Exceptions to the fact that the coastal plain is composed mostly of soft sediments occur in southern Florida and in Yucatan, where coral rock has been upraised. Southeastward from Yucatan a coastal plain is wanting and rocky bluffs separated by stream-cut valleys come boldly down to the surf line. Partially drowned valleys on each side of Central America bear record of a recent but moderate downward movement of the land.
From New York northward along the border of the continent the coastal plain is mostly lacking, or if recognisable, is greatly modified by glacial deposits, and the Piedmont plateau, as it is known farther south, swings eastward and becomes a coastal plateau with a more or less roughened surface, which extends northward to Labrador and the Arctic Ocean.
The geologically recent oscillations of the continent, as stated in the preceding chapter, have been greatest in high latitudes, where the last movement, as there are reasons for believing, was upward and is still continuing. This rise, although it has not fully counteracted the changes produced by a preceding downward movement, has caused the shore-line to recede and a great area on the arctic border of the continent which was previously submerged has thus become exposed. The coastal plain on the west side of Hudson Bay, as described by T. B. Tyrrell, is about 50 miles wide in the vicinity of Fort Churchill, latitude 55°, and broadens rapidly northward of that locality. In latitude 64° the boundary between these new lands and the older plains of the interior is about 300 miles from the present
shore; thence northwestward it has not been traced, but may be expected to cross the Mackenzie some 250 miles from its mouth and pass westward into Alaska.
This arctic coastal plain is known in part as the Barren Grounds, but in general may be designated as a tundra, as over extensive areas it is similar to the still greater tundras of Siberia. This tundra forms the extreme northern and northwestern border of the continent in arctic Canada and northern and northwestern Alaska, and although but imperfectly explored, has a length of probably 2,000 miles and a width of from 50 to 60, and in places of over 100 miles. On the west coast of Hudson Bay the tundra region slopes gradually from 500 to 600 feet above the sea down to the present coast, and is traversed by sand and gravel terraces and beaches or ridges which mark the former positions of the sea margin. The lower ridges referred to are thickly strewn with shells of molluscs belonging to species still living in the adjacent ocean waters, thus indicating the recency of the emergence of the land. This arctic coastal plain has the same general geographical features as the coast plain on the southern Atlantic and Gulf border of the continent; but, owing mainly to different climatic conditions, differs from its southern representative in nearly every detail.
The tundra may be briefly defined as a vast frozen morass. The dense mat-like vegetation consists principally of mosses and lichens (but not noticeably of Sphagnum or peat-moss, as is sometimes stated), and during the short and not infrequently hot summers is beautified by a multitude of low flowering herbaceous plants. Trees are absent, except along the inland border, where the tundra merges with the subarctic forest. To the north, or seaward from the isolated groves of stunted spruce-trees marking the "continental timber-line," the only representative of arboreal vegetation is usually the slim osier-like arctic willow which grows in sheltered localities and attains a height of 3 to 5 feet. Near the streams there are in some localities broad areas covered with dark-green meadow-like growths of rushes (Equiseta). The luxuriant flowering plants spring
into existence as if by magic as soon as the winter's snow melts, and under the warmth and light of the nightless arctic summer grow with wonderful rapidity. In winter the tundra is snow-covered, but the snow is less deep than in more humid regions, and the cold is intense. The bog becomes deeply frozen, and is not completely thawed during the succeeding summer. Even in midsummer, when the surface is a luxuriant garden of flowers and fresh gray-green moss, ice exists a foot or two beneath the luxuriant carpet and extends to a great but unknown depth. Excavations made in Alaska have shown that the perennial, dirt-stained ice beneath the tundra is at least 25 feet thick, but this is by no means its maximum depth. On the shore of Eschscholtz Bay and along the Kowak River sections of the tundra exposed in cliffs indicate a thickness of 150 to 300 feet of ice, covered by a thin layer of black peaty soil. The similar region in Siberia, as shown by borings, is known to be permanently frozen to a depth of 380 feet deep. The subsoil ice is sheltered by the vegetation and the peaty soil resting on it, from the heat of the short summers, and the part softened by the summer's sun is refrozen during the long intensely cold winters. It is probable that under the present climatic conditions a sheet of perennial ice would be formed beneath the tundra, but the suggestion that the ice now present is in part an inheritance from a former period of greater cold is not without support. The vegetation of the tundra grows each year at the surface, while the partially decayed material below is frozen and preserved. This increase in depth of the vegetable matter is much the same as the growth of peat in temperate latitudes, except that the partially decayed material is preserved in cold storage. It was in the tundra of Siberia that the completely preserved bodies of the mammoth and the woolly rhinoceros have been discovered. Similar finds are to be looked for in the tundra of North America, where the bones of these animals have already been found.
On the Pacific border of the continent the shores are mostly bold, and coastal plains comparable with those on
its eastern and northern margins are absent. In southern California, however, in the vicinity of Los Angeles, a modern and apparently local elevation of the land has produced a highly fertile plain, now, owing to the magic touch of irrigation, beautified by gardens and orchards.
The Piedmont and Coastal Plateaus.—Adjacent to the western margin of the Atlantic coastal plain, and extending from Alabama northward to New England, there is a plateau region about 150 miles broad in its central part, but narrowing towards its extremities so as to be from 40 to 60 miles broad in Maryland and New Jersey, and of about the same width at the south, in Georgia. The slope of the plateau surface is seaward from an elevation of about 1,000 feet along its western margin to 250 or 300 feet at the fall line where it joins the coastal plain.