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Photograph of trench cut

Photograph of trench cut by the USGS in 1979 across Strand C of the Cucamonga Fault zone (as used by Morton and Matti, 1987) on the Day Canyon fanhead at the base of the southeastern San Gabriel Mountains. The fault zone dips about 35 north (toward the left), and forms a zone about 2 m wide of disturbed sediment and aligned cobbles and boulders (visible behind the person). The dark-colored zone at the top of the trench wall is the thick A horizon of a soil profile that caps the alluvial-fan gravels exposed in the trench. This profile drapes down the face of the scarp and thickens at its base, where a jumbled mass of colluvial material is present just to the left of the trench spoils. This colluvial wedge has grown in two ways throughout the 12,000-year history of fault-scarp development: (1) by gravitational collapse of the over-steepened land surface that occurred during each of several successive earthquake ground ruptures that formed the scarp and contributed to its cumulative development through time, and (2) downslope movement by colluvial processes as a result of weathering and sediment transport.

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