1922] Vaughan: Geology of San Bernardino Mountains 



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periphery, however, topographic forms are strikingly new. Several of the 

 streams are not reduced to grade; they meander through broad uplands in the 

 central part of the range, then plunge over falls into steep canyons, which they 

 follow to the valleys that border the ranges. 



For these striking topographic differences there seems to be no adequate 

 explanation in the rock types, in the relative masses of the ranges, in their 

 relation to major drainage lines, nor in their relation to precipitation. It is 

 concluded, therefore, that since each is a faulted, uplifted block, the San Ber- 

 nardino mass, in which old topographic forms are well preserved, is much later 

 in origin than its neighbor, the San Gabriel range, in which none of these old 

 forms are now to be found. 



While it is thus evident that there are considerable areas of an old 

 erosion surface preserved in the higher portions of the San Bernardino 

 Mountains, the case is not so simple as stated by Mendenhall. Two 

 erosion cycles have been recognized by Mendenhall, but evidence will 

 be adduced to show that four distinct cycles of erosion are represented 

 besides several subcycles. 



The First Cycle 



The long, even summit ridge extending from San Bernardino 

 Mountain to San Gorgonio Mountain is so striking that it at once 

 suggests the remnant of an old peneplain ; nor are we disappointed 

 on closer observation (pi. 17 A) . True, San Bernardino Mountain itself 

 culminates in a serrate ridge, but half a mile to the east this gives way 

 to a narrow flat summit ; and a mile and a half farther this broadens 

 out to a considerable area which is strikingly distinct from the rounded 

 topography of the second cycle sloping away from it on the north. 

 Still farther eastward this area narrows to a broken ridge with occa- 

 sionally a flat area. About a mile west of San Gorgonio Mountain 

 glacial cirques have been effective in giving the ridge a rugged aspect, 

 but at the summit of the mountain we again find a portion of the old 

 surface. The fact that these flat areas extend across gneisses and 

 schists, as well as granite, precludes the possibility of their being due 

 to some dominant structural feature. They are therefore regarded as 

 the remnants of an old peneplain. 



East of The Pipes are several flat-topped summits capped with 

 basalt (pi. 17B) which are perhaps correlative with the peneplain rep- 

 resented on the summit ridge. The granite surface below the basalt 

 and the surface of the basalt itself are both flat. The latter, however, 

 is traversed by a few broad, shallow gullies which cut through some 

 of the basalt flows, thus showing that we have here a surface actually 

 carved from the basalt and not merely the top of the basalt flow. 



