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32°, — while it would waste slowly at its surface by spontaneous evapo- 

 ratiom, and more rapidly at its surface by solar heat — it would waste at 

 its base also by the upward transmission of earth heat. But this 

 waste would be represented by so much water, which under an immova- 

 ble glacier would form a lake. Under a movable glacier it helps to form a 

 river, and the river which issues at the terminal surface moraine brings 

 out the evidences of the lubricant, as "mountain meal." 



Every glacier must be made cavernous by its river, and along the 

 caverns produced by the river and its branches are collected and de- 

 posited or rolled forward all the stones in the glacier while those upon its 

 surface (or melted out to its surface, by the upper waste), ride down to its 

 lower end. 



The much larger part of the erosive action of a glacier must therefore be 

 of the nature of river erosion ; while a certain percentage of it may be of 

 the nature of engraving. But if so, then our knowledge of river erosion 

 must direct us in the investigation of glacial erosion. 



River erosion is local and interrupted. Parts of a river bed are filling 

 up, while reefs and barriers are being cut away. So, under a glacier, the 

 loci of erosion must be few and of limited extent. Behind these the 

 rolled glacial debris are covering and protecting the bed rock instead of 

 eroding it. Our kames show therefore not only that Glaciers are feeble 

 eroders, but that they are great depositors and protectors of the earth sur- 

 face. 



We may go one step further, and show how in the age of ice the usual 

 erosion of our topography was almost stopped and forbidden by the ice. 



For, the topography of the earth's surface is evidently due to rain, 

 softening the surface — to rills, removing the softened surface — to brooks, 

 sweeping the collections made by rills, down through the brook-vales and 

 ravines which they have made, — until the process of erosion is reduced 

 to a minimum where river deposits commence. Rivers never erode, except 

 at rock barriers — or, in rainless regions, where they saw strait down, 

 using their whole debris. 



Now, in the ice age, the ice-covering protected the whole country from 

 rain, rill and brook erosion, and the process of topographical modification 

 of the earth's surface ceased, and was not resumed until the close of that 

 age. What erosion took place, must have been exclusively confined to 

 the lines of subglacial rivers and theii' branches, along the subglacial 

 caverns. In a continental ice-flow crevasses were impossible, except along 

 a few lines of escarpment. 



The rain, therefore, in the ice age must have constituted a great riseau 

 of superglacial drainage incapable of eroding the subglacial topography ; 

 in fact removed from it hundreds and even one or two thousand feet from 

 it vertically. If the Canadian ice had a surface slope southward, towards 

 Pennsylvania and Ohio, or south-westward up Lake Erie and across Illinois, 

 then mighty rivers, heading in the Laurentian mountains and the Adiron- 

 dacks, must have flowed for a long time over Ihe upi)er surface of tiic ice 



