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University of California Publications in Geology [Vol. 8 



of these ranges. Both are evidently antecedent streams. The exact 

 nature of the movements during Pleistocene times was not studied in 

 detail. The Santa Ana River, Santiago Creek, and Arroyo Trabuco, 

 a tributary of the San Juan, all show well-developed stream terraces. 

 (See plate 27, figure 2 ; text-figure 1) . Two distinct terraces were noted 

 on the Santa Ana River and three very prominent ones on Santiago 

 Creek and the Arroyo Trabuco. These stream terraces apparently 

 record successive uplifts of the coast, as the two lower terraces of the 

 Arroyo Trabuco are directly traceable into the ocean terraces at the 

 mouth of its master stream, the San Juan. 



The development of Santiago Creek appears to be due to a con- 

 dition antecedent to the movements of the Pleistocene. The course 

 of the main stream roughly marks the division between the Cretaceous 

 and the Miocene strata. (See plate 27.) 



The geological divisions recognized in this area were a basement 

 complex of Triassic (?) age; upper Cretaceous, Chico; lower Eocene, 

 Martinez ; the upper Eocene, the Tejon ; the lowermost Miocene, the 

 Vaqueros ; and stream deposits of probable Pleistocene age. The range 

 is faulted on the northeast side, the Miocene being brought in contact at 

 one point with rocks of Triassic ( ?) age. This fault apparently merges 

 into an anticline to the northwest in the vicinity of Sierra Peak. In 

 its southern extension the displacement does not seem to be so great as 

 the maximum displacement northeast of Santiago Peak. The age of 

 this fault is evidently post-Miocene, and some of the physiographic 

 features, such as Lake Elsinore in its southern extension, suggest at 

 least a Pleistocene age for a part of its development. If our corre- 

 lation of the Perris Peneplain on the south with some of the plateaus 

 mentioned is correct, the probable age of its development would be 

 later Miocene or Pliocene. Mr. Buwalda suggests that the Perris 

 Peneplain is of the same age as the Ricardo Peneplain described by 

 Baker.' 1 If this could be demonstrated, we would have a valuable 

 datum plane in this part of the state. 



On the southwest slope of the Santa Ana Mountains the Cretaceous 

 and Vaqueros in most of the range dip toward the west from the 

 central canoe-shaped area of the basement complex. A small anticline 

 in the Chico was recognized in the northern portion of the area. 



o Baker, C. L., Physiography and structure of the Western El Paso Bange 

 and the Southern Sierra Nevada. Univ. Calif. Publ. Bull. Dept. Geol., vol. 7, 

 pp. 137-139, 1912. 



