376 



University of California. 



[Vol. i. 



Diller* has described a well-marked plain of erosion bordering 

 the upper portion of the Sacramento Valley. It is said to begin 

 near the 40th parallel, on the west side of the valley, and to con- 

 tinue northward, bending eastward around the head of the valley 

 to near the Great Bend of the Pitt River, where it passes beneath 

 the lavas of the Lassen Peak district — a distance in all of about 100 

 miles. The width of the plain varies from 1 to 14 miles. On the 

 west, this peneplain has been carved mainly upon the upturned 

 edges of Cretaceous strata, but in a number of places "extends for 

 several miles into the area of the harder and more durable meta- 

 morphic rocks of the Klammath Mountains." Its steepest average 

 slope is less than two degrees, and its lower or easterly edge dips 

 under the recent alluvium of the valley at elevations ranging from 

 600 to 1,300 feet, while the rear of the slope terminates against the 

 Klammath Mountains at elevations from 1,100 to 2,600 feet. The 

 transition of the peneplain into the mountain slopes at its rear is 

 said to be sometimes abrupt and sometimes very gradual. It is 

 believed to antedate the uplift of the Klammath Mountains, ,and 

 to be still recognizable, within that group, in isolated tables up to 

 6,000 feet in elevation. 



On the eastern side of the valley, the peneplain merges into, and 

 becomes identical with, the general slope of the Sierra Nevada 

 previously referred to. It is supposed to have been originally con- 

 tinuous with the level platform of the Interior or Basin Region- 

 In the words of Mr. Diller, " The erosion plains that we have 

 traced upon the borders of the Sacramento Valley, in the Klammath 

 Mountains, upon the western slope of the Sierra Nevada, and prob- 

 ably also in the interior region of northeastern California, join one 

 another in such a way as to show that they are parts of one exten- 

 sive erosion plain which formerly spread over most, if not the 

 whole, of middle and northern California and southern Oregon." 

 The age of this baselevel is considered to be Miocene, and its 

 formation regarded as being contemporaneous with the deposition 

 of the lone formation about its edges. The latter beds apparently 

 never overlap the peneplain, and sometimes end abruptly against 



*Loc. cit. 



