240 0. H. Hershey — River Terraces of California. 



Art. XXII. — Certain River Terraces of the Klamdth Region, 

 California ; ^^ Oscar H. Hershey. 



Remnants of terraces occur in all the principal valleys of 

 the Klamath Mountains. Heretofore, the writer has not con- 

 sidered them of any particular significance as they appeared 

 not to constitute a definite system. In the down-cutting of 

 the deep Pleistocene valleys, remnants of the old valley floor 

 were left at various levels above the present streams and do 

 not necessarily indicate an "uplift of the region by stages. On 

 a recent trip between the coast at Humboldt Bay and the high 

 mountains near the head of the South Fork of Salmon River, 

 the writer had the opportunity of observing a more definite 

 system of river terraces than are developed farther east in the 

 Klamath region and they seem to tell a story woithy of con- 

 sideration, especially through its bearing on the problem of the 

 cause of glaciation of the high mountains. 



These terraces are situated on the- Trinity River below the 

 mouth of New River, on the Klamath River below the mouth 

 of Salmon River, and on the Salmon River and its South 

 Fork as far up as Summerville. These streams in this region 

 flow in Pleistocene, canons which have an average depth of 

 3,000 fee,t. They are trenched into comparatively resistant 

 metamorphic rocks such as schists and slates, intruded by 

 batholites and dikes of gabbro, diabase, diorite and allied 

 igneous rocks. There is considerable diversity in the resistant 

 properties of the different formations and in consequence the 

 canons vary greatly in w^idth. Throughout the greater portion 

 of their courses they are extremely narrow at the bottom, 

 usually no wider than the streams, and for miles in places are 

 practically impassable except high up on the slopes. The 

 rivers are superimposed on the structure and traverse indiffer- 

 ently hard and soft belts. The downward progression of the 

 valley floor is controlled by the rate of erosion of the hard 

 rock barriers in the gorges. In tjie soft belts, the streams 

 exert their energies on the walls of the valley rather than its 

 floor and excavate small basin-like valleys with flat floors from 

 ten to twenty -five times as wide as the gorges in the hard rocks. 



Through the gorges the streams flow straight and swift and 

 hurry along the gravel and boulders. In the basins, the streams 

 until recently w^ound about in meanders, here and there touch- 

 ing and undermining the valley walls. The gravel carried 

 out of the gorges was spread over the flat floors of the basins 

 in sheets from 5 to 20 feet in depth, constituting ordinary 

 gravelly alluvial plains. As the rock barriers in the gorges 



