196 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



Hazelton, but the present stream continues westward across a rock point 

 into the Wabash Valley. A map showing the changes which this drainage 

 system has experienced, and also changes in smaller tributaries of the 

 Wabash in southwestern Indiana, appears in the monograph on the Illinois 

 Glacial Lobe.^ The changes are there discussed in considerable detail. 



WESTERN TRIBUTARIES OF THE WABASH IN ILLINOIS. 



The western tributaries of the Wabash are all comparatively small. 

 Those whose courses lie within the limits of the Wisconsin drift are not gov- 

 erned by the preglacial drainage, for the dritt has filled the region to a 

 higher level than the old divides. But south from the border of the Wis- 

 consin drift the courses of streams are governed to a large extent by the 

 preglacial drainage lines. The few changes or departures from the old 

 drainage are discussed in Monograph XXXVIII, as are also the influence 

 of morainic ridges of Wisconsin age upon the course of the streams. 



CAUSES FOR CHANGES IN DRAINAGE. 



Of the several factors which are influential in causing changes of 

 drainage, glaciation is known to have been widely operative in this region. 

 Piracy seems to have been operative, at least to a limited extent, and possibly 

 has had wide influence. In addition to these the influence of uplift or earth 

 movements should perhaps be considered. 



GLACIATION. 



With the extension of the earliest glaciation into the lower courses of 

 a northward-flowing stream there would have been a ponding of water in 

 front of the ice field. This ponding would eventually reach a height at 

 which discharge could take place over the rim of the drainage basin, and a 

 new system of drainage would be inaugurated. With the advance of the ice 

 field many streams would be thus aff'ected, and in some cases the influence 

 of the ponding might be felt at points many miles beyond the limits of the 

 ice field. In the region under consideration streams which had been flow- 

 ing from the Appalachian region northward into the basins now occupied 

 by Lakes Erie and Ontario are likely to have experienced ponding in their 

 lower courses before the ice field had encroached greatly on the basins of 



1 Mon. U. S. Geol. Survey Vol. XXXVIII, PI. VIII. 



