72 



Sierra Club Bulletin 



the field represented in Figure i these levehng operations may- 

 be studied to excellent advantage. The furrows between the 

 several ridges are leisurely filled up by the inblowing and wash- 

 ing of leaves and the finer material of the adjacent ridges. As 

 the weathering of the surface boulders goes on^ the crumbling 

 material which falls from them collects about their bases, thus 

 tending to bury them, and produce that smoothness of surface 

 which characterizes all the more ancient moraine-fields of the 

 Sierra. The great forest soil-belt of the west flank has not 

 been hitherto recognized as a moraine at all, because not only 

 is it so immensely extended that general views of it can not be 

 easily obtained, but it has been weathered until the greater por- 

 tion of its surface presents as smooth an appearance as a farm- 

 er's wheat-field. 



It may be urged against the morainal origin of the forest belt 

 that its sections exposed by freshet streams present a quite dif- 

 ferent appearance from similar sections of more recent mo- 

 raine-beds unmistakably such; but careful inspection shows the 

 same gradual transition from the boulder roughness of the one 

 to the crumbled earthiness of the other that we have already 

 traced between the superficial roughness and smoothness of 

 moraines according to age. 



Under certain conditions moraine boulders decompose more 

 rapidly beneath than upon the surface. Almost every section 

 of the forest belt presents specimens in every stage of decay, 

 and, because those that are water-rounded and polished are 

 more enduring than others, they occur in comparatively greater 

 abundance as the soil becomes more ancient. The position of 

 the soil-belt is given in the ideal cross-section of the range 

 (Fig, 2)'. 7^^ upper limit nearly coincides with the edge of a 

 comparatively level bench, A B, extending back to the summit 

 peaks. Upon this lofty, gently inclined bed the waning ice- 

 sheet lay nearly motionless, shallowing simultaneously across 

 its whole breadth, and finally broke up into distinct ice-streams 

 which occupied the present river canons. These have left their 

 lateral moraines in the form of long branching ridges of soil, 

 several miles apart, extending from the summit ice-wombs 

 down to the main soil-belt, into which they blend and disap- 

 pear. But if the ice-sheet had maintained its continuity to the 



