﻿I46 NEW YORK STATE MUSEUM 



The genesis of this prevailing orientation probably involves factors 

 which cover the entire geologic history of the region. In an earlier 

 chapter Professor Cushing has shown that during the time of the 

 earliest sedimentation in the region there was alternately a tipping 

 to the northeast and the southwest, the fulcrum of motion lying 

 across our district, initiating what is called the Frontenac axis 

 [p. 95]. The broad depression of the valley is thought to be partly 

 the result of sagging, accompanied by jointing, one main trend of 

 joints having fair agreement with the trend of the valley. 

 Cushing also shows that some slight folding occurred in Paleozoic 

 time and stronger folding in Precambric time which probably had 

 some directive influence on the drainage [p. 108-115]. 



The larger existing features and general stream directions were 

 developed during Tertiary time under subatmospheric erosion. Dur- 

 ing Pleistocene time the St Lawrence valley, being closely in line 

 with the spreading flow of the ice sheet over the region, served as a 

 trough for the advancing and the waning ice lobes. We do not know 

 the number of ice invasions but it seems quite certain that the latest, 

 or Wisconsin, ice sheet was preceded by others of probably greater 

 effectiveness in erosion. The striking parallelism of the minor 

 features of the topography is probably due in some degree to re- 

 peated glaciation, the alternation of ice flow of the glacial epochs 

 and the stream erosion of the interglacial epochs mutually assisting 

 or guiding each other. 



Dominant types. The topographic features in the sedimentary 

 rocks are naturally an expression largely of the stratigraphic char- 

 acters. This has already been discussed in a former chapter by 

 Cushing [p. 1 21-136]. In the present connection we have to con- 

 sider the topography in its relation to the glacial and glacio- 

 aqueous history. 



Leaving out of account for the present the localized and scanty 

 moraine deposits, we may distinguish two dominant types of the 

 surface relief in the area, (1) the rounded rock hills or knobs oi 

 rather striking relief in the northern part of the area, in the district 

 of Potsdam and Precambric rocks, and (2) the broad level stretches 

 which characterize the southern half of the area, where the rocks 

 are well stratified. 



Rock knobs. In the northern part of the area, covered by the 

 Grindstone and Alexandria sheets and the northeast part of the 

 Theresa sheet, the crystalline rocks and the lower Potsdam appear 

 commonly in the form of knobs or bosses, singly or in clusters and 

 chains, as illustrated in plates 6 and 7. Cushing has shown [p. 54] 



