The Geological Story Briefly Told 81 



imposing form, one of the most stupendous and imposing moun- 

 tain ranges of the continent, and indeed of the world. The 

 great mountain mass stands as a huge block of the crust of the 

 earth, uplifted more at its eastern side and tilted toward the 

 west, having a gentle westward slope, sharply defined crest, 

 and abrupt eastward-facing escarpment. 



Widespread uplifting of the land in Quaternary time brought 

 about important climatic changes, due to change of wind cur- 

 rents. The Pleistocene epoch of the Quaternary period was 

 the Ice Age, the period in which widespread glaciation occurred 

 over northern North America. During the Ice Age the crest 

 of the Sierra Nevada above 5,000 feet altitude was covered with 

 a mantle of ice. Ice, despite its hard and brittle character as 

 we commonly know it, when in great masses possesses something 

 of the character of a plastic substance. Under the pressure due 

 to its own weight the ice of glaciers flows in the manner that 

 may be likened to cold stiff pitch or tar. The accumulation 

 of ice upon the crest of the high Sierra was so great that vast 

 tongues of ice ploughed down the valleys of the Sierra slope both 

 west and east, excavating and widening them, and carrying vast 

 quantities of earth materials which were deposited on the lower 

 slopes as the ice finally melted. Loose soil and broken rock were 

 swept from the higher reaches of the range, and the surface left 

 often naked and bare. Marks of ancient glaciers, naked, 

 scoured, and polished rock surfaces on the slopes below the crest 

 of the range, cirques or rock basins in which glaciers originated 

 high on the crests of peaks and ridges, bear testimony to the 

 work of ice. Deep V-shaped canyons which had been cut in 

 the hard rocks by swiftly flowing streams that descended the 

 western slope of the great tilted Sierra block were widened and 

 made U-shaped by the power of the great ice streams that 

 coursed through them. 



While these great activities were in progress in the Sierra 

 region tremendous changes were occurring in the region of the 



