0. H. Hershey — River Terraces of California. 241 



were eroded, the basin floors were cot down, remnants of the 

 alluvial plains left as terraces, and a new gravel plain formed. 

 That is the explanation of the terraces but it is not by any 

 means the whole of the story. 



The terraces are confined almost exclusively to the basins. 

 The system is chiefly developed below a height of three or four 

 hundred feet above the streams. Occasional alluvial remnants 

 occur higher but there is nothing definite about them. Three 

 fairly persistent levels may be traced out in the different basins, 

 although intermediate terraces are occasionally detected. The 

 lower is the most prominent, stands from 20 to 75 feet above 

 the present streams at low water, and in reality forms the flat 

 floor of the basins. After a terrace level is abandoned by the 

 streams, the rock debris from the neighboring steep mountain 

 slopes works down and builds up talus deposits, giving the 

 older terraces a sloping surface. Hydraulic mining demon- 

 strates that the river deposits extend to the inner edge of the 

 terraces under the talus debris. The lower terrace level has 

 been abandoned so recently that talus deposits and alluvial fans 

 on it are not conspicuous, so that its evenness of surface is a 

 marked feature. 



Above Hawkin's Bar, the Trinity River issues from a gorge 

 cut largely in gabbro, and enters upon a belt of Bragdon slate, 

 the least resistant of all the pre-Cretaceous formations of the 

 Klamath region, and has eroded in it a valley from one-fourth to 

 over one-half mile in width. The stream now flows in a nar- 

 row canon trenched from 50 to Y5 feet below the flat valley 

 floor. The walls of the canon consist of highly inclined slates 

 capped by gravel. 



From the mouth of Willow Creek to Waterman, three miles, 

 the valley is wide and the terrace system well developed. 

 Several hills of rock rise like islands, centrally on the valley 

 floor. The stream flows in its usual very narrow canon trenched 

 30 to 40 feet below the lower terrace which forms the flat 

 valley floor. 



The river passes out of the Bragdon slate belt, for several 

 miles traverses a narrow gorge and then returns to the slate 

 belt and enters Hoopa valley. This is about seven miles in 

 length and one mile in average width. Its altitude is about 

 350 feet above sea level. "^ The floor is even enough to consti- 

 tute a fine body of agricultural land. The river traverses it 

 in a meandering course, touching each side alternately, but the 

 meanders are trenched 20 to 30 feet below the flat valley floor. 



* Altitudes given in this paper are derived largely from aneroid deter- 

 minations by J. S. Diller. The figures of the height of terraces are mainly 

 estimates drawn from memory, but are sufficiently accurate not to vitiate the 

 arguments. 



