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.