The Geology of a Portion of the San Jose Hills

Citation

Cutsforth, David Harrison (1949) The Geology of a Portion of the San Jose Hills. Master's thesis, California Institute of Technology. doi:10.7907/B9NJ-HV96. https://resolver.caltech.edu/CaltechETD:etd-10112005-081503

Abstract

The area discussed in this report, consisting of sixteen square miles, embraces a portion of the San Jose Hills approximately three miles south of the town of Covina, Los Angeles County, California. The area was mapped on a base map prepared from U. S. Geological Survey topographic maps, and the Brunton compass-pacing method of mapping was used. The region is part of an upland that rises above the Los Angeles and San Gabriel basins, and consists of a group of rolling hills trending in an approximate east-west direction. The relief throughout the region is moderate, and as a result of the semi-arid climate vegetation in the area is sparse. Bedrock is well exposed, excepting in some of the areas underlain by shale, where slumping has distorted the rocks and soils and a dense grass growth further hinder exposure of the underlying formations. All of the rocks exposed in the area are of sedimentary origin, and, with the exception of Recent and Pleistocene alluvium, are part of the Puente formation of upper Miocene age. The Puente formation is divided into three members--a lower member of shale, a middle member of sandstone and conglomerate, and an upper member of shale, sandstone, and conglomerate. The subsurface rocks, knowledge of which has been derived from wells drilled in the area, consist of the Topanga formation, the Glendora volcanics, the Mountain Meadows dacite porphyry, and the basement complex in that order with increasing depth. There appear to have been two major periods of deformation in the San Jose Hills area during Tertiary time--one at the close of the Pliocene and one during and after the deposition of the Miocene Puente formation. The Puente deformation seems to have involved only gentle folding, but the post-Pliocene deformation was more severe, involving steep folding and some faulting. The general structural trend of the region is approximately N 60 E, and is parallel to the boundaries of the higher hills. Folding in the area has created a series of parallel anticlines and synclines, with the San Jose anticline in the northern half of the area being the major structure. The one fault of any magnitude in the area is the San Jose fault, which enters from the east and apparently dies out in the central part of the area. It is a vertical or steeply dipping reverse fault with considerable downthrow on the south side. There are other smaller faults in the southern half of the area. The geologic history of the region largely involves erosion after the intrusion of the basement complex in Mesozoic time until the Miocene period, when the area became one of deposition. The Glendora volcanics were deposited in early middle Miocene time, followed by submergence of the area and deposition of the Topanga, Puente, and Pliocene formations. After the close of the Pliocene the region emerged from beneath the sea and again became an area of erosion.

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