GEOLOGY OF THE LAKE PLACID QUADRANGLE 69 



flow through this deep, narrow valley, the site of the present Wil- 

 mington notch having then been a division of drainage or col which 

 has been cut through by the West Branch Ausable river, the latter 

 having taken this course as a result of the glaciation of the region 

 (see chapter on the pleistocene geology). In fact the position of 

 the whole valley of the river between Mt Whiteface and the Sen- 

 tinel range has been primarily determined by the presence of the 

 great zone of weakness in the rocks. 



In the bed of the river three-fourths of a mile west-southwest 

 of Owen pond, there is a big ledge of crushed rocks, especially 

 pegmatite and diabase, which are distinctly brecciated. Some frag- 

 ments of diabase have been mingled with the pegmatite. This zone 

 strikes N 20° E. Along the river within the granite area in the 

 notch, the rock is badly broken up in a wide zone parallel to the 

 stream. Just northeast of this along the river, the Grenville gneiss 

 is faulted with slickensides visible. 



At High fall the granite of the gorge is highly jointed, the joint 

 surfaces being from one to several feet apart, forming a zone of 

 weakness which has determined the stream course. The joints 

 strike N 40° E and dip 65° E. Slickensided joint faces are com- 

 mon. Just above High fall a diabase dike in the stream bed is 

 brecciated, and close by is a distinctly slickensided scarp 4 feet high. 



At the so-called Little High fall, about one-half of a mile below 

 High fall, there is a very prominent crushed zone as wide as the 

 river bed with a strike N 30° E. 



In the river bed about half way between High fall and The 

 Flume, there are two well-developed crushed-rock, or highly jointed, 

 zones with strike parallel to the river. In The Flume the rock is 

 considerably jointed, but just above the bridge there is an exces- 

 sively jointed zone parallel to the course of the stream. Evidently 

 the gorge development here has been greatly aided by the rock 

 structure. 



Faults in the town of Keene. At the southern end of the gorge 

 one-half of a mile south of Keene, the rocks are badly broken 

 up and somewhat slickensided, indicating a prominent crushed-rock 

 zone here with strike about east-northeast by west-southwest. By 

 the roadside at the edge of the gorge, this fault zone is well exposed 

 (see plate 21). The topography strongly suggests the continua- 

 tion of this fault zone some distance northeastward as indicated 

 on the map, but this is not verified by actual outcrops. 



By the roadside nearly 2 miles north of Keene, the Whiteface 

 anorthosite and Grenville mixed rocks are much broken up, some- 



