GEOLOGY OF THE BLUE MOUNTAIN QUADRANGLE 59 



the depression now in part occupied by Tirrell pond. If so, the 

 very steep mountainside just east of the pond (see plate ii) may 

 be accounted for in a similar manner. 



Where weak Grenville beds overlie the granite or syenite, and 

 both show moderate dips in the same direction, the effect has been 

 to produce longer and more gentle slopes due to removal of the 

 Grenville. In a general way, the whole southern side of the Cedar 

 river valley from Indian Lake village westward illustrates this 

 principle. 



If granite or syenite and overlying Grenville both show steep 

 dips in the same direction, removal of the Grenville may leave a 

 steep slope, as seems to be the case on the mountainside just north- 

 west of Blue Mountain lake. 



The Blue Mountain region, unlike the eastern and southeastern 

 Adirondacks, shows no prevalent tendency of mountain ridges to 

 trend northeast-southwest because of faulting. Faulting has, how- 

 ever, been a prime factor in the production of a few important 

 topographic features of the quadrangle, the known faults having 

 . been described above. We need only to repeat here that the straight, 

 narrow depression the full length of Long lake; the straight, deep 

 channel of Cedar river north of Indian Lake village; and the great 

 depression which contains Indian lake all have been developed 

 along fault zones of weakness. Very little of the topographic 

 eflfect of the last named fault, however, shows within the Blue 

 ^Mountain quadrangle. 



Influence of Exfoliation 



Exfoliation has produced some interesting, though comparatively 

 minor, topographic effects. The steep eastern face of Water- 

 barrel mountain is almost impossible of ascent because its upper 

 400 feet is completely covered with smooth, mostly barren exfolia- 

 tion slabs from 50 to 100 feet across and several feet thick. As a 

 result of the sliding of such great rock slabs down the scarp, and 

 their breaking up into large angular blocks, an extensive talus 

 deposit has been built up toward the base of the mountain. Because 

 of gradual removal of weak Grenville strata which dip under the 

 granite of this mountain, the face of the mountain has, for a long 

 time, been retreating westward by splitting off of exfoliation slabs. 



Other fine exhibitions of exfoliation on large scales are on the 

 steep mountainsides northeast of Tirrell pond, east of Salmon 



