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NEW YORK STATE MUSEUM 



blanketing. It lies below the Cretaceous peneplain level, having 

 been worn down below that in the Postcretaceous erosion cycle. 

 The Precambric boundary presents quite different characters 

 on the north from those seen at the south. It is much more 

 sinuous and much more abrupt. There is no Potsdam inface, 

 and the Precambric rocks are apt to tower somewhat abruptly 

 from 25 feet to 100 feet or more above the Potsdam level. 

 Between these outlying ridges embayments run, carrying the 

 slope of the plain in between the ridges, and in many of these 

 embayments the Potsdam is found, lying between the gneisses 

 of the adjoining ridges. Faulting has undoubtedly played some 

 part, perhaps a major part, in the production of these features, 

 but it does seem quite clear, nevertheless, that the shore had a 

 steeper slope than on the south, and that the surface was much 

 less even. 



Northern hills and valleys 



The ridges and valleys of a large part of the Adirondack 

 region show a general north-south to northeast-southwest trend, 

 this being more prominently the case in the eastern half of the 

 region. The precise cause for this general trend is not clear. 

 The larger faults have this direction and are undoubtedly influ- 

 ential factors in tjae topographic control; the strike, both of 

 the foliation and of the Grenville bedding, has often the same 

 direction, and has no doubt its share in determining the topo- 

 graphic alinement; one main joint set has the same trend and 

 may also be a factor; finally, the ice sheet moved over the district 

 with a south to southwest direction; and, though the direction 

 of the basal currents was mainly controlled by the existing topog- 

 raphy rather than the topographic trend a result of the ice 

 direction, yet some share in the general shaping of the region 

 must be allotted to it. It seems likelj^ that all these factors 

 have combined in the production of the present trend; but it is 

 as yet wellnigh impossible to determine definitely their relative 

 importance. The frequent independence of this trend shown by 

 the strike naturally suggests that it is of less importance than 

 the others. The prominence of other joint sets, in addition to 



