72 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



Oak Orchard Creek, a tributary of Lake Ontario; and the eastern portion by 

 Black Creek, a tributary of the Genesee River. The portion north of the 

 axis is chiefly underlain by the Lockport limestone, but the portion south 

 is developed in the soft rocks of the Onondaga salt group. The amount of 

 drift is so great that the preglacial valleys are completely concealed. 

 Whether the Onondaga formation was sculptured by the ice sheet in a 

 manner similar to the sculptui'ing in the Medina shale can not easily be 

 d(}termined on account of the great amount of di'ift. On hypothetical 

 grounds the material removed by the ice sheet may be inferred to be of 

 large amount, for the formation would be likely to offer but little more 

 resistance to erosion than was offered by the Medina shale. 



The western portion of the plain carries a few drumlins, while the east- 

 ern portion is characterized by numerous drift ridges, some of which are of 

 drumlinoid and others of morainic type. 



Although this plain was entirely covered by the waters of the glacial 

 Lake Warren for a period sufficiently long to form a well-defined shore line, 

 the waves have only removed or toned down the lesser irregularities of the 

 drift surface and cut slight benches and built small bars and spits on the 

 borders of the drift knolls.' 



Upon the lowering of the lake level from the Warren to the Iroquois 

 beach this plain appears to have been occupied by a shallow lake, which 

 discharged across the Niagara escarpment at several points. Eventually 

 the lake was drained and the discharge from the Lake P]rie Basin found its 

 way across this plain by two main channels, one u£ which crossed the 

 Niag'ara escarpment along the line of the Niagara River, while the other 

 took the course followed by the Erie Canal to Lockport, as pointed out by 

 Grilbert.- The volume of water along the channel followed by the Erie 

 Canal is thought b}' Gilbert to have been great and to have been main- 

 tained for a considerable time, for it produced a well-defined channel and 

 carried a large amount of material over the escarpment to the lower plain. 

 The Niagara route finally absorbed the entire drainage, and the falls are 

 now receding through the plain between the Niagara and Corniferous 

 escarpinents. 



iSee Fairchild: Bull. Geol. Soe. America, Vol. VIII, 1897, pp. 272-284. 



'' Old tracks of Erian drainage in western New York, by G. K. Gilbert; Bull. Geol. Soc. America, 

 Vol. VIII, 1897, pp. 285-286; also Vol. X, 1899, p. 129. 



