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WAVE-LIKE PROGRESS OF AN EPEIROGENIC UPLIFT.
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loess to have been deposited while the ice-sheet that spread the upper portion of the early till was melting away. The very remarkable paha of that district, which are eskers of loess, were accumulated while the waning ice-sheet walled them in at each side.[1] That the later part of the loess deposition was contemporaneous with the formation of the Altamont moraine, belonging to the later drift and marking its limits, I ascertained in northwestern Iowa, where this moraine along a distance of seventy-five miles, from Guthrie county northwestward to Storm Lake, is bordered on its west side by an expanse of loess as high as the crests of the morainic hills, while its elevation above the expanse of till eastward is from fifty to seventy-five feet. During the time of deposition of this part of the loess the ice-sheet reached to the Altamont moraine and was a barrier preventing the waters by which the loess was brought from flowing over the lower area of till that reaches thence east to the Des Moines river.[2] On three widely separated tracts the loess, as elsewhere the coarser portions of the modified drift forming sand and gravel plains, was in progress of deposition upon successive areas as fast as the ice-sheet supplying these stratified drift beds receded. Immediately after the land was bared by the retreat of the ice, and even while the ice itself occupied the adjoining land, the loess was being laid down, contemporaneous successively with the early till on the southern border of the drift, with the till of intermediate age in northeastern Iowa, and with the later till enclosed by the Altamont moraine. The loess deposition I believe to have been mainly continuous, accompanying the gradual and widely extended but wavering departure of the ice-sheet from its farthest boundary to this outermost of the conspicuous morainic belts.[3]

  1. ↑ U. S. Geol. Survey, Eleventh An. Rep. for 1889-90, pp. 435-471.
  2. ↑ Geol. and Nat. Hist. Survey of Minnesota, Ninth An. Rep. for 1880, pp. 307-314, 338.
  3. ↑ The interpretation of the loess and glacial history of the Mississippi basin which I here present differs widely, it must be acknowledged, from the opinions of Professors Chamberlin and Salisbury, and Messrs. McGee and Leverett, to whom we owe so much of the critical investigation of this area. These observers have been led by their
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