Alluvium

Alluvium is young sediment—freshly eroded rock particles that have come off the hillside and been carried by streams. This top image shows part of an alluvial terrace in the southern Sierra Nevada, where boulders as large as 1 meter mix with sand and gravel, washed down a side canyon into the Kern River valley. The Kern has undercut the terrace here and carried away part of the alluvium deposit, starting a new cycle of transport and deposition somewhere downstream.

Alluvium is pounded and ground into finer and finer grains each time it moves downstream. The process can take thousands of years. The photo below is from a small alluvial fan at the foot of the Temblor Range in west-central California. Here the alluvium is all of gravel size and smaller. Each of the visible layers in this streambank exposure was probably laid down in a single flood event. The feldspar and quartz minerals in alluvium weather slowly into clays and dissolved silica. Most of that material eventually (in a million years or so) ends up in the sea, to be slowly buried and turned into new rock.


_{(c) 2001 Andrew Alden, licensed to About.com, Inc.}

More about California Geology

Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
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