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gravimetric surveying, Yucaipa, California, San Andreas Rift, faults, Mill Creek


Earth Sciences | Environmental Sciences | Physical Sciences and Mathematics | Tectonics and Structure


The Yucaipa Valley is located approximately 70 miles east of Los Angeles, California. It is situated between the Transverse Range Province and the Peninsular Range Province. Cutting through the Mill Creek area to the north is one of the most significant structural features of Southern California, the San Andreas Rift.

Gravimetric surveying of the valley portion of the 7.5 minute Yucaipa Quadrangle has continued for two years under the direction of Dr. S.W.Dana at the University of Redlands. Gravity measurements were made with the use of the Worden gravimeter while reductions of the data were performed with an IBM 1130 computer system. The reduced gravity is interpreted as a rough approximation of the basement configuration.

Many interesting features are noted in the gravity contouring in Yucaipa Valley. There are several locations where the gravity increases around a central point being interpreted as a rise in the basement. Similar decreases in gravity might represent a basin-like depressions. Numerous faults in the nonconformable basement were also noticed on gravimetric data while no surface expression was evidenced.

The Mill Creek area is highly faulted with many parallel north-west trending faults. These are mostly within the San Andreas Rift. Most of the faults were detectable only by gravimetric data as they were buried beneath the Quaternary alluvium.

Isogals in the area show a complex pattern with a general decrease in value towards the west. This westerly decrease in gravity would cause ground water to flow primarily in this direction.


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