NIAGARA FALLS AND VICINITY 57 



Throughout the greater part of the district, the pohshed rock sur- 

 faces are covered by a coating of drift of very varying character 

 and thickness. This was the ground moraine or till of the Lauren- 

 tian glacier, and represents the rock debris which was frozen into 

 the bottom of the ice, and carried along in its motion, till liberated 

 by the melting of the ice. This ground moraine, either in its 

 original heterogeneous character or modified by the agency of 

 running water, filled most of the old river gorges through which the 

 drainage of preglacial times found its exit. Some of the shallower 

 lowlands, like that of the Tonawanda, were also filled with drift, 

 while the more profound ones, like the Erie and Ontario lowlands, 

 received only a partial drift filling. 



The partial obliteration of the old drainage channels, which was 

 thus brought about, together with a depression of the land on the 

 northeast to a depth below that at which it now stands, converted 

 the unfilled lowlands into lake basins, apparently reversed the drain- 

 age of many streams, forcing them to cut gorges where their old 

 channels were drift-filled, and finally became the immediate factors 

 in the formation of Niagara. 



Lacustrine period^ 



During the slow melting of the glaciers in the Laurentian region, 

 and the resultant northward retreat of the front of the ice, large 

 bodies of w^ater, of varying depth and extent, were held in front of 

 the ice sheet, which formed a dam across the northeastern part of 

 the lowland country, the general slope of which was now toward 

 the ice instead of away from it. The elevations of these glacial 

 lakes were determined by the lowest uncovered passes in the 

 margins of the lake basins across which the discharge took place, 

 and, as during the continued melting of the ice dam, lower passes 

 were progressively uncovered, the outlets were successively trans- 

 ferred to them and the levels of the lakes sank correspondingly. 



^For a detailed account of the successive stages in the development of 

 the great lakes, the shore lines, outlets and extent of each, the reader is 

 referred to the papers by Gilbert, Spencer, Taylor, Leverett, Fairchild and 

 others, cited in the appendix. 



