1898.] WILLIAMS — OX KAXSAK" DRIFT IX PENNSYLVANIA. 87 



the surface consists of Kansan drift, and it is also allowed that this 

 surface was last deposited. It follows, therefore, that the earliest, 

 or Kansan, drift was deposited after the Allegheny river had reached 

 its present level. This is but one of hundreds of similar cases 

 found for 200 miles along the Allegheny, and with streams under 

 both glaciers — eastern and western — cut to present levels pre- 

 glacially, the great antiquity of the ice age falls. 



It may be asked, however, how the reversal of streams and cut- 

 ting of cols are disposed of, as these are matters of considerable cer- 

 tainty. 



When we consider that the ice advanced up stream in all cases over 

 the northern Allegheny region, we can see that extreme high water 

 would obtain and the water would pour over the cols into adjacent 

 systems long before the actual presence of the ice at the spot. In 

 fact, the actual presence at a given spot is unnecessary. If we next 

 consider that the advancing ice would confront the loftiest part of 

 our highlands, we can see that it would be aided in its efforts to 

 produce high water by a large snow cap whose ablation would pro- 

 duce torrential conditions in all the drainage systems, and fill those 

 systems with local trash, more or less rolled, which would saw down 

 the cols over which the empounded waters escaped, long before the 

 ice reached the region, and that when the glacier did make its 

 appearance it would discharge into abnormally deep water. We 

 have thousands of evidences from the north to the south of the 

 State, in elevated beach lines, and similar remains, that the water 

 exceeded 1600 feet above tide, and only on the highest mountain 

 tops do we find unmodified till. In all other cases it is ordinary 

 overwash or slack water modifications. The dead slack of the 

 original water is shown throughout the region by the clean iceberg 

 clay which sometimes reaches 100 feet in depth, and underlies all 

 other deposits. 



The matters touched upon here will be more fully discussed in the 

 final report of the survey. 



