GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE 5/ 



' ORIGIN OF RELIEF FEATURES 

 Influence of Rock Character 



In common with the Precambrian rock area of northern New 

 York in general, differences of rock character have been very 

 influential in the production of the existing relief features of the 

 quadrangle. Most important is the relatively weak Grenville forma- 

 tion which almost invariably occupies valleys or lowlands. As a 

 result of the long Preglacial time of weathering and erosion, the 

 Grenville strata were much more readily worn down than the very 

 hard and resistant syenites and granites. Fine illustrations of this 

 principle are the two prominent belts of Grenville represented on. 

 the southern half of the accompanying geologic map. One of these 

 belts extends for ii miles from the vicinity of Indian Lake village 

 westward through the Cedar river valley, and the other from the 

 vicinity of Pine lake westward nearly across the quadrangle to 

 Blue Mountain lake, these two belts being connected through the 

 valley south of Rock lake. There are smaller areas of Grenville 

 from L^nknown pond northeastward, and in the vicinity of the third 

 of the Chain lakes. These Grenville areas include most of the 

 lowest land of the southern half of the quadrangle, the prevailing 

 rocks of the lowlands being Grenville limestone with more or less 

 closely associated hornblende and pyroxene gneisses. 



About lYz miles northwest of Indian Lakp village the Grenville is 

 much more resistant than usual, being a white feldspar-quartz gneiss 

 which stands out 300 to 500 feet above the valley bottom. It is a 

 very unusual example of this kind. The deep valley immediately 

 south of Blue ridge has probably been developed by removal of a 

 belt of Grenville. If so, the removal of the Grenville has been 

 almost, if not quite, complete since no outcrop of such rock was 

 anywhere found. The small fault at the eastern end of the valley 

 may have been a factor in determining the location of the valley, 

 but its influence was probably not sufficient to account for so large 

 a valley. A similar explanation probably applies to the long, narrow 

 depression between Blue mountain and the sharp ridge immediately 

 to the south of it, and also to the ,deep depression now in part 

 occupied by Tirrell pond. 



The mixed gneisses, because of their considerable content of rela- 

 tively weak Grenville, also tend to occupy the lower lands. Within 

 such areas, the bolder relief features are due to the local presence 

 of the more homogeneous and resistant igneous masses, the weaker 



