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Santa Fe Dam Recreation Area lies at the mouth of San Gabriel Canyon on an alluvial fan. The quarries [pits] in the area have provided much of the aggregate [crushed rock] used to build freeways and construct homes in California. Indeed, the City of Irwindale claims:

The City is known as "Jardin de Roca," Spanish for "Garden of Rocks" because of the high quality of rock and gravel found here. Almost every highway in the State of California, and many highways west of the Mississippi River, have Irwindale rock as part of their foundation.

"The San Gabriel Mountains are traversed by deep, steep-sided canyons cut into highly fractured crystalline basement rocks that form the bedrock underpinnings of the mountains. The sides of most canyons are blanketed by unstable hill-slope rock debris that constantly is being stripped away by slope failures and by runoff and washed out to the range fronts, where sediment is deposited on surfaces and channels of alluvial fans."

More from Geolologic Setting of the Transverse Ranges Province

"In a city like Los Angeles, nearly all of our time is spent - whether we are standing, sitting, sleeping, or driving - on an underlayment of concrete or asphalt. This manufactured ground material has its origins in the earth, at specific locations around the city. The aggregate that makes up the bulk of these bulk materials tends to be found in great abundance in the river valleys, where the disintegration of the mountains spills into channels, and falls downslope over the millennia, forming deep deposits in the ground. The material even sorts itself out, based on the distance from the mountains, with heavier, coarser material near the base of the slope, and progressively finer material further away.

More from Margins in our Midst, A Journey Into Irwindale The Center for Land Use Interpretation Newsletter, Winter 2003

Most of the rocks found at Santa Fe Dam are granite. To help identify them, use this fine web site Igneous Rock Identification provided by the Alert Project NASA/Cal Poly Pomona.

Web Page by Jane Strong for CNPS-SGM, April, 2004