246 O. H. Hershey — Rivev Terraces of Calif oimia. 



The upper Trinity River between Trinity Center and Bragdon 

 falls about 10 feet per mile. This is so nearly a stable grade for 

 that stream that it has not in recent times materially trenched 

 the floor of its comparatively broad valley, but, on the contrary, 

 it is able to build alluvial plains of coarse gravel which it floods 

 nearly every year. This portion of the Trinity River is a 

 larger stream than the Salmon River above Forks of Salmon, 

 and a smaller stream than the Klamath River from the mouth 

 of the Salmon River to Weitchpec, but it is about the equiva- 

 lent of the average of those two streams. It is encumbered 

 with gravel of similar coarseness to that on the Salmon and 

 Klamath Rivers. I wish to derive from this comparison the 

 suggestion that while the streams were forming the flat valley 

 floor in the basins, the fall from Summerville to Weitchpec 

 was probably no greater than 10 feet per mile. Indeed, on the 

 lower Trinity where the tiny canon is well developed, the fall 

 now scarcely exceeds that amount and at the time of the 

 development of the broad valley floor must have been consider- 

 ably less. 



The difl^erence between the theoretical 10 feet per mile and 

 the present fall of 35 feet per mile or 25 feet per mile, may be 

 the differential amount of the uplift, which would mean an 

 absolute elevation of the Summerville basin exceeding 2,000 

 feet (which, in reality, is probably a minimum). This implies 

 that the central portion of the Klamath region, particularly 

 that area which is occupied by the high mountains which were 

 once extensively glaciated, has suffered an elevation relative to 

 the present coast-line, late in the Quaternary Era, of several 

 thousand feet. 



The higher terraces seem also to indicate uplift, but not of 

 such a pronounced differential character, as a stable grade was 

 resumed without great depth of cutting. This series of dis- 

 turbances occurred within the last one-flfth and probably the 

 last one-tenth of the Quaternary Era. Similar uplifts may 

 have occurred earlier in the era, but their effects inland have 

 been mostly destroyed. 



The terrace system is developed on Redwood Creek and on 

 the lower Mad River, where it connects with the lower marine 

 terraces along the coast. Opposite Korbel the flat-topped hills 

 mark the floor of an ancient valley a number of miles wide, as 

 described by Mr. J. S. Diller.* It is traceable as the principal 

 upper terrace to the mouth of the valley where the latter 

 enters on the lowland of the Humboldt Bay region. The river 

 has trenched it to the depth of several hundred feet, excavat- 

 ing a broad valley in soft Tertiary strata but a narrow gorge 



* ''Topographic Development of the Klamath Mountains." U. S. Geol. 

 Sur. Bull., No. 196, p. 54. 



