268 Chamberlin and Leverett — Studies of the 



But when the waters of the upper Grand river basin were 

 forced over the col in large volume and descended the steep 

 slope of the little valley (from about 1025 feet A. T., to about 

 850 feet A. T., in three miles), deep scouring at the mouth 

 would naturally result and the formation of cascades or falls 

 would readily follow. These would work up stream as the 

 erosion progressed. They appear to have reached a point just 

 above Edenburg when a later incursion of the ice stopped the 

 process and filled the deep valley with debris. After episodes 

 that need not be noted here, the stream assumed its present 

 horizon. 



Passing by several minor features of interest, the phe- 

 nomena seem to clearly point (1) to very notable erosion (200 

 to 250 feet), after the drainage was reversed by the ice, and 

 (2) to the filling of the trench so cut 200 feet or more with 

 drift by a later ice incursion, and (3) to only a very moderate 

 erosion since this late filling. This valley which was cut after 

 the early reversal bears on its sides morainic debris, and also at 

 some points in the bottom of the stream. The stream has not 

 here cut into the later drift to exceed 75 feet. 



The profile along the Shenango river shows phenomena in 

 harmony with these. Only that portion of the pool which 

 lies below the junction of the two streams appears as a notable 

 phenomenon. It shoals rapidly to the north as though the 

 Shenango merely bevelled the up-stream edge of the Mahon- 

 ing pool. 



The formation of the pool was favored by the softness of 

 the rock in this portion of the valley and the hardness of the 

 Homewood sandstone near the mouth of the Connoquenessing. 



This reference of the puzzling phenomena presented by the 

 closed basin in the bottom of the Mahoning and Shenango 

 valleys at their junction to the exceptional scouring action of 

 the Mahoning consequent upon its precipitous descent from 

 the Lowell ville col when forced over it by the invading ice 

 seems to fit the phenomena much better than previous hypoth- 

 eses,* though the other hypotheses may find partial applica- 

 tion here and more full application elsewhere in the region. 



So far as these observations go, they are in consonance with 

 the hypothesis of northward discharge advocated by Dr. Fo- 

 shay, differing from it only in suggesting that the probable 

 watershed may be some 50 miles further south on the Ohio, 

 and in restricting the application of the hypothesis to the ele- 



*The scouring of streams of ice, I. C. White. Second Penn. Geol. Survey, 

 Q, p. 17, QQ. pp. J 6-20, Q3, p. 20. Q4, pp. 37-38, and elsewhere in these reports. 

 Ice and under-iee currents, J. F. Carll. Second Penn Geol. Survey, III, p. 362 

 (Footnote). IIII, pp. 234-5 (Footnote). Northward differential uplift, P. Max 

 Foshay. This Journal, Nov., 1890. p. 400. Northward differential depression, 

 ice, and subglacial waters, Frank Leverett. This Journal, Sept., 1891, pp. 208-9. 



