68 G. F. BECKER STRUCTURE OF THE SIERRA NEVADA. 



will be rapid and heavy showers will move the blocks, while masses of large 

 size will remain unmoved and decompose slowly. The patches and zones 

 where the shattering has been relatively thorough will thus be carved into 

 hollows and ravines, while the more solid parts of the mass will remain as 

 hills or mountains. 



Formation of Canons. — The influence of these relations is very sensible in 

 the Sierra and every step of the process can be traced on a large or small 

 scale. The shattered zones commonly follow the direction of one of the 

 main fissure systems, and so do many of the canons, large and small ; but 

 the zones sometimes jump across from one set of fissures to another parallel 

 set belonging to the same system, and so, too, do the canons. Such zones 

 also sometimes end abruptly — a fact due, no doubt, to variations in the com- 

 position of the rock. The canons do the same. 



Ice seems to have played a considerable part in clearing the canons of 

 fragments and in excavating shattered and decomposed patches, so that in a 

 sense one must ascribe a large erosive eflTect to the glaciers ; but the ice 

 seems, nevertheless, to have been incapable of cutting into solid masses to 

 any extent, or even into much fissured rock where little decomposition had 

 preceded and whei-e the blocks were tightly wedged together. 



In many cases the glaciers have polished rock surfaces, the contours of 

 which are so thoroughly characteristic of surface exfoliation, due to weather- 

 ing, that no observer could doubt their character, and some of these surfaces 

 are such that ice could not possibly have modeled them. Such evidence, 

 together with that derived from the occurrence of glaciated lavas near the 

 bottoms of the present canons, indicates very clearly that the present system 

 of canons was established long before glaciation began, and probably during 

 the warm and no doubt very wet Pliocene epoch. 



In the area here dealt with there are gorges resembling the Yosemite val- 

 ley in the most striking manner, though on a small scale, but so exposed as 

 to show that their existence is due simply to local intensification of the shat- 

 tering process. In one especially striking instance in the district known as 

 Border Ruffian the gorge is on the east-northeast vertical fissure system, but 

 the fissures are slightly inclined to the southward through some irregularity 

 in resistance. Faulting has consequently produced irregular cross-fractures 

 of the mass and reduced it to more than ordinarily small fragments. 

 Weathering, water and ice have then done their perfect work and cut the 

 mass down to the lowest possible level, leaving, however, the rocky floor 

 exposed. The Yosemite, too, is on the same fissure system ; the fissures there 

 also are somewhat inclined, and the walls still show that the fragments are 

 in part of unusually small size. The bottom of the Yosemite valley is occu- 

 pied by alluvial deposits, but the floor of the deep alcove into w^hich the 

 Yosemite creek falls is solid rock. The Yosemite was once at least partially 



