134 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



in the abandoned valley which leads from Sandy Creek northward to French 

 Creek, the rock floor is shown by several oil wells to have an altitude about 

 1,050 feet above tide. This valley is filled with drift apparently of early 

 glacial age, and its rock floor has not suff'ered excavation since the drift 

 deposition. Its rock floor is the probable continuation of the elevated rock 

 floor of the headwaters aiad indicates a descent of somewhat more than 200 



feet in 18 to 20 miles. This rate 

 of fall would be natural in such a 

 small stream descending from the 

 elevated table-land, and difi"ers but 

 little from the rate of fall in south- 

 ern tributaries of the upper Alle- 

 gheny of corresponding size — e. g., 

 the fall on the Tuna, a similar 

 stream, from De Golia, Pa., to the 

 mouth of the stream, a distance of 

 14 miles, is 215 feet.^ At the north 

 end of the abandoned valley, where 

 it opens into French Creek, a rock 

 floor is struck in wells at an alti- 

 tude about 975 feet above tide, 

 which seems to mark the continua- 

 tion of the old valley floor. 



Turning now to the main 

 stream of the old Middle Alle- 

 gheny, it appears probable that the 



FIG. 5.-Present drainage of part of the Middle Allegheny ««! at ThompSOu's had a height of 

 drainage system. ^^ ^e'^^^ 1,220 fect. At TidioUtC, 8 



miles below, early glacial gravels rest on a rock shelf that represents the 

 old river bottom at 1,160 feet above tide. At Reno a similar shelf stands 

 only 1,040 feet; while at Franklin, in an oxbow filled with early glacial 

 gravel (see PI. VIII), one boring reached rock at 1,040 feet above tide, 

 and another penetrated to a level only 1,015 feet above tide without reach- 

 ing rock. The gravel at these points rises to a level much above that of the 

 terraces connected with the outer moraine of the Wisconsin or late ice inva- 



Scale of mileG 

 ■■■'•' 



'See Carll: Second Geol. Survey Pennsylvania, Eept. I\ p. 334. 



