1 62 OSCAR H. HERS HEY 



that of the Klamath river, which has attracted some attention 

 and given rise to speculation as to its origin. I have never vis- 

 ited it, but it is described as just such a narrow, steep-sided, and 

 very deep valley as that of the lower Trinity. The mountains 

 near the so-called canyon rise to altitudes of 6,000 and 7,000 

 feet, and the valley has a depth below these higher summits of 

 4,000 to 5,000 feet. Probably 3,000 or 4,000 feet of this erosion 

 belongs to the time following the deformation of the Neocene 

 surface. I suggest in explanation of it the same hypothesis as 

 has been applied to the lower Trinity, namely, that the Klamath 

 river flowed in practically its present course in late Neocene 

 time, and that during the subsequent orographic disturbance a 

 broad arch was formed athwart its course, but the river main- 

 tained its position by cutting a deep valley through the arch. 

 Virtually the same opinion has been expressed by Mr. Diller. 



In the case of the Trinity river, there must have been first a 

 short, rapid tilting of the Neocene baselevel to cause the river 

 to migrate and then followed a long, gradual uplift ; for, had the 

 movement been a simple one and gradual, the river would not 

 have abandoned its course, and had it been rapid throughout, 

 the river would not have been able to maintain its course by 

 cutting a gorge through the arch. 



The south fork of Trinity river and the main Trinity river 

 below Hawkins' Bar roughly mark the western limit of this arch. 

 The Post Creek mountains stand on the crest of the arch south 

 from Hay Fork river. From their summit southwestward to 

 near the south fork of Trinity river there is a long, compara- 

 tively gentle slope of the general mountain surface, then a sud- 

 den descent into the canyon of the river, and on its southern 

 side a comparatively abrupt rise of about 3,000 feet to the 

 even-crested summit of South Fork Mountain. There is here 

 apparently a fault with a throw of several thousand feet or a 

 sharp monocline. I believe that I am correct in referring the 

 long southwestward slope northeast of the river to the late Neo- 

 cene grade level of Trinity valley, but I am not so sure that it is 

 an equivalent of the Klamath peneplain remnant at the summit 

 of South Fork Mountain. The Hyampom Neocene deposit lies 



