﻿I3O NEW YORK STATE MUSEUM 



platforms are well shown on both the Theresa and Alexandria 

 sheets. 



Though the Potsdam as a whole is strong, the uppermost beds, 

 together with the sand beds in the basal Theresa, form a weak com- 

 bination, in which the massive bed of the Potsdam summit is rela- 

 tively strong. The overlying Theresa is also stronger than this weak 

 zone, and hence the Theresa edges form rather prominent infaces, 

 with these weak beds at their base; not infrequently also the strong 

 summit bed of the Potsdam forms a narrow platform of its own, 

 part way up the inface [pi. 33]. The Theresa rocks weather to 

 iron stained crusts and their exposed edges have a thin bedded 

 look, giving these infaces a peculiar and unmistakable look of 

 their own. Above the base the Theresa shows rather rapid alter- 

 nations of thicker and thinner bedded layers, the former somewhat 

 more resistant, so that low infaces of these various layers are fre- 

 quent throughout the Theresa country. 



The sandy basal layers of the Pamelia formation, some 25—30 feet 

 thick, constitute the weakest zone within our map limits, and are 

 readily . stripped away from the Tribes Hill underneath, while the 

 overlying limestone is more resistant, so it is not surprising that the 

 Pamelia cliff front is one of the most conspicuous topographic fea- 

 tures of the district, a feature which the contour maps clearly bring 

 out. In front lies a flat Tribes Hill platform. The cliff ranges from 

 20 feet to more than 100 feet in hight, but is usually from 50 to 60. 

 Higher up in the formation the occasional very massive limestone 

 beds form frequent low infaces of their own, as in the case of the 

 Theresa formation. The Lowville differs but little from the Pamelia 

 in resistance, and has no zone of weakness at its base, hence is not 

 fronted by a prominent inface of its own, and is the only formation 

 which lacks one. It has its own minor fronts, but these are of the 

 same order of magnitude as those of the upper Pamelia beneath. 



The Leray is a thin formation, but because of the massive- 

 ness of its beds, and the abundance of chert in its lower portion, it 

 everywhere forms infaces with distinct characters of their own, of 

 which the curious blocky type of weathering is the most conspicuous 

 [pi. 20]. The 7 foot tier above also has a front of its own. 



The thin bedded Trenton limestone is considerably less resistant 

 than the Watertown, hence the Watertown platform in front of the 

 Trenton inface is comparatively broad, especially when the small 

 thickness of the formation is taken into consideration. Notwith- 

 standing the weakness of the Trenton, its inface to the south of 

 the Black river is far the highest and most commanding of the 



