ICE-SHEET EROSION IN NEW YORK 53 



the crest of the cliff' is acceptable when we include in the removal the 

 weathered rock. No distinction was drawn between the weathered and 

 loose rock and the firm rock, and this is an important point in discus- 

 sion of ice erosion. It is certain that along this cliff almost no erosion 

 of the live, unchanged rock occurred. At many localities a weathered 

 layer may be seen in position. Sometimes the ice removed all the 

 weathered rock down to a hard bed, while the latter is merely scratched. 

 Plate 17, figure 1, shows an example ; the derrick stands on live, striated 

 rock, while at the left is the remnant of an upper and rotted layer. 



It is recognized, of course, that subterranean drainage and solution of 

 the limestones probabl} 7 extended to great depths; but the unequal re- 

 sistance of the upper beds has frequently produced a very distinct plane 

 between the severely weathered beds and an underlying bed that was 

 slightly weathered (see plate 18, figure 2, plate 22, figure 2). 



All through western and central New York the preglacially weathered 

 rock may be found in situ in numerous and critical localities. It would 



Figure 3.— Typical Profiles of the Niagara Escarpment. After Gilbert. 

 1. Shale. 2. Lockport limestone. 3. Drift. 



seem to be conclusive evidence of lack of erosion, and will be referred to 

 in discussing other localities. Plates 17-20, 22-23 are a few illustrations, 

 selected from many photographs, which show the preglacial weathering 

 on sandstone, shale, and limestone. Brief descriptions are appended 

 to the cuts. Along the Niagara escarpment the weathered rock fre- 

 quently appears where the drift is removed. 



After looking at the figures showing the serrated cliff the reader might 

 ask if the peculiar profile might not be the final effect of deep erosion, 

 by lowering, or backward ice-cutting, of the cliff. Doctor Gilbert pre- 

 sented conclusive topographic argument against this, and the above 

 facts proving failure of erosion will place the matter beyond cavil. 



The slight erosion of the limestone cliff, and the failure to remove 

 even the crest of shale where this was unprotected by a limestone cap, 

 as shown by Gilbert, pages 123 and 124 of his article, and illustrated in 

 figure 3, is satisfactory proof that here, at least, the ice had little erosive 

 power. And why not here ? Every condition for vigorous erosion seems 

 to have been fulfilled — an opposing cliff, not too high, composed of soft 



VIII— Bull. Geol. Soc. Am., Vol. 10, 1904 



