Leverett — Weathering and Erosion as Time Measures. 367 



ing and erosion to be at least as old as the Kansan drift, and 

 possibly the correlative of the Pre-Kansan drift. Its aspect 

 is decidedly older than the Illinoian drift exposed in the west- 

 ern part of the field covered by the Labrador ice sheet. The 

 topography of the quadrangles within the limits of the Jer- 

 seyan drift and of the old drift of northwestern Pennsylvania 

 is controlled to so slight an extent by glacial deposits that the 

 topographic sheets are not likely to have much value in setting 

 forth the amount of Post- Jersey an erosion, and hence are not 

 drawn upon for illustrations. 



European Equivalents of the Pre-Kansan. 



Upon turning to the European fields conditions somewhat 

 similar to that of the Kansan and Pre-Kansan have been 

 brought to notice. Thus in the Alpine region the oldest drift, 

 known as the Gunz, is almost completely overridden by another 

 old drift, the Mindel. In that region, as in the American, 

 just discussed, it is difficult to determine the difference in age 

 because of the meager exposure of the Gunz drift. The 

 studies of Penck and Bruckner, however, have shown that 

 there was an advanced state of weathering of the surface of 

 the Gunz drift before the Mindel glaciation buried it. The 

 Mindel and Gunz drifts are thought by Penck to be separated 

 by a shorter interval than that between the Piss and Mindel.* 

 In the Skandinavian field observations by Nathorst and others 

 indicate that in southern Sweden a set of .striae out of harmony 

 with the ice movement which carried the lower bowlder clay of 

 Germany is likely to have preceded that movement. While 

 the earlier movement may not have been sufficiently distinct 

 from the later to constitute a separate stage of glaciation, 

 Geikie has suggested applying to it a distinct name, Scanian, 

 from the district in Sweden where it was brought to light. 



Iowan drift? 



Since it is through a study of weathering and erosion that 

 the presence of the so-called Iowan drift has come to be 

 questioned, it seems fitting that this discussion should include 

 a brief statement concerning the leading features in the region 

 that drift has been supposed to cover. The region lies in 

 northeastern Iowa immediately east of the area covered by the 

 Des Moines lobe of Wisconsin drift. The topographic maps 

 which lie wholly or partly within its limits are the Decorah, 

 Oelwein, Elkader, Winthrop, Farley, Stan wood and Rock 

 Island. Pig. 7, which represents part of the Oelwein quad- 



* Die Alpen im Eiszeitalter, p. 1168. 



