of 



ravines became widened, deepened, and ex- 

 tended into enormous U-shaped glaciated gorges. 



Glaciers have gouged or scooped many basins 

 in the solid rock. These commonly are made at 

 the bottom of a deep slope where the descending 

 ice bore heavily on the lever or against a reverse 

 incline. The size of the basin thus made is de- 

 termined by the size, width, and weight of the 

 glacier and by other factors. In the Rocky 

 Mountains these excavations vary in size from 

 a few acres to a few thousand. They became 

 lake-basins on the disappearance of the ice. 



More than a thousand lakes of glacial origin 

 dot the upper portions of the Rocky Mountains 

 of Colorado. Most of these are above the alti- 

 tude of nine thousand feet, and the largest, 

 Grand Lake, is three miles in length. Landslides 

 and silt have filled many of the old glacier lake 

 basins, and these, overgrown with grass and 

 sedge, are called glacier meadows. 



Vast was the quantity of material picked up 

 and transported by these glaciers. Mountains 

 were moved piecemeal, and ground to boulders, 

 pebbles, and rock-flour in the moving. In addi- 



253 



