ransome.] The Great Valley. 375 



fans of the numerous streams which flow into the Sacramento River 

 from the west, north, and east. Its upper limit varies somewhat, as 

 would be expected. It appears to attain a maximum elevation of 

 1,300 feet* on the Cold Fork of Cottonwood Creek, on the west 

 side of the valley, where it overlaps the old baselevel of erosion 

 presently to be described. 



On the east, the Sacramento Valley is bounded by the gentle 

 slopes of the Sierra Nevada proper, until we reach latitude 39 40', 

 approximately that of the town of Chico. Here the more ancient 

 rocks of the Sierra begin to disappear under the late tertiary vol- 

 canic accumulations of the Lassen Peak volcanic ridge, which con- 

 tinues to form the boundary of the valley on its eastern and north- 

 eastern sides. Similarly on the west, the Coast Ranges proper give 

 place as we go north, near latitude 40 , to the irregular and ill- 

 defined group of little known mountains where the Coast Ranges 

 and Cascade Ranges unite, and for which the name Klammath 

 Mountains has been proposed. f 



As regards the* character of the slopes which the bounding 

 mountain ranges present to the valley, it may be noted that 

 whereas those on the west rise with comparative abruptness from 

 the plain, the slopes of the Sierra, on the other hand, when we allow 

 for the vigorous trenching of the present streams, rise toward the 

 east with a remarkably regular and very moderate grade, and at 

 their foot merge gradually into the plain. This long western slope 

 has been emphasized by nearly every observer who has had occa- 

 sion to describe the range as a whole. Newberry, indeed, says 

 that "above Chico Creek the Sacramento Valley rapidly narrows, 

 that portion lying east of the river forming a nearly level plain, 

 four to six miles wide, from which the foot-hills of the Sierra rise 

 abruptly."| But an inspection of the Chico topographic sheet of 

 the U. S. Geological Survey hardly bears out his statement of 

 abruptness, while immediately to the south of Chico Creek the 

 gentle regular slope is very strikingly shown by the contours. 



*Diller, Topographic Revolution on the Pacific Coast. 14th An. Rept. 

 U. S. Geol. Survey, p. 407. 

 fDiller, Inc. cit., p. 404. 



{Pacific R. R. Reports, Vol. VI, Geology, p. 25. 



