LAWSON ~1 



I'alacheJ 



The Berkeley Hills. 



To the southeast of Skyline Ridge the Orindan formation is 

 traceable for many miles paralleling the Monterey formation on the 

 northeast. On the ridge on the south side of Telegraph Canon, 

 over which is the summit pass of the Telegraph Canon wagon 

 road, the contact of the two formations is sharply defined; and 

 here again the normal superposition of the Orindan conglomerates 

 upon the cherts and shales is reversed, the latter lying upon the 

 former with the beds dipping at an angle of about 6o° S. W. 



Owing to the softness of the rocks composing the Orindan 

 formation, they yield a heavy soil and exposures are not abundant 

 except along road cuttings and occasional steep slopes, so that the 

 proportion in which the conglomerates, sandstones, and shales 

 enter into its composition can not be definitely stated ; but, judg- 

 ing from the great abundance of pebbles in the soil, the conglom- 

 erate greatly preponderates. 



Thickness. — In attempting an estimate of the thickness of the 

 formation, it must be again borne in mind that any result arrived 

 at will be a minimum value for the formation as a whole. The 

 observed volume of the Orindan beds within the limits of the area 

 mapped may be placed at 800 to 1,000 feet. But it seems probable 

 that the rocks here exposed represent the shallower margin of the 

 basin in which these deposits accumulated; and it further seems 

 not improbable that the upper limit of the formation represents a 

 surface of erosion evolved after the basin of accumulation had 

 been filled to the brim or had been drained. 



Intrasives. — Some small basaltic dykes cut the Orindan forma- 

 tion as seen in the cuttings of the Telegraph Canon road. The 

 largest is a dyke, not more than 10 feet wide, with a slight northerly 

 hade, which cuts the conglomerates a little to the south of the 

 summit of the road. It is traceable for about a quarter of a mile 

 directly across the strike of the beds which it cuts. The dyke 

 rock is rather decomposed and affords a good illustration of 

 spheroidal weathering. Two other dykes cut the sandstone of the 

 Orindan formation in Telegraph Canon near the chert quarry, but, 

 being only from two to four inches wide and much decomposed, 

 they are only observable when the rains have washed the face of 

 the cutting clean. 



