Page:Popular Science Monthly Volume 34.djvu/31

Rh ; and (4) that the earlier epoch, of cold was the longer, though the cold was the more intense and the climate more variable during the later epoch.

The latest researches in the Atlantic slope have shown more definitively (1) that the interval of mild climate separating the two cold epochs was from five to ten times as long as the post-glacial interval; (2) that the cold epochs themselves were brief in comparison with the inter-glacial and post-glacial intervals; and (3) that the earlier and longer epoch of cold was attended by continental submergence reaching four or five hundred feet in the latitude of New York and extending to South Carolina, while the land depression of the later refrigeration was but forty or fifty feet at New York, and scarcely extended beyond the great terminal moraine of Long Island and northern New Jersey.

During the first epoch of cold and wet local glaciers formed in the Rocky Mountains and in the Sierras, the Great Basin (which these ranges bound) was flooded and the now extinct Lakes Bonneville and Lahontan were formed, and into the lakes great volumes of sand and silt—the lower lacustral beds of Gilbert and Russell—were swept by the flooded rivers; at the same time the northern ice-sheet stretched down into the Mississippi Valley as far as the Missouri River, the land was depressed, and both glacial and aqueo-glacial deposits were laid down; and it was at the same time, too, that the Atlantic coast was depressed until the high hills overlooking New York, Philadelphia and Washington were half submerged, and that the rivers built great deltas of gravel and loam along the shore of the expanded ocean, while the waves dropped shallow-water sediments all over the lowlands. With the interglacial warmth the glaciers of the Western mountains were melted, the lakes were dried, and river-gravels were deposited by the shrunken streams over and canons were cut into the old lake-bottoms; in the Mississippi Valley the glaciers retreated and the drift-plains became forest-covered; while in the East the land underwent re-elevation, and there was erosion of such extent as to afford a rough measure of the duration of the warm interval. During the later epoch of cold and wet glaciers again formed in the Rocky Mountains and in the Sierras, Lakes Bonneville and Lahontan were refilled—the former to overflowing—and the upper lacustral beds were laid down within them; the northern ice again invaded the Mississippi Valley and formed two or more drift-sheets, together with the peculiar glacial-mud deposit (or loess) into which they graduate, as well as the great terminal moraine stretching from Ohio to Dakota; and in the East the ice again overrode the Adirondacks and the New