SUPPLY OF MATERIALS 495 



where shales and sandstones of the Medina border the shore. On the 

 south side there are frequent exposures of the Medina strata at the 

 west end and for some distance east of the mouth of the Niagara river. 

 The intervening shore as far east as Sodus point is not known to the 

 writer in detail, but it is understood to be made up largely of glacial 

 materials overlying bedrock which usually outcrops below water level. 

 East of Sodus point and as far as the Saint Lawrence there are a number 

 of places where limestone ridges extend out into the lake as points or 

 islands. Except in the two localities on the north shore at the east 

 between the Saint Lawrence and Presque Isle, and at the west between 

 Lome Park and Burlington, the rock exposures are very small and are 

 usually overlain by a considerable amount of glacial debris, which sup- 

 plies the great bulk of the shore materials on these shores. 



On lake Erie the rock exposures on the north shore are fewer in 

 number than on lake Ontario, and the glacial and lacustrine deposits over- 

 lying the rocks are thicker. The principal rock exposures occur near the 

 east end of the lake, east of ISTormandale, where low rocky points are 

 the rule. At the west end of the lake and on the islands south and 

 west of point Pelee, rock outcrops from beneath the loose glacial debris. 

 Along the south shore bedrock underlying glacial deposits is frequently 

 noted at the base of the line of sea-cliffs. The material supplied to the 

 waves from the bedrock is on the whole very small in quantity. 



Glacial materials, till, interglacial sands, and gravels. — By far the 

 larger portion of the loose materials which are operated on by the shore 

 processes of both lakes are of glacial origin — till derived either from 

 drumlins or moraines, and sands and gravels derived from kames, eskers, 

 sand plains, or more frequently from the lacustrine deposits of the inter- 

 glacial epochs. 



Lacustrine materials — secondary. — Lacustrine materials of post-glacial 

 date, derived from the earlier glacial deposits, are also a minor source of 

 shore materials ; but it is usually impossible to distinguish them from the 

 similar deposits from which they were derived, and, so far as the study 

 of the processes on the shore are concerned, it is unnecessary to make any 

 distinction between them and the earlier glacial materials. 



SOURCE OF THE SHORE DEBRIS 



The materials, whatever their type, are brought within the range of 

 the shore processes by various agencies. 



The waves cut out much waste by direct action on drumlins and other 

 topographic forms of glacial origin. Indirectly they are aided by numer- 

 ous other agencies, animate and inanimate. 



Where there are sand beds of interglacial origin underlying till or 



