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University of California Publications. 



[Geology 



their basins mainly to the solution of the metamorphosed lime- 

 stone in which they are situated. The western and largest, Bear 

 Lake, has been greatly enlarged by the construction of a dam 

 at the head of Bear Creek Canon until it is now five miles long 

 and, at the maximum, a mile broad. The dam is now being 

 heightened so as to increase considerably the area of the lake. 

 Other broad valley flats near the heads of streams and upland 

 areas between the streams are : Horsethief , Cactus, Monarch, 

 Burnt, Union, Little Pine, Big Pine, Arrastre, and Broom flats; 

 Ilolcomb, Little Bear, and Grass valleys ; and other flat areas to 

 which no particular names are applied. Not all of these are in 

 limestone areas, although it is not known how many may pos- 

 sibly owe their origin to solution of calcareous rocks. The im- 

 portant point, however, is that the upper courses of these streams 

 were developed and are still in an older cycle of erosion than 

 their rejuvenated lower courses, and that many, at least, of these 

 broad open valleys cannot be accounted for by solution of lime- 

 stone. There is, for instance, the broad flat known as Big 

 Meadows, near the head of Santa Ana River, which merges into 

 a terrace which has a height four miles farther down the river 

 of nearly four hundred feet above the present river bed. Few 

 changes in topography can be more startling than will be noted 

 by one who, after climbing the steep slopes of a canon in the 

 northern foothills of the San Bernardino Range — as, for in- 

 stance, any one of a half dozen canons in the area mapped on 

 the northwestern portion of the San Gorgonio Atlas Sheet — 

 suddenly comes upon a broad divide and finds broad valleys of 

 low gradient, separated by low rounded hills, leading down in 

 the opposite direction. Or he may look at the difference in the 

 character of the topography on the opposite sides of the divide 

 separating the drainage tributary to the Pacific Ocean from that 

 sloping towards the Mohave Desert, in the area mapped on the 

 northern portions of the San Bernardino and Redlands Atlas 

 sheets. The southern side of this divide is characterized by 

 its extreme youth and by its deep canons which reach back to 

 the very head of the divide ; the country and streams have much 

 less relief to the north of the divide, where the topography bears 

 a much maturer aspect. Doubtless much of this difference in 



