Vol. 4] Lawson. — Tehachapi Valley System. 459 



With this limited catchment and an annual rainfall of only 10^ 

 inches it seems quite certain, under present climatic conditions, 

 that the lake could not have attained the expanse necessary for 

 it to reach the level of the lowest part of the rim of the basin. 

 Assuming a run-off to the lake of 50 per cent, of the rainfall 

 for a rocky region such as we have to deal with, and an evapora- 

 tion of 60 inches per year under a more humid climate than now 

 obtains, it would require a rainfall of at least 35 inches to enable 

 the lake to attain the necessary expanse to escape at the present 

 outlet. Such a rainfall is not an improbability for this end of 

 the Sierra Nevada during glacial times. But it is to be noted 

 that the present drainage outlet of the valley is through a gorge 

 which has been cut down probably 300 feet since the enclosure 

 of the basin by faulting. To attain this increased altitude for 

 the original lowest place in the rim of the basin, the expanse of 

 the lake would be considerably greater than the present area of 

 the valley floor and the rainfall necessary to effect this would be 

 correspondingly greater. A further objection to the lake ever 

 having had an overflow at the original level of the outlet is that 

 we should expect to find strand lines scored on the sides of the 

 valley well above its present floor and no such strand lines have 

 been detected. It seems thus that we are not warranted in assum- 

 ing that the lake which once occupied Cummings Valley ever had 

 an outlet, and that the drainage outlet which now exists has been 

 evolved without the aid of an overflow. The outlet is a gorge 

 about 300 feet deep at the point where it leaves the valley. Here 

 the geomorphically mature surface to the north of the gorge abuts 

 upon the precipitous mountain scarp which bounds the valley 

 on the south. Where these two surfaces meet there is an asym- 

 metric notch in the mountain profile which constituted, at the 

 formation of the valley, the lowest part of its rim. This notch 

 lay in the line of the fault scarp and could not be missed by 

 a stream cutting back into the very precipitous mountain slope 

 which rises from the Great Valley only a few miles distant. Cum- 

 mings Valley has been tapped, then, by the headwater erosion 

 of a tributary of Tejon Creek cutting back in a structural 

 trough. A consequence of this conclusion is that the great moun- 

 tain wall which confines the south end of the Great Valley on 



