1917] 



Moody: Breccias of the Mariposa Formation 



385 



walls of forty degrees. Little difference is to be found in the rate 

 of erosion, measured in terms of steepness of slope, in the canon ; 

 amphibolite seems no more resistant than the slates of the Mariposa. 

 A great number of youthful, tumultuous streams are tributary to the 

 North Fork. Robbers Ravine and the nameless gorge just east of 

 Colfax drain the rugged region around Cape Horn. South of Colfax 

 the drainage is chiefly into Bunch Canon. Live Oak Creek, a tributary 

 of Bunch Creek, and Bushy Creek, which flows directly into the North 

 Pork, drains the high region around New England Mills. The eastern 

 tributaries are Indian Creek, Bushy Creek and Owl Creek, all drain- 

 ing the Forest Hill divide just beyond the limits of the map. The 

 average grade of the North Fork of the American River is 58 feet 

 to the mile. The disparity in grade between the Bear and the American 

 rivers has been shown by Lindgren to be due to the weakening of the 

 activity of the former caused by the capture of its headwaters by the 

 South Fork of the Yuba River in the vicinity of Emigrant Gap. 



At least two cycles of erosion, separated by a period of extensive 

 deposition are represented in the region, and each controls in a greater 

 or less degree the development of present physiographic features. The 

 first cycle appears in the old surface upon which the superjacent series 

 was laid down. This surface has been extensively exhumed by the 

 stripping away of the gravels and volcanic rocks which have accumu- 

 lated since Cretaceous time. The surface is one of subdued relief 

 carved in the massive, schistose and slaty rocks of the basement com- 

 plex. Tbe remnants of the superjacent series which are found on 

 some of the hills determine a second surface of low relief, but of a 

 totally different character ; it is a graded, depositional surface. The 

 present topography was evolved by the dissection of this graded sur- 

 face by streams consequent upon a late uplift of the Sierra Nevada 

 block. The present elevation of the range may have been attained in 

 a double uplift, the first stage of which caused the removal of most 

 of the superjacent series, and the second instituted the vigorous 

 erosion that is still continuing in the carving out of the narrow, 

 groove-like canons in the basement rocks. The present streams flow 

 in V-shaped trenches with abrupt approaches; lateral or tributary 

 drainage has not been sufficiently advanced as yet to remove all of the 

 once extensive superjacent series ; outliers such as Howell Hill and 

 Colfax Hill are abundant and characteristic of this, as of other parts 

 of the foothill belt of the Sierra Nevada. In a word the present 

 geomorphic cycle is still in a stage of early youth. 



