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While the ice was retreating and supplying the loess, the land thus uncovered and relieved from the ice weight had been gradually rising, until it had attained approximately its present height in Wisconsin, Iowa, and southern Minnesota, before the formation of the moraines. This altitude has endured, excepting minor
studies to conclude that between the deposition of the early till in southeastern Illinois, with its accompanying loess, and that of the till and attendant paha or eskers of loess in northeastern Iowa, there intervened a very long and diversified history of glacial recessions and re-advances, including at least one prolonged interglacial epoch. A summary of these views in relation to the glacial succession in Ohio is well stated by Mr. Frank Leverett in this Journal of Geology, Vol. 1, pages 129-146, Feb.-March, 1893. From my early study, "Modified Drift in New Hampshire" (Geol. of N. H., Vol. 3., 1878, chapter i., pp. 3-176, with maps and sections), and from my later work on the Glacial Lake Agassiz, I am strongly impressed with the conviction that the deposition and ensuing erosion of the drift, both till and stratified beds, as the loess, went forward very rapidly. What these authors have ascribed to interglacial epochs, one or more of them of great length, seems to me to be more probably referable to geologically very short stages of fluctuation of the mainly waning ice-sheet.
Professor Salisbury, in the report cited, shows that there were two successive deposits of till, and a corresponding division of the loess, on and near to the boundaries of the drift; these seem to me probably due to two closely consecutive stages of ice advance, instead of the long time interval which he thinks to be indicated. Again, in the report on northeastern Iowa, to which reference was given, Mr. McGee clearly shows, chiefly by the forest bed intercalated between two sheets of till, that likewise there the ice advanced twice, with a considerable intervening time, which he supposes to have been far longer than the Postglacial epoch. To my mind, however, the forest-covered borders of the Malaspina glacier or ice-sheet in Alaska leave no doubt that forest beds enclosed in till may be due to oscillations of the ice-front within distances of no more than a few miles or even less than one mile, and that they may have required no longer interval than a few tens of years or at most a century, sufficient for the forest growth, between the times of ice retreat and re-advance.
When the depression of the ice-loaded land brought it down to so low altitude that the borders of the ice-sheet began to be melted more rapidly than they received increase by snowfall and onflow from the thicker central portion of the ice, a general recession of the glacial margin ensued. On the southern part of the drift in the Mississippi basin no continuous moraines were accumulated, and I attribute their absence principally to the attenuated condition of the ice there and its lack of a steep border. During the glacial retreat, wherever the wavering climate caused the mainly waning ice-border to remain nearly stationary during several years the vigorous outflow of the ice to its then steep frontal slope brought much drift, forming belts of irregular morainic hills and ridges, and leaving many hollows which enclose lakes. The fluctuations of the general glacial retreat seem to me to have been due principally to variations of snowfall, some long terms of years having much snow and prevailingly cool temperature, therefore allowing considerable glacial re-advance, while for the greater part other series of years favored rapid melting and retreat.