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Oblique aerial photograph looking northwest at a thrust-fault scarp

Oblique aerial photograph looking northwest at a thrust-fault scarp of the Cucamonga Fault zone along the base of the southeastern San Gabriel Mountains (Cucamonga Strand C of Morton and Matti, 1987). The fault scarp trends east-west across the San Gabriel Mountain front, and disrupts sand and gravel deposits of alluvial fans that have debouched from the mouths of the mountain canyons (in this view, Day Canyon on the left and East Etiwanda Canyon on the right. The largest and best-preserved alluvial fans are latest Pleistocene and Holocene in age (15,000 to about 1,500 years), and are thought to have been deposited mainly as a result of the climate change that occurred across the Pleistocene-early Holocene boundary (Bull, 1991). Note that older strands of the Cucamonga zone form scarps north of Cucamonga Strand C (toward the mountains); these are not as well preserved as the Strand C scarp, and can be mapped only intermittently along the base of the mountains. Click here to view a trench cut by the USGS across Cucamonga Strand C. The trench showed that the Strand C scarp is formed by a low-angle thrust fault that dips north toward the San Gabriel Mountains at about 35. Photo by D.M. Morton, USGS, July, 1973.

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