SIERRAN VALLEYS OF THE KLAMATH REGION I 57 



and in consequence this arching is not so apparent in the topog- 

 raphy as it is in the Coast Range region farther south. Indeed, 

 it is only by studying the cause of the nearly complete destruc- 

 tion of the Neocene river deposits that one is led to recognize 

 the fact and comprehend the nature of this arching. 



The main Neocene channels for the most part had courses 

 across this subsequently developed system of arches and troughs. 

 Over the arches they have been carried so high above the pres- 

 ent stream level that subsequent erosion has completely removed 

 the old river deposits, and in most cases even destroyed the form 

 of the old valleys. But where some of the main rivers crossed 

 the deeper and broader troughs, their deposits were not lifted 

 high enough above the present stream level to be completely 

 destroyed by erosion. That is precisely the position in which 

 we find the present Neocene remnants of the Trinity basin. 



The direction of the axes of the arches and troughs is gener- 

 ally a little west of north. The preservation of twenty miles of 

 the length of the old Trinity river deposit was due to the fact 

 that the portion of it from Weaverville northward ran nearly 

 parallel to a trough. The floor of the old channel rises very 

 gradually toward the north, but where the channel deposit turns 

 westward at Weaverville. the bottom rises rapidly. Apparentl}^ 

 the floor of the channel at Weaverville has an altitude less than 

 1,000 feet above the sea and at the distance of three miles or 

 less this has risen to probably 2,500 feet above sea-level. 



The Weaverville Neocene area terminates northward about a 

 mile south of Swift creek, but the broad basin in which it lies 

 continues onward for many miles. The floor of the Neocene 

 valley seems to have reached such an elevation at Trinity Cen- 

 ter that the present Trinity river has trenched below it and 

 removed its filling. It is probable that the summit of the low 

 hills just west of Trinity Center reaches nearly or quite to the 

 level of the floor of the old channel, and some of the river 

 deposits near the top of these hills may be actual remnants of 

 the Neocene deposit. The present river valley is abnormally 

 wide at this point, which is due to the fact that here the old and 

 new courses coincide, and further that there was at this locality 



