OCR Text |
Show igoo] PHYSIOGRAPHY 15 Many of the basins and valleys of the system are Vast lava beds, forming in certain regions extensive low plateaus, as in the drainage area of the Columbia River. -^ matter of prime importance for the geologist and for the student of race distribution is the record of a great glacial sheet which in very recent geological times spread over practically all of Canada and a large part of the United States, " it extended south over New England and New York to the Ohio River, and westward over the prairies and a portion of the great plains. The erosive action of this glacier has been of great significance. With its retreat was discovered the great level extent of prairie land, where the drift deposit has produced most fertile soil. \ Along the northern frontier of the United States, and through eastern and central Canada, the ragged track of the ice sheet is marked by the thousands of lakes and^ vatercourses which distinguish that area. Jrhe mineral deposits of the continent are rich and varied. Of these coal is one of the most important, and, in the east at least, is so universally distributed that jhere is no habitable portion of the United States many miles distant from a natural supply r The interior and far west are less favored, but there are, nevertheless, many spots in which coal is found. The Mexican deposits are rich but undeveloped. In 1902 the United States produced 1 Shaler, United States of America, I., 428. |