386 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 Wis- 

 consin 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.' 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 interpre- 

 tation. 



On the drift border, in some parts of southern Illinois and 

 Indiana, the loess was deposited, according to Salisbury, imme- 

 diately 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.'' In the northeastern part of Iowa, McGee similarly finds the 



' " Preliminary Paper on the Driftless Area of the Upper Mississippi Valley," 

 Seventh An. Rep., U. S. Geol. Survey, for 1884-85, pp. 199-322. 



^"The Geology of Crowley's Ridge" (1891), Geol. Survey of Arkansas, An. Rep. 

 for 1889, Vol. 2, pp. 228, 229. 



