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THE JOURNAL OF GEOLOGY.

fully at the evidence of this progressive earth movement in the chronologic and geographic order of its successive portions.

Between the chief time of deposition of the Mississippi loess and the formation of the prominent moraines east of the Wisconsin driftless area, there intervened an uplift of the upper Mississippi region to a vertical extent estimated by Chamberlin and Salisbury as probably 800 to 1,000 feet.[1] On the western portion of the driftless area and southward to the Gulf of Mexico, the loess had been spread by very slowly flowing river floods, and partly in temporary lakes, due to the greater depression of the basin toward the north, while in the opposite direction the subsidence was insufficient to carry the low southern part of the valley beneath the sea level. The ensuing uplift probably scarcely increased the altitude of that southern area about the mouth of the Mississippi, but thence it extended northward as a differential epeirogenic movement, raising the depressed country of the central and northern portions of this great river basin several hundred feet. As a result of the changed slope, in the former place of the quiet water whose sediment was the loess, strong currents, bearing sand and gravel, flowed down the valleys from the ice-front when it amassed the moraines mentioned in Wisconsin. The duration thus represented has been supposed to comprise a long interglacial epoch, but the observations on which this belief rests seem to me to admit a different interpretation.

On the drift border, in some parts of southern Illinois and Indiana, the loess was deposited, according to Salisbury, immediately after the till which immediately underlies it, and was in part contemporaneous with the till. As soon as the ice-sheet retired from the positions where this relationship exists, the glacial drift was covered by this finer silt of the modified drift supplied by streams that flowed from the melting and retreating ice.[2] In the northeastern part of Iowa, McGee similarly finds the

  1. ↑ "Preliminary Paper on the Driftless Area of the Upper Mississippi Valley," Seventh An. Rep., U. S. Geol. Survey, for 1884-85, pp. 199-322.
  2. ↑ "The Geology of Crowley's Ridge" (1891), Geol. Survey of Arkansas, An. Rep. for 1889, Vol. 2, pp. 228, 229.
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