Hershev. ] 



Quaternary of Southern California . 



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Pliocene sandstone, and then deep, narrow, rocky gorges before 

 they enter Piru Creek. It is evident that once the soft rocks 

 extended as far as the present site of the stream, which, in the 

 course, of its wanderings on a broad late-Pliocene flood-plain, 

 aided perhaps by a slight tilting' of the basin, came to flow along 

 the southwestern border, and subsequently, when uplift enabled 

 it to trench far beneath the level of the old plain, it encountered 

 the hard gneissic rock and cut its present canon while the area 

 of softer rock on the northeast was being generally reduced. 

 This is one of the finest cases of a superimposed stream that has 

 come under my observation. 



For many miles Pirn Canon has an average depth of 1,000 

 feet. In the crystalline rocks, granite and gneiss, its bottom is 

 little wider than the stream-bed. For more than five miles it is 

 trenched into Cretaceous shales and sandstones, and here widens 

 in places to several hundred yards, but throughout it remains a 

 rocky, winding canon abounding in precipitous bluffs, a sheer 

 precipice of 500 feet height not being uncommon. It is identical 

 in character and dimensions with Sierran canons eroded in the 

 Sierra Nevada region in similar rock as the granite and gneiss of 

 the Alamo mountain, by streams of similar size. Piru Canon, 

 we know, dates entirely from a time succeeding the uplift and 

 tilting of a late Pliocene formation, thus corroborating the 

 post-Pliocene age of the Sierra Nevada canons as heretofore 

 inferred from other data. 



As in the Sierra Nevada canons, the erosion of Piru Canon 

 has continued practically uninterrupted to the present day. No 

 line can be drawn in the canons corresponding to a widely 

 distributed, rather late Quaternary deposit in neighboring low- 

 land areas. The term Sierran can have no epochal value, as it 

 is derived from these canons and must be applied to the whole 

 time occupied by their erosion. It is simply the designation of 

 a type of deep, narrow valleys in the California mountains which 

 are of later age than the Tertiary era, but it can have no part in 

 the scheme of classification which is going to be proposed. 



The Santa Clara River, between Acton and Lang, is also a 

 superimposed stream, flowing in a deep, narrow Sierran canon 

 trenched in the hard crystalline rocks on the south of the main 



