126 G. K. GILBERT — GLACIAL SCULPTURE IN WESTERN NEW YORK 



Pekin. After an interval of 4 miles another lens ai)pears at about the 

 same horizon and continues eastward for 20 or more miles, being accom- 

 panied near Lockport by two other lenses, one above and the other below. 

 In preglacial time the topography of the country was presumptively 

 of subaerial type. As the Niagara escarpment was then in existence, 

 the land was not at baselevel, and the general features of a larger dis- 

 trict show that the slope of the country was then, as now, toward the 

 north. We may assume, therefore, that the land north of the escarp- 

 ment was traversed by north-flowing streams and was characterized by 

 a system of parallel valleys and interstream ridges. In passing each 

 stream valle}^ the outcrops of the ledges were deflected southward, and 

 in passing the intervening ridges northward, so that the outline of each 

 outcrop comprised a series of reentrants and salients. This scalloped 

 pattern is not now found, and its absence is to be ascribed to ice sculp- 

 ture. Each ledge has lost its projecting capes, and may have lost a large 



lQ l5 . ilO MILES. 



FiiiL'EE 5.— Plan of the northern Boundaries of resistant Ledges. 



The lower (south) full line is the boundary of the Niagara limestone. The broken line is the 

 boundary of the Clinton-limestone. The upper full lines are boundaries of sandstone lenses of the 

 Medina formation. N, portion of Niagara river. P, Pekin. L, Lockport. 



territory in addition. At two points near Pekin, where the Clinton lime- 

 stone has no support from underlying sandstone, it has been eaten back 

 to the ver}^ base of the Niagara cliff, and the same is true for a space of 

 several miles near Lewiston, although there its resistance was reeuforced. 

 by that of a strong sandstone ledge. On the Canadian side of the Niagara 

 river not only are the limestone and sandstone worn back to the Niagara 

 escarpment, but the underl3'ing shale has been removed to a depth of 

 about 40 feet, so that the whole escarpment has a height of 250 feet, with 

 the outcrop of the Clinton limestone midway on the slope. No data 

 have been found on which to base an estimate of the extent to which 

 these ledges have been eroded, but it seems clear that the belt of removal 

 was much broader than in the case of the Niagara limestone, so that the 

 Niagara escarpment was rendered more prominent than formerl}' by the 

 erosion of material at its base. 



Sculpture of the Medina Shale 



The main bod}^ of the Medina formation below the sandstone ledges 

 consists of red shale and has a thickness of several hundred feet. This 



