140 University of California Publications in Geology [Vol. 9 



of about three and one-half miles, tapering to a point at a distance of 

 about thirteen miles to the southeast. On the eastern side of the range 

 the strip is narrower, being about a mile in width and three in length. 



The structure of the Cretaceous of the Santa Ana Mountains 

 appears to be that of an asymmetric anticline, the eastern limb of 

 which has been faulted down and partly concealed by subsequent 

 formations. The strata of the western limb of this fold south of Black 

 Star Canon have a strike of about N 45° W and a westerly dip of 25° 

 to 30°. On the west side of this canon the beds are gently inclined, 

 the dip in places not exceeding ten or fifteen degrees. Still farther 

 north the structure becomes complicated by gentle minor folds. North- 

 east of Coal Mine Hill a carbonaceous stratum marks one limb of a 

 small anticline. Coal Mine Canon, north of Sierra Canon, is nearly 

 parallel to the axis of a pre-Tejon fold which involved the Martinez 

 and Chico strata. 



THE TEABUCO FOEMATION 



The basement complex comprising the core of the Santa Ana 

 Mountains is unconformably overlain by a massive red conglomerate, 

 which is traceable as a distinct mapable unit for a distance of about 

 ten miles along the strike. It occurs as a narrow belt three or four 

 hundred feet in width, extending from North Star Canon nearly to 

 Trabuco Canon. The term Trabuco formation is here proposed as 

 a local name for these red beds so well exposed along the western 

 flank of the Santa Ana Mountains. 



The best section of the Trabuco formation may be seen in Harding 

 Canon, where the beds have a dip of about 45° S and a strike of N 

 15° W. The metamorphosed sedimentaries of the basement complex 

 dip about 45° N and strike N 15° W. In places the low dip carries 

 the red basal conglomerates some distance up the slope of the ridges 

 into the area of the basement complex in such a way that small isolated 

 patches of the beds may be found, separated by erosion from the main 

 mass, or connected with it by a thin veneer of residual gravels. 



The loosely cemented conglomerates of the Trabuco formation de- 

 velop upon weathering a rounded topography which is in marked 

 contrast to the abrupt cliffs formed by the gray conglomerates of the 

 overlying Chico. The conglomerates of the basal Chico lie in apparent 

 conformity upon those of the Trabuco, but in most localities the change 

 from the red to the gray beds is very abrupt, suggesting an erosion, 

 interval. Direct evidence for such a structural break is lacking. That 



