Yosemite National Park 



199 



canyons have been deepened and broadened. The valleys, V- 

 shaped as cut by streams, have been generally rendered U-shaped 

 by the plucking and wearing of the ice on the bottoms and 

 walls of the canyons. Rock fragments were plucked from 

 walls and floors, carried along by the moving ice, and when the 

 ice melted the rock debris carried along was thrown down. 



The most important deposits, and probably the most signifi- 

 cant of all the effects of the ice, are the moraines heaps and 



Photo by F. E. Matthes, U. S. Geol. Survey 

 FIG. 60. Old moraine, near Wawona Road south of Turtleback Dome. 



ridges of boulders, gravel, sand, and clay thrown down by the 

 melting ice. The most reliable record of glacial activity, on the 

 whole, is that embodied in the deposits of rock waste left behind 

 by the glaciers. These, wherever well preserved, accurately 

 define the limits reached by the ancient glaciers. Terminal 

 moraines mark the ultimate limits reached by the ice. Lateral 

 moraines mark the limits of trunk glaciers and tongues of ice 

 on the sides of valleys down which the ice moved. Medial 

 moraines mark the joining or coalescing of glaciers which con- 



