196 Adventures in Scenery 



like grasp of the ice, cut grooves and scratches on rock floors 

 over which they passed. The glacial scratches, called striae, 

 thus made, tell the direction of ice movement. The finely 

 powdered rock dust served as a polishing agent, and expanses of 

 highly polished rock surfaces show unmistakably the work done 

 by the ice in abrading hard surfaces. Such polished and striated 

 surfaces high on rock walls and on the tops of high promon- 

 tories give indisputable evidence of the depth of the ice mass. 

 Erratic boulders perched high above the valley bottom, or 

 strewn in moraines, or as isolated fragments, show by their 

 character (texture and composition) the parent ledges or 

 sources from which they came, and the direction of the move- 

 ment of the ice. 



Sierran Glaciation Independent of 

 Continental Glaciation 



The formation of glaciers in the High Sierras was indepen- 

 dent of the formation of the Continental Ice-Caps in the Hud- 

 son Bay and Labrador regions. Snowfall in the High Sierras is 

 very great at this day, the moisture-laden winds from the Pacific 

 Ocean, rising above the Coast Ranges and moving landward, 

 lose their moisture in the cold air of the high crests of the 

 Sierras, and the Great American Desert of the Basin region to 

 the east results. In Quaternary geologic time, when con- 

 tinental glaciation occurred, continental conditions may have 

 been such that the precipitation was greater than at present. 

 At any rate snowfall was great enough in Quaternary time 

 so that glaciers formed along the Sierra crest. Snow gathered 

 among the high peaks in protected basins and on the shaded 

 sides of mountain crests. More snow fell during the winters 

 than melted during summers. Thus bodies of snow gradually 

 changing to neve and ice came to lie in great masses. Basins or 

 slopes so situated as to become gathering places for snow-ice are 

 called cirques. Such gathering-grounds for snow, the head- 

 quarters so to speak of glaciers, were, and are, common in the 



