachian Mountains from New York to central Alabama and the Labrador plateau. The eastern border of the Piedmont plateau is determined by the fall line described above, where the hard crystalline rocks of the Piedmont region meet the softer rocks of the Atlantic coastal plain. The rivers flowing eastward from the Appalachian, such as the Delaware, Susquehanna, Potomac, and the James, cross the Piedmont plateau in well-defined but narrow channels, usually from 100 to 200 feet deep, leaving the interstream spaces with generally level surfaces, although etched as it were by the lateral tributaries of the master streams. These rivers are shallow and rapid in their courses across the plateau, or in somewhat technical geographical language are not as yet graded, but on crossing the fall line become sluggish tide-water streams which widen into estuaries, as already described. Owing to the warm humid climate of this region, the rocks in the interstream spaces are usually deeply decayed and furnish clay soils which have characteristic red and yellow colours. Much of the cotton and tobacco of the South Atlantic States is grown on these residual soils which were left as the more soluble portions of the rocks were removed in solution.
Labrador, although in great part unexplored, is known to present the characteristic features of an irregular plateau, with a general elevation of 1,500 to 2,000 feet above the sea. The surface is undulating and has hills and hollows, the latter frequently holding lakes and swamps, but the inequalities seldom exceed 500 feet in vertical range. Although the western boundary of the Labrador plateau is indefinite, its area may be taken at about 500,000 square miles. In its western part, and apparently rising from the plateau as a group of residual hills left by erosion, are the so-called Laurentian or Laurentide Mountains. The eastern border of the plateau forms the bold and excessively rugged Atlantic coast-line of Labrador, characterized by steep cliff with a fringe of small rocky islands. The adjacent
sea is deep and the continental shelf narrow. On the south the plateau is bordered by a series of terraces which lead down to the St. Lawrence River and on the west it merges indefinitely with the plains of the continental basin.
The rocks of Labrador are largely metamorphic, but include ancient igneous intrusions, and are hard and resistant. The present surface is the result of deep erosion which has removed a great but unknown thickness of material and left exposed what was once the deeply buried basal portion of a mountainous region. This is a part of the oldest known land of the continent, and, so far as can be learned, has never been covered by the sea since a very ancient geological period. In addition to the long eras of erosion, during which the débris removed was deposited in part farther south, and contributed to the formation of the stratified rocks of the Appalachian region and interior continental basin, there was a comparatively recent extension of great glaciers over the plateau which removed the previously disintegrated and decayed rocks and left the present bare, rounded, and generally subdued hills with intervening basins. The soils are thin, for the reason that under the present climatic conditions rock decay is retarded, and are confined principally to the depression where peaty material has accumulated. Owing to the lack of soil on the uplands, the excess of water in the hollows, and to the severity of the climate, the forest is not continuous, the trees are small, and the vegetation generally of a subarctic character. This vast region is without agricultural possibilities, and thus far has been of value to man almost solely on account of its fur-bearing animals and the fisheries of its coast.
The northern border of the Atlantic plateau cannot at present be accurately defined. Seemingly it should include the Arctic archipelago, which provisionally may be considered as a deeply dissected plateau region, at present less elevated than formerly, thus allowing the sea to enter the valleys and to transform old uplands into islands. The bold and highly instructive explorations conducted in
recent years by Robert Bell, for the Canadian Geological Survey, have shown that in the region adjacent to Hudson Strait the plateau features characteristic of the greater part of the Labrador peninsula are absent and mountains occur which rank as the highest on the eastern border of the continent.