V GLACIAL EROSION OF LAKE BASINS 125 



of the Rhone crept on down its valley past Martigny and 

 St. Maurice till it reached the lake ; it is then supposed 

 not to have marched on with an ice-wall, say five hundred 

 or more feet high, but to have at once spread out like so 

 much soft pitch, and to have filled the lake to its present 

 water-level or thereabouts. Then, over this great plain 

 of ice, the sub-glacial torrent of the Rhone is supposed to 

 have flow^ed, carrying with it and depositing at the end 

 of the lake that ancient alluvium which, somehow, has 

 got to be accounted for ' ^ 



Having thus filled the lake v/ith ice instead of water, 

 the main body of the glacier is supposed to start afresh 

 and to travel over the ice, and thus obviate the imaginary 

 difficulty of a glacier moving up hill, though every 

 student of glaciers now admits that they did so, and 

 though it is universally acknowledged that this very glacier 

 of the Rhone moved over higher, steeper, and more irregular 

 hills on its way to the Jura and to Soleure. 



Now this extraordinary theory involves two difficulties 

 which are passed by in silence, but which seem to entirely 

 contravene all that we know of the nature of glaciers, and 

 to be entirely unsupported by facts. The first is — the 

 glacier ceasing to move onward as a glacier, but spreading 

 out to fill up a lake basin, as if the lake were simply 

 frozen to the bottom. Is this conceivable or possible ? 

 I think not. When glaciers come down to a fiord or to 

 the sea they do not spread out laterally, but move on till 

 the water is deep enough to buoy them up and break off 

 icebergs, and no reason is given why anything different 

 should have happened in the case of the great Swiss and 

 Italian lakes, supposing they existed before the ice age 

 came on. That the glacier should afterwards slide over 

 this level plain of ice is equally inconceivable, in view of 

 the property of regelation of ice under pressure. Owing 

 to this property the glacier and the lake ice would become 

 one mass, and would move on together under the law of 

 decreasing velocity with depth. This, however, is of 

 little importance, if, as I conceive the supposition of the 

 formation of an ice-sheet at the water level for fifty miles 

 1 A, Falsan, La Periode Glaciaire, p. 135, 137. 



