Q6 G. F. BECKER — STRUCTURE OP THE SIERRA NEVADA. 



trations of all tliese statements. " So far," says Heira, " there is no proof 

 whatever that these [Norwegian and Greenland] glaciers tear their ground 

 moraines from rock in place." Finally, he concludes that "glaciation is 

 equivalent to relative suspension in the process of valley formation."* 



The action of the ice and snow fields on the Sierra during its glacial period 

 was thus chiefly to protect the underlying rock from decomposition and 

 erosion, and this action must have exerted an influence upon the erosion at 

 lower altitudes. Imagine the range to-day to be covered with canvas down 

 to 5,000 feet above sea-level. Then erosion could take place only below this 

 level, and the only eflect which erosion could produce would be to deepen 

 the stream beds and cut steeper slopes than those which now characterize 

 the topography. You may say that a canvas-covered range is highly hypo- 

 thetical, but there are plenty of hills covered, if not with liuen, yet with 

 sheets of lava. The following diagram shows the outlines of two hills near 

 Pence's ranch taken from j^hotographs. They are near together, in the same 

 formation, and the lower portion of each is composed of the same material, 



Figure 7 — Lava-capped hill and bare hill. 



but one has a lava cap and the other has none. The unprotected portion 

 of the lava-capped mass shows relatively very steep slopes, and one cannot 

 doubt that if such protection were now added to the rounded hill its lower 

 exposures would gradually assume contours similar to those of the mesa- 

 topped butte. 



The formation of the ice cap on the Sierra must have acted very much as 

 the superimposition of a lava cap or a canvas cover would do on the rounded 

 hill shown in the figure 7, or it must have produced an effect similar to that 

 actually observed and indicated in figure 6. I do not think that the action 

 of the ice cap can be pronounced quantitively insufficient to produce the 

 observed eflPect. One has only to imagine this covering to have persisted for 

 a sufficient time to produce any increase of general slope or any amount of 

 canon erosion. The great moraines form a rough index of the duration of 

 the glaciers, and there is nothing seemingly unreasonable in supposing that 

 while these vast masses were accumulating the observed caiion erosion might 

 have taken place. 



The subject may be regarded from a slightly different though nearly 

 equivalent standpoint. Duriug the Pliocene the waters flowing from the 

 crest must have become loaded with detritus high up on the range and the 

 coarsest portion of this load must have been deposited as soon as decreasing 

 inclination or increasing width of the stream beds caused a slackening of 



*"Haudbuch der Gletcherkuude," 1885, pp. 174-189. 



