BORDER OF THE OLDEST DRIFT. 221 



valleys or is disposed in loops with protrusions down or into the valleys 

 and with reentrants on the uplands. It is to be expected, however, that 

 such disposition in loops would result from the topographic conditions. 



An occasional bowlder has been found so far beyond the well-defined 

 glacial deposits as to arouse suspicion of transportation through human 

 agencies, though the object of such an expenditure of labor is not always 

 clear. For example, a single bowlder is reported b};- W. S. Gresley to lie 

 in a ravine on the south side of the Ohio River near Pittsburg in such 

 situation that it can not be refei-red to stream transportation down the 

 Allegheny and Ohio valleys.^ The writer observed a small bowlder on an 

 upland south of the Ohio, near the Pennsylvania and West Virginia line, 

 that was evidently beyond the reach of the Ohio. Other bowlders that 

 seem referable to stream action have been found on high terraces along 

 the Ohio near Pittsburg and near Beaver, Pa. In the j)resent state of 

 knowledge the writer is not prepared to decide whether or not glaciation 

 reached the Ohio at Pittsburg and Beaver. These scattering bowlders seem 

 to be only strays that have been carried beyond the limits of glaciation, 

 bat there is a chance that they are a part of a very attenuated glacial 

 deposit. The limit of this old drift, as given on the glacial map (PL II), 

 represents merely the approximate margin of a well-defined, easily traced 

 deposit. 



The border of this old drift sheet appears to emerge from beneath the 

 Wisconsin drift near the New York and Pennsylvania line on high uplands 

 east of the Conewango River. It passes southward across the Allegheny 

 River nearly to the bend of the Tionesta River at Barnesville, Pa., while 

 the Wisconsin border passes westward across northern Warren County at a 

 distance of several miles north of the Allegheny River. There was appar- 

 ently a small lobe of the earlier ice field extending from the Allegheny 

 Valley southward toward the bend of the Tionesta, for westward the limits 

 of glaciation appear to be along or near the Allegheny Valley from near 

 Warren down to Tidioute. From Tidioute to the vicinity of Oil City the 

 border appears to lie a short distance north of the Allegheny, though 

 probably nowhere over 6 miles from the river. In that part of its course 

 it is 15 to 20 miles outside the limits of the Wisconsin drift. Near Oil 

 City the border crosses to the south side of the Allegheny River, but may 



'Am. Geologist, Vol. XVIII, 1896, pp. 331-332. 



