Alluvium is young sediment—freshly eroded rock particles that have come off the hillside and been carried by streams. Alluvium is pounded and ground into finer and finer grains (by abrasion) each time it moves downstream. The process can take thousands of years. The feldspar and quartz minerals in alluvium weather slowly into surface minerals: 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.
Alluvium
Mechanical or Physical Weathering Gallery
Alluvium is sediment that has carried by and deposited from running water. Like this example from Kansas, alluvium tends to be clean and sorted. (more below)
Photo courtesy Ron Schott of Flickr under Creative Commons license
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