igoo-i.] Physical Geology of Central Ontario. 163 



renewal of the process of subaerial degradation, and the development 

 of topographic forms and an adjusted drainage system appropriate to a 

 region underlain by alternate series of strong and weak rocks at low 

 angles of dip. During the extensive Pleistocene glaciation the topo- 

 graphic features, the product of the preceding cycle, may have been 

 largely modified, destroyed, or otherwise obliterated, and new forms 

 produced. 



A'leasiire of erosive work. — Our measure of the work performed 

 during these two periods must necessarily be derived from a knowledge 

 of the present features, and of the conditions existing before the oper- 

 ation of the erosive agents. The proportion to be assigned to either 

 period depends upon a knowledge of the relative competence of the 

 processes of degradation, and of the time during which they were oper- 

 ating. The amount of work performed by either process, and by both, 

 will vary with the locality, and with the conditions under which the 

 process is in operation, e.g. geographical position, elevation, position 

 with reference to baselevel, character of the rocks, relation to the ice 

 front and to the neve of a glacial lobe. At present the knowledge of 

 the total effects of both processes, and of the method of operation of 

 sheet-glacier ice, seem too limited to warrant the assignment of a 

 definite portion of the work to either, except in local cases. 



Present Features. — General Description. — The Niagara cuesta 

 is a prominent topographic feature extending along the south shore of 

 Lake Ontario from east of Rochester to Hamilton, thence northward 

 across Ontario to the Manitoulin Islands, thence curving southwestward 

 to the east of Green Bay and across parts of the States of Wisconsin, 

 Illinois and Iowa. Lakes Erie, Huron, and Michigan are situated upon 

 the outer lowland ; Lake Ontario, Georgian Bay, and Green Bay lie 

 upon the lowland in front of the cuesta; Lake Superior lies in a 

 position outside of both lowland and cuesta. 



The cuesta-front forms one boundary of a great inner lowland. The 

 southwestern loop of this lowland is best developed in the State of 

 Wisconsin, and may thus be appropriately designated the WISCONSIN 

 LOWLAND. The eastern part, the ONTARIO LOWLAND, includes the 

 basins of Lake Ontario and Georgian Bay, as well as the adjacent land 

 areas. The two parts of the inner lowland are connected by a narrow, 

 more or less submerged belt, passing across the Manitoulin Islands. It 

 has been found convenient to refer to the present unsubmerged part of 

 the Ontario lowland, within the Province of Ontario, as the CENTRAL 

 Ontario Lowland. (Map I.) 



