SURFACE DEPOSITS. Ill 



the mo st-northerly point at which the strata of the Upper coaL 

 measures are found, is fully seventy-five miles. Along this 

 entire distance no rock is to be seen, and only occasionly can 

 the presence of the drift be detected. Beyond these two 

 points in either direction, the strata appear so seldom, and so 

 near the base of the bluffs, and the exposures are so small 

 that they never modify the peculiar aspect which the Bluff 

 Deposit imparts to the bluffs everywhere. 



Sometimes the presence of the drift may be detected 

 at the base of the bluffs by an indistinct terrace, but usually 

 its presence is known only by the appearance of clay, sand, 

 or gravel upon the surface, and by the issuance of springs. 

 Along a great part of the whole line, the drift is evidently 

 beneath the level of the great flood-plain, because nothing 

 but the material of the Bluff Deposit appears to view from 

 base to summit of the bluffs. 



The drift is evidently very thin where the Bluff Deposit 

 is thickest, and the latter has been deposited in a broad 

 depression Jiollowed out of the general surface, principally in 

 the drift alone. The topmost strata of the Upper coal- 

 measures are, however, occasionally found scored by glaciers, 

 which indicates that the hollowing process extended into the 

 stratified rocks also. Indeed the drift is nowhere known 

 to be very thick in the vicinity of the Missouri river, and in a 

 couple of instances at least the Bluff Deposit was found 

 resting directly upon the limestone. Within twenty-five 

 miles east of the eastern border of the Bluff Deposit there 

 is evidence that the drift reaches a thickness of about two 

 hundred feet, 



The valleys of all the streams of western Iowa, which reach 

 the flood-plain of the Missouri river within the State, traverse 

 the region occupied by the Bluff Deposit. The streams and 

 their branches not being very far apart, the surface is often 

 much broken; but it nowhere presents those peculiar outlines 

 that are so striking a feature of the bluffs of the Missouri 

 river. The valleys being of considerable depth, and not 

 far apart, the aggregate slope from the watersheds to the 



