352 



University of California. 



LVOL. 3. 



two magnificent cirques which lie at the base of the cliffs encir- 

 cling Mt. Whitney, the one to the south and southwest and the 

 other to the northwest. The southerly cirque is shown in Plates 

 32 a and 42. This is a fine example of the way in which these 

 glaciers of the Sierra Nevada have eaten into the heart of the 

 mountains. The companion cirque on the northwest of the 

 mountain is similar in character, and the two glaciers were con- 

 fluent immediately on emerging from the cirques. Below the 

 point where the two glaciers became confluent there is a low, 

 well glaciated median ridge, which is probably nearly coincident 

 with the line of confluence of the two ice streams. This rocky 

 ridge extends for about a mile and rises from 50 to 100 feet above 

 the glaciated sloping floor which it traverses. The crest line of 

 the ridge pitches with the general slope and has a descent of over 

 GOO feet. Besides its roche mouionnee character of surface, the 

 ridge presents another feature of interest. The granite of which 

 it is composed is traversed by three systems of joints, one roughly 

 horizontal and two vertical, of which one is parallel to the axis 

 of the ridge and the other at right angles to it, or nearly so. 

 These joints thus divide the granite into parallelopipeds, some of 

 which are elongated in the direction of the ridge, while others 

 are elongated transverse to it. In several places an aggregation 

 of these parallelopiped blocks of granite have been removed from 

 their places and carried away by the ice stream. The result is that 

 there is a series of vertically walled troughs from 10 to 40 feet 

 deep abruptly interrupting the smooth roche mouionnee surface of 

 the ridge. At the upper end and middle parts of the ridge these 

 box-shaped troughs are transverse to the ridge; while at the 

 lower end of the ridge the troughs are longitudinal. These 

 troughs have evidently been formed by the removal of blocks of 

 granite after the main sculpture of the ridge had been completed 

 since the vertical walls of the troughs rise to its surface abruptly ; 

 yet, while this is true, the ice has glaciated the walls of the 

 troughs to some extent, glacial polish and striation being per- 

 fectly distinct upon some of the even, vertical sides of the troughs 

 which were closely observed. 



About a mile below the cirque shown in Plate 42 the walls 

 of the canon on Whitney Creek on the south side are polished 



