368 Leverett — Weathering and Erosion as Tim,e Measures. 



rangle, illustrates well the topography of the part of this 

 region where the valleys are excavated entirely in the drift. 

 Farther east the drift becomes so thin that rock ridges dominate 

 the topography. It will be observed , that the surface presents 

 broad slopes, as in the Kansan drift, but the valley bottoms 

 are less well defined, the slopes on either side extending nearly 

 to the streams. Tabular divides are also wanting and con- 

 siderable inequality is found along the lines of water parting. 

 This region differs also from neighboring parts of the Kansan 

 in being nearly free from loess. 



The inspection of such a map as that given in fig. 7 naturally 

 raises the question whether the departures which it exhibits 

 from the typical Post-Kansan erosion, shown in figs. 5 and 6, 

 may be interpreted as a superimposed youth, such as might 

 result from a fresh invasion of the ice into a much eroded 

 region of older drift. A study in the field would naturally 

 be directed to the solution of the question of the existence of 

 a drift sheet too fresh to be consistent with the interpretation 

 that the valleys were entirely formed after its deposition, and 

 a special search should be made to discover whether a fresh 

 drift is so situated in the valleys as to indicate clearly that it 

 was deposited after the valleys were excavated. Upon visit- 

 ing the field in 1906 and again in 1907, and spending about 

 two months in careful examination of the exposures on uplands, 

 slopes, and valley bottoms, the writer was unable to discover 

 any trace of fresh drift. The valleys were found to be filled 

 with a concentrate of the more resistant constituents of the 

 drift, the soluble constituents having apparently been removed 

 just as in the weathered portion of the Kansan drift. The 

 valleys are shallower than in the Kansan drift of southern 

 Iowa and northeastern Missouri, and run out at their heads in 

 so-called sloughs. But so far as discovered this shallowness is 

 not occasioned by the deposition of a fresh sheet of drift, but 

 rather seems due to a filling by slope wash. The ramification 

 of the valleys into the uplands is not strikingly different from 

 that found in the Kansan drift and seems to call for a maturity 

 of stream work of a similar rank. In view of the fact that 

 my own studies have not been exhaustive and that the Iowan 

 invasion is still held by the Director of the Iowa Geological 

 Survey, who has had a longer acquaintance with the region, I 

 still consider the question of an Iowan invasion an open one. 



Ann Arbor, Michigan, February 22, 1909. 



