Stebbins Gulch Weathering to Erosion

Weathering, mass wasting and erosion

What most people would call “erosion” in everyday speech is subdivided into three distinct processes by geologists:  weathering, mass wasting, and erosion.  Weathering is the chemical and physical decomposition of rock near the Earth’s surface.  Mass wasting is the downslope movement of weathered debris by gravity.  The term erosion is applied only to the actual transport of sediment by water, wind or ice.  All three processes are suggested in the photo above.  Let’s begin by assuming that the large blocks of sandstone now lying in the creek bed were once part of the cliff face high above the creek.  The joints in the rock allow rainwater and plant roots to penetrate into the cracks, weathering the surface and weakening the rock’s hold on the cliff.  Alternate freezing and thawing in winter would further widen the joints.  Eventually the rock breaks away and slides or rolls down the slope.   Finally, the boulder begins moving downstream a few feet at a time during heavy flooding.  

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