5g2 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



son, Kernbuts. These peculiar topographic forms were de- 

 fined by Lawson 30 in his paper upon the Geomorphogeny of 

 the Upper Kern Basin. Lawson's Kern cols, the low passes 

 which the Kernbuts connect with the main mountain mass, 

 are characteristically separated from one another in this 

 region by elongate depressions, the fault sags of Dr. G. K. 

 Gilbert. 31 "Considering the Rift as a physiographic type, I 

 find it convenient to have a specific name for one of its ele- 

 ments, the small valley ; and in some of the descriptions which 

 follow I shall speak of it as a fault-sag. 



The general relation of the Rift to the greater valley is 

 illustrated by the cross profile in Fig. 7. Along the north- 

 eastern side it lies everywhere lower than the adjacent slope 

 of the greater valley, the produced profile of the valley slope 

 passing the fault-ridges (kernbuts of Lawson) as well as the 

 fault-sags. Along the southwestern side some of the fault- 

 ridges appear to project above the restored profile of the 

 greater valley, while the fault-sags lie below. If I interpret 

 the structure correctly, the great compound fault concerned 

 in making the valley includes a certain- amount of step fault- 

 ing which is responsible for some of the western ridges of 

 the rift belt; but with that exception, the ridge and sags of 

 the rift are occasioned by the unequal settling of small crust 

 blocks along a magnified shear zone." 



The conditions along the Hay ward Rift are essentially the 

 same as Dr. Gilbert has outlined but since this feature is a 

 recent one the topographic forms which are present are on 

 a small scale but were apparently developed "by the unequal 

 settling of small crust blocks along a magnified shear zone." 

 Many of the small hills are elongate-oval in form with major 

 axis parallel to the Rift and the writer is inclined to regard 

 some of them as small "sliver" fault blocks due to minor 

 "scissors" faults along the Rift. The Hayward Rift in 

 this region traverses the long southwestern slope of Sonoma 

 Mountain into which it has cut a shallow trench in basalt. 

 A view from a point a mile southeast of the Petaluma reser- 

 voir looking along the rift toward the northwest shows a 

 slightly notched skyline at the end of the Mountain School 



s » Lawson, A. C, Univ. Calif. Publ., Bull. Dept. Geol., Vol. 3, No. 15, p. 332, 336, 

 337, 1904. 



81 Report of the State Earthquake Investigation Commission, p. 33, 1908. 



