Page:The Journal of geology (IA journalofgeology21894univers).pdf/410
Niverville beaches, lay undisturbed. The loess region of the Mississippi valley, having been earliest and permanently uplifted, suffered no further change during the progressive elevation of the Lake Agassiz basin; and that in its turn was at rest while the great area of Hudson bay has been undergoing elevation.
Having already shown that the entire duration of Lake Agassiz was about 1,000 years, we must conclude that the uplift of its area, probably to heights ranging from 100 feet to mainly about 500 feet, occurring first at the south and later at the north, took place, when in most rapid movement upward, at rates of a half a foot to one foot per year. A century, therefore, would comprise an elevation of 50 to 100 feet. The movement, however, was evidently more or less intermittent, with pauses of slower uplift or stages of rest, when the successive beach ridges were formed. Nowhere else in the records of present or past epeirogenic movements of any region have so rapid changes of level of large tracts been ascertained; and these changes seem clearly to have occurred through a gradual deformation of the earth's crust by quiet flexure, not by faulting and earthquakes, which would break the regularity and continuity of the ascents of the beaches when traced long distances. The preglacial epeirogenic uplifts of drift-bearing areas, also apparently taking place without faulting, was probably much slower; but their final depression beneath the ice-sheet may have been even considerably more rapid. Very sudden and great, yet not seismic, uplifts of extensive areas, as supposed by Prestwich for southern England and Wales, to account for the "head" or "rubble drift,"[1] and by Shaler for the coastal border of New England, to explain the origin and preservation of the kames,[2] seem, at least in my opinion, to be physically impossible.
The probable nature of epeirogenic movements, in their dependence on conditions of the earth's crust and interior,