EROSION OF THE OLDEST DRIFT. 235 



or more above tide. The amount of drift is meager, both on uplands and 

 in valleys, south of the divide which limits the Grand Valley drainage. In 

 a few places well-defined channels underlain by gravel cross the divide and 

 aiford evidence of the discharge of glacial waters across it. 



Along Oil Creek Valley from Titusville to Oil City there are several 

 exposures of the old drift on the face of the bluffs, but the valley bottom 

 appears to be underlain by gravel of Wisconsin age which connects with 

 the outer Wisconsin moraine a short distance from Titusville. 



The valley of Sugar Creek is bordered in a similar manner by deposits 

 of old drift, while the Wisconsin drift covers the valley bottom. The trib- 

 utaries of Sugar Creek which lie outside of the limits or influence of the 

 Wisconsin glaciation carry only deposits of the old drift. These deposits 

 are shown by wells to reach a depth of 100 feet or more. The material is 

 very stony, so that it is commonly classed as gra^^el, but the writer found 

 here, as well as in many other localities, a slight clay admixture and also 

 angular striated stones with the well-rounded stones. 



A heavy deposit of old drift is found in the abandoned valley that 

 leads from Sandy Creek at Polk northward to French Creek. This deposit 

 is exposed at the north end in the bluff of French Creek, and also in ravines 

 immediately south of French Creek. There is some blue silt in the lower 

 part, which was probably deposited in advance of the ice sheet. The sur- 

 face portion is a stony drift similar to that found so widely in this region. 



AMOUKT OF EROSIO]Sr. 



In addition to the evidence of great age found in the advanced stage 

 of weathering of the pebbles, there is the evidence from erosion. As 

 indicated in the description of the drift, there are several valleys in which 

 the drift is preserved only in narrow strips in recesses or places along the 

 valley borders where erosion is at a minimum. The distribution of these 

 strips is such as to indicate that they are remnants of a sheet which once 

 filled the valleys nearly if not quite to the height at which they occur. 

 This is very clearly shown in the portions of old valleys which have been 

 abandoned. The abandoned Grand Valley, which connects Brokenstraw 

 and Oil creeks, has a filling of drift 200 to 300 feet in depth occupying its 

 entire width. But Oil and Brokenstraw valleys, which, apparently, received 

 a similar filling, now cai'ry only narrow strips in recesses along their borders. 



