Studying the Yosemite Problem 



143 



Reference has been made to "earlier glacial epochs." It 

 may be asked, how were the records of the different epochs 

 distinguished from each other? Chiefly by the degree of 

 disintegration of their constituent materials. The moraines 

 of the last epoch are relatively fresh-looking, at least suffi- 

 ciently well preserved to retain their characteristic ridge 

 form. Sharp-crested as a rule, they have the appearance in 

 many places of railroad embankments following a definite 

 alignment of straight stretches and smooth curves, but left 

 in a rough, unfinished state, with large boulders perching 

 upon them or protruding from their sides. A large proportion 

 of the rock fragments, cobbles and boulders in them are but 

 little weathered and retain the smooth surfaces characteristic 

 of ice wear. The older moraines, on the other hand, have 

 for the most part lost their original sharp-crested form, and 

 have been converted to shapeless masses covered with residual 

 soil and interrupted in many places by little valleys or ravines. 

 Only occasional large boulders of resistant rock survive upon 

 them, and even these have lost most of their ice-worn sur- 

 face and are surrounded by fragments scaled off from their 

 sides. Dim moraines, such as these, it will readily be under- 

 stood, can scarcely be traced by their configuration or align- 

 ment. The only reliable method is to identify them by their 

 rock materials. Moraines as a rule contain mixtures of dif- 

 ferent kinds of rock samples, as it were, of all the rock 

 formations over which the ice has passed. Some of these 

 rock types are apt to be distinctive and readily recognized; 

 for the glacialist they are truly diagnostic. When he en- 

 counters one of them he knows at once he is dealing with 

 indubitable moraine. But in order to be able thus to iden- 

 tify obscure glacial deposits by the rocks contained in them, 

 it clearly behooves the glacialist to be thoroughly familiar 

 with the rock formations occurring in each glacier basin. It 

 was in this connection that the simultaneous carrying on of 

 petrographic and physiographic studies proved particularly 

 advantageous. It was the petrographer's duty to study each 

 separate rock formation and to show the area occupied by it 

 on the map. He was therefore often in a position to tell the 

 physiographer, who perhaps was working farther down 



