Salt River Cobbles

December 15, 2005

Salt_river_cobbles

Provided and copyright by: LuAnn Dahlman, Center for Earth and Space Science Education at TERC
Summary authors & editors: LuAnn Dahlman

These stunning river cobbles were collected along the Salt River, northeast of Phoenix, Arizona. Over time, the river and its tributaries have carried an incredible variety of rocks from Arizona’s Central Highlands down to the Basin and Range province in the southern part of the state. This collection represents years of gathering rocks on occasional walks along the river. All the rocks are totally natural; a sprinkling of water brings out their intense colors.The largest cobbles in the photograph are about 8 inches (20 cm)across. The rocks reveal examples of storm-deposited sediments, near-melting conditions, the presence of crack-filling hydrothermal fluids, intrusions of slow cooling magma, and extended series of regular deposition events. While most of the rocks exhibit sedimentary and igneous features, virtually all of them have experienced some degree of metamorphism; without exposure to heat and pressure, they wouldn't have been hard enough to withstand the grinding action of the bedload transport that moved them down-stream. Rock units represented by the cobbles include the Mazatzal Quartzite (purplish conglomerate with white quartz and red jasper clasts), Barnes Conglomerate (rounded white and gray pebbles in reddish cement), and Payson Granite (coarse-grained pink/orange granite).

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