10 



University of California. 



[Vol. 3. 



Tertiary depression. The Sierra Madre and San Bernardino 

 ranges abound in such deep, narrow gulches and canons. They 

 are strongly contrasted on the one hand with the broad basins 

 which separate the ranges and on the other with the insignificant 

 valleys eroded from the Pleistocene deposits of the neighboring 

 regions. 



THE SANTA CLARAN EPOCH. 



The Soledad Canon, between Saugus and Lang stations, is a 

 steep-sided valley, from one-half to one mile in width and 400 to 

 GOO feet in depth. It is terraced, and of these terraces there are 

 remnants to indicate four principal flood-plains. The upper and 

 best defined has an estimated height above the river of 400 feet, 

 and the others are about equally spaced down to twenty-five feet 

 above the river. These terraces are benches cut into the inclined 

 Upper Pliocene strata and covered with five to fifteen feet 

 of horizontally-stratified, coarse alluvial gravel and boulders. 

 Each marks a halt in the down-cutting of the valley, perhaps 

 dependent upon an uplift by stages. Tributary canons are 

 proportional in size to the main valley. Among them they have 

 cut up the basin into an extremely broken country, with a 

 youthful type of topography. The erosion accomplished since 

 the uplift of the first alluvial plain is about equal to that on 

 certain portions of the Illinoiau area in the Mississippi Basin. 



Throughout the Upper Pliocene basin the hills have been 

 reduced to a comparative uniformity, none falling much below 

 or rising much above a plane which is far below the summits of 

 the surrounding mountains. This uniformity was produced by 

 degradation nearly to a base-level represented by the highest 

 river- terrace. It was approximating to a peneplain. I should 

 hardly like to call it a peneplain, because, although the streams 

 flowed leisurely in very broad, shallow valleys which were 

 thoroughly graded and occupied the larger part of the surface, 

 these valleys had distinct bluffs and the interfluvial surfaces were 

 in places sharply undulating. It was a land in the condition of 

 topographic old age. 



A careful study of the structure will make it evident that in 

 places as much as several thousand feet in thickness of the 



