﻿GEOLOGY OF THOUSAND ISLANDS REGION 1 65 



eastern region. The drift of northwestern Pennsylvania lying in 

 advance of the Wisconsin drift, is believed to be as old at least as 

 the Kansan. For an ice sheet to so expand as to reach either 

 northwest or southeast Pennsylvania without trespassing on New 

 York seems impossible. Hence we are forced to the belief, apart 

 from any evidences on the ground, that the State has been more 

 than once in the climatic condition of Greenland at the present 

 time. 



If the State has been overrun by ice sheets more than once it 

 seems rather strange that geologists have not recognized the phe- 

 nomena and discriminated the records. It must be admitted that we 

 now lack the evidence afforded by multiple till sheets, separated by 

 temperate climate deposits such as are found in the Western States. 

 With attention directed to this subject it is probable that some con- 

 clusive proofs will be discovered. 



But while no single fact or class of phenomena yet found fur- 

 nishes conclusive proof of more than one ice epoch, we have a 

 variety of indirect evidenaes ? and many features are well ex- 

 plained only on that supposition, and several lines of study converge 

 toward that conclusion. Moreover, to attribute all the glacial 

 phenomena to a single ice sheet involves inconsistencies, such as 

 the evidence of impotence in erosion of the latest ice, with indica- 

 tion of vigorous erosion formerly; and the lack of glaciation sur- 

 faces on ice-shaped rock as well protected as places showing hairline 

 striae and polish. 



The glacial features of the Thousand Islands region which are 

 not satisfactorily referred to the latest ice work probably can not 

 be attributed to an ice sheet as ancient as the Kansan, but would 

 seem to be the effect of some recent ice epoch. Whether it was 

 one of the later Prewisconsin invasions or only an early Wis- 

 consin episode we may not now decide. 



Anomalous physiography. South of our area, in the central 

 part of the State, many channels of ancient drainage are found 

 which are not Postwisconsin. In the area under discussion these 

 features do not occur because the whole region was drowned in 

 deep water during the ice recession. But the region has its own 

 peculiar topographic features that are difficult of full explanation 

 under the conception of a single ice transgression. The valley, 

 basin and scarp topography has already been briefly discussed 

 [p. 146]. Other points will be touched on below, but a full dis- 

 cussion of the difficult problem requires more fieldwork specially 

 directed to the particular features. 



