CORRASION BY GRAVITY STREAMS. 295 



extensive peneplain. Lawson 1 refers this dismantled up- 

 land to the upper surface of a large batholith which has 

 been stripped of its cover of sediments. The fact, however, 

 that this dismantled peneplain surface occurs indifferently 

 in both intruded sediments and intrusive granites must be 

 accounted for. In this old surface, ordinary streams appear 

 to have developed broad valley floors now uplifted to form 

 plateau surfaces (see also Lawson, pp. 315 - 328). Especially 

 well is tbis latter feature to be seen in the neighbourhood 

 of Evolution Valley where the wide upland valley surfaces 

 apparently are, or rather were, continuous with the wide 

 upland valley levels of the plateaus drained by the Lower 

 Merced, the latter almost certainly being due to the long 

 continued action of ordinary streams. Later came a period 

 of stream revival and the high plateau surfaces were deeply 

 trenched by valleys. In some spots canon cutting may be 

 seen to be due to a double period of stream revival. 



After this period of canon-cutting came several ice 

 visitations. The earliest period of which we have certain 

 knowledge appears to have been one of strong glacial 

 action. (A still earlier and important ice period is sug- 

 gested by the peculiar profiles of the topography 2 which 

 have been obliterated in part by the later ice masses. 

 Of this however, we have no certain information). It 

 was productive of long and steep-walled trenches within 

 the more rounded forms of the older topography. [The 

 trenches which occur in valley constrictions have spurless 

 walls, cliff bases in alignment, and huge cirque-like heads.] 

 Large local depressions and interruptions occurred in the 

 channel bases, and the heads of these were caused to recede 

 by the combined action of abrasion, quarrying and sapping 

 until the valley floors were cut up into a series of gigantic 

 "steps" — either high or low — with correspondingly large 



1 Lawson, A. C, pp. 300, 301. z Gilbert, Systematic Asymmetry. 



