356 Leverett — Weathering and JLrosion as Time Measures. 



marks a very important position of the ice border, whether a 

 mere halt or a readvance is not yet clear. The results of my 

 own studies are in full accord with those of the German stu- 

 dents in placing the limits of the last glaciation far enough 

 south to embrace the prominent moraines along the south 

 border of the region characterized by lakes. 



Illinoian Drift. 



The Illinoian drift of the Labrador icefield is exposed out- 

 side the limits of the Wisconsin drift in Illinois, Indiana, and 

 Ohio, and at its farthest extent reached slightly into Iowa.* 

 The entire outlying portion of this drift is covered by loess 

 except a small district in northwestern Illinois and southern 

 Wisconsin, where the loess is of rather patchy distribution. 

 Beneath the loess there are widespread occurrences of a black 

 soil and muck known as the Sangamon soil, which rests upon 

 the leached surface of the Illinoian drift. It thus appears 

 that the Post-Illinoian history is somewhat complex. The 

 occurrence of the black soil and muck is suggestive of cool, 

 damp conditions, while the overlying loess suggests conditions 

 of aridity. The loess is found to pass beneath the outermost 

 moraines of the Earlier Wisconsin drift and hence antedates 

 the Wisconsin ice invasion.f 



The leaching which the Illinoian drift has experienced 

 appears to have been largely accomplished prior to the deposi- 

 tion of the loess, for the loess is sufficiently thick over much 

 of the Illinoian area to preserve an unleached basal portion. 

 Yet in spite of this cessation of the leaching at the time of 

 the loess deposition it had reached a depth, and weathering 

 bad gone to a degree, far in advance of the leaching and 

 weathering thus far experienced by the Wisconsin drift. The 

 removal of calcareous constituents, pebbles as well as fine 

 material, was nearly complete to a depth of 4 to 6 feet, while 

 weathering often extended to two or three times these depths. 

 Indeed pipes and weathered seams extend far down into the 

 unleached portion, as is not found to be the case in the Wis- 

 consin drift. One cannot well escape the conclusion that the 

 time involved in this leaching was at least two or three times 

 as great as that of the Wisconsin, and it may considerably 

 exceed this amount. In addition to the weathering and leach- 

 ing there is a general induration of the entire deposit, pro- 

 duced probably by a partial cementation, a feature which is 

 very sparingly exhibited by the Wisconsin drift. 



Turning now to the amount of erosion which the Illinoian 



* See The Illinois Glacial Lobe, Monograph xxxviii, U. S. Geological 

 Survey, 1899, pp. 24-130. Also Monograph xli, 1902, pp. 253-294. 

 f See Monograph xxxviii, pp. 128-129 and plate xi. 



