140 University of California Publications in Geology [Vol. 7 



towards the desert one notes that it becomes incised in a deep 

 and narrow canon which is totally unlike anything developed 

 along the lower courses of the drainage to the eastward. 

 Recent rejuvenation has evidently taken place in the valleys 

 of the Jawbone Canon drainage. At the foot of one of 

 these lower summits there is a rather deep valley coming out of 

 the Sierra into the Mohave Desert. The lower course of this 

 valley is a box canon which comes to an abrupt head at the place 

 where the Los Angeles aqueduct crosses it. In the eastern wall 

 of this canon is exposed northwestwardly-dipping Rosamond 

 greenish tuff-breccia, overlain unconformably by unconsolidated 

 alluvium of light reddish-brown color and derived from the 

 granitic bedrock of the Sierra. In the opposite canon wall is 

 exposed the granite. Between the granite and the Rosamond is a 

 fault along which recent displacement has taken place. The 

 plane of the fault has been followed by the canon. The fault 

 traces approximately north and south, swinging to the westward 

 farther south, and its plane dips from fifty to sixty degrees to 

 the east. The cutting of the box canon has been accomplished 

 since the deposition of the unconsolidated debris forming its 

 upper walls. The topographic expression of the fault is marked 

 by an abrupt break in the profile. The granite rises above the 

 general level as abrupt hills and mountains while the lower-lying 

 Rosamond directly up to the fault contact is eroded into a much 

 gullied topography. This zone of recent deformation continues 

 westward to Tehachapi and Cajon passes. 



Between Indian Wells on the east and Jawbone Canon on the 

 west there is no evidence of recent faulting such as appears in 

 the region west of Jawbone Canon. Between Indian Wells and 

 Jawbone Canon there is a section of the southeastern front of 

 the southern Sierra Nevada exhibiting older topography than 

 the regions of the front to the northeast and to the west which 

 have undergone more recent uplift. 10 



10 For a description of the older topography of this section see the 

 second paragraph under the heading of the Ricardo Erosion Surface. 



