70 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



NIAGARA ESCARPMENT. 



This escarpment, which marks the northern hmit of the Lockport Hme- 

 stone in western New York, has a height ranging from about 200 feet in 

 the vicinity of the Niagara River to 50 feet or less in the vicinity of the 

 Genesee River. As shown on the topographic map (PI. Ill), it is prominent 

 for 60 miles east from the Niagara River, its height seldom falling below 

 100 feet, but in the remaining 20 miles to the Genesee it is comparatively 

 inconspicuous. The altitude above tide declines from about 640 feet in the 

 western portion to scarcely more than 600 feet in the vicinity of the Genesee 

 River. 



At the brow of the escarpment the limestone is present only in small 

 amount, and, as noted by Gilbert, is absent from the immediate edge of the 

 cliff through considerable spaces. It thickens rapidly upon passing south- 

 ward, though not sufficiently to overcome the dip of the strata. The 

 southern boundary of the outcrop is 60 to 75 feet lower than the northern. 

 Below the Lockport limestone there is about 80 feet of Rochester shale, 25 

 feet of Clinton limestone, and then a great body of Medina shale containing 

 a few sandstone ledges near the top. The sandstones are discontinuous 

 and lie at various depths below the Clinton limestone, the range as given 

 by Gilbert being from 40 to 100 feet. In places the Medina and Clinton 

 beds are worn back to the base of the Niagara escarpment, while in other 

 places they extend out short distances to the north, forming a lower escarp- 

 ment rudely parallel with the main- escarpment. 



Gilbert's studies lead him to think that these resistant ledges of the 

 Clinton and Medina have been worn back so far by glacial erosion as to 

 lose the contours typical of subaerial erosion. They do not present the 

 sharply outlined salients and reentrants which characterize the brow of the 

 escarpment. He found the brow of the escarpment to be more regular in 

 outline where it stood nearly in line- with the ice movement than it is where 

 it faced against the ice movement. In the latter case its contours are deeply 

 serrated or inflected, the axes of serration being parallel with the glacial 

 striae. On the serrated face furrows were formed, which range in depth 

 from 10 to 30 feet and which have usually a breadth of several hundred 

 feet. The longest of them probably extends more than half a mile back 

 from the escarpment. These rock furrows are now largely filled with drift, 

 yet are plainly discernible on the topographic sheets (see PI. III). The 



