158 OSCAR H. HERS HEY 



even in Neocene times the junction of the main Trinity river and 

 the east fork of Trinity river. 



I think I can trace the old valley up the main Trinity river to 

 and beyond the mouth of Coffee creek, a distance of half a dozen 

 miles from Trinity Center. There are near the river a number of 

 small mountain ridges rising to about the same height. Back of 

 them the slopes of the high mountains rise very abruptly. 

 Usually this does not indicate a halt in the down-cutting of a 

 deep mountain valley, because the tributary streams in approach- 

 ing a trunk stream ordinarily reduce the summits of the inter- 

 vening ridges to much lower levels than the main divides, pro- 

 ducing the appearance of a basin, beneath the floor of which has 

 been trenched a later system of valleys : but in this case the 

 observer gains the impression that the ridges near the river have 

 sufficient regularity in height to indicate that they are remnants 

 of the floor of an old valley which was several times as wide as 

 the present river valley. This apparent old valley floor rises 

 upstream more rapidly than the present river, and before it 

 reaches the point beyond which later erosion has so completely 

 destroyed it that one fails to recognize a trace of it, the elevation 

 above the river may be a thousand feet. 



For a long time I have entertained the idea that the old 

 Trinity river drained Scott valley. There is no definite evidence 

 of this, but a number of facts in its favor. The abrupt termina- 

 tion of Scott valley at the southern end indicates differential 

 uplift of the Scott Mountain region on the south. This we may 

 call the Scott Mountain arch. The Trinity Neocene valley dis- 

 tinctly rises on the southern slope of this arch. On the summit 

 of Scott Mountain there is a depression which may be a portion 

 of the old valley floor. In that case the amount of the differ- 

 ential uplift would be between 2,000 and 3,000 feet. Some of 

 the important tributaries of Scott river (which flows north), 

 have southerly courses, hardly explainable by the known struc- 

 ture of that region, and suggest a reversal of the direction of 

 drainage in the main valley. 



The new valley trenched by Trinity river between Trinity 

 Center and Junction City has an average depth below the late 



