448 



University of California. 



I Vol. 3. 



streams build broad, low alluvial fans of fine material on the 

 borders of the river's flood-plain. The characteristics of this 

 group of terraces are the evenness of surface, the fineness of 

 the materials, the uniformly great thickness of the deposits, the 

 absence of landslides, and the deep red color. 



The so-called 120-foot terrace buries an upper group of 

 channels having common characters, and even the bed-rock floor 

 of which may be more than 120 feet above the river. The 

 channel gravels are comparatively fine and thin. They are 

 overlaid by the usual flood-plain silts, which in two mines are 

 fossiliferous. These silts are overlaid by remarkable torrent 

 fans, which contrast strongly with the alluvial fans of the 

 Upper Group. The former are small, thick torrent fans 

 built up on the flood-plain at the mouths of small ravines, 

 while the larger creeks, such as Pearch and Camp, did not build 

 any corresponding fans. On the higher terraces similar small 

 ravines did not yield such fans, but the large creeks built fans. 

 It is because of this diference that the 120-foot terrace every- 

 where is characterized by a markedly undulating surface, quite 

 unlike that of any other terrace. Further, where the torrent 

 fans are absent, landslides commonly rest on the inner border 

 of the flood-plain. 



The lower channel under the 120-foot terrace has coarse 

 and thick gravels, and they are not covered by thick flood-plain 

 silts, although red clay generally forms the surface deposit. 

 This is the outer edge of the terrace, and has an average height 

 above the river of 110 feet in the Upper Basin and 145 feet at 

 Graham Flat, but is due at Orleans at 120 feet, hence my deri- 

 vation of the term "120-foot terrace." The surface sweeps 

 up over the torrent fans in an unbroken slope. But the torrent 

 fans and landslides do not overlie the lower channel, "from which 

 I infer that they were forming on the flood-plain while the 

 river occupied the lower channel. 



Bed is he prevailing color of the flood-plain deposits of this 

 terrace, and even of the torrent fans where there is much clay, 

 hut it is not as intense a tint as in the Upper Group, and does 

 not penetrate as deeply into the gravel. Where the torrent 

 fans are thick, gray and light brown are the prevailing colors. 



