1913] The Moraine Systems of Southwestern Ontario. 61 



ping done is also quite uneven in degree of detail; in some parts, espe- 

 cially the northwest, the work was mainly of the nature of reconnaissance 

 and was done early in the studies. Other parts, as the moraines on the 

 Niagara peninsula, those along the escarpment southward from Colling- 

 wood, and those north of London and north of Toronto were studied 

 in much more detail. This was the natural result of work carried on 

 through many years on an independent basis. Since 1908 the work 

 has been done under the direction of the Director of the Geological 

 Survey of Canada. 



The Uncovering of "Ontario Island". 



One of the results of the influence of topography upon the move- 

 ments of the ice was the formation of what may be called "Ontario 

 Island". At its maximum the ice was probably not less than 2,000 or 

 3,000 feet deep over the region around Dundalk on the top of the high- 

 land south of Collingwood. But as the ice melted off, this covering 

 grew thinner, until at last it became so thin that it ceased to move. 

 Streams melted out tunnels and canyons in it and reached down into the 

 dirty basal layers of the ice. From these the streams gathered gravel 

 and sand, and where they filled the tunnels and canyons with this material 

 they made eskers, like the magnificent ones which extend southeast from 

 Flesherton and Mount Forest, or, where they dumped their load in an 

 expanded cavity or recess in the ice they made kames, as in the great 

 hills of gravel and sand northeast of Stratford and north of Barrie. 

 But soon this thin ice covering melted off and a large area extending 

 northeast from London was freed of ice, but was still surrounded by ice 

 on every side. 



During the warm season each year the ice sheet was affected by 

 melting, not alone along its edge, but over a marginal belt several hun- 

 dreds of miles wide. It is to be remembered also that great ice streams 

 like those which flowed past this island are always hundreds of feet 

 higher along the line of their central axes than at their sides. On this 

 account, an extensive high obstruction in the path of the ice sheet, but 

 over which the ice moved, always caused a depression in the surface of 

 the ice sheet, and where the obstruction was so great as the highland 

 between Lake Ontario and Lake Huron, it formed a correspondingly 

 large depression. The surface of the ice sheet for 50 to 100 miles all 

 around sloped toward this depression, and all the water from the melting 

 on these slopes flowed into it. It is manifest that there was no chance 

 * for an outlet for this water anywhere north of London. But along a 



