Soil Erosion Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 4, 2011

Regolith And Slopes

Regolith is a collection of many different soil and rock types. It’s the loose rock matter, like volcanic ash and wind-driven deposits that are scattered around on bedrock.

All rock surfaces, except for the super steep and the fairly new (geological time) are covered by a layer of weathered material. Generally, the growth of plants in a specific area contributes to soil development by holding it in one place. This allows the soil that builds up on a rock to take place in a layered way.

At the deepest level is bedrock, with different types of overlying regolith and finally a scattering of soil on top. This is shown simply in the series below.

bedrock Weathering and Topography Regolith And Slopes regolith Weathering and Topography Regolith And Slopes residual regolith Weathering and Topography Regolith And Slopes transported regolith Weathering and Topography Regolith And Slopes soil

Of course, depending on the region, this soil progression might not all be visible. But just knowing how it all stacks up can give you an idea of what is missing in any one rock formation. Table 15-2 describes what each layer in the soil development column contains.

Table 15-2 Bedrock and regolith layers are topped with a thin cover of soil.

Rock type



top layer of regolith (1–3 m) mixed with organic matter

Transported regolith

sediment transported and laid down by erosion (water current, waves, wind, ice) and mass wasting

Residual regolith

weathered rock from settled lower bedrock


layer of weathered rock


solid unchanged rock

Mass Wasting

In general, mass wasting doesn’t happen with a lot of flow-type movement. It moves material a fairly short distance, compared to the longer distances that sediments are carried by rivers.

Mass wasting describes the slow or sudden movement of rock downslope as a result of gravity.

Mass wasting has several factors that affect it. These include gravity, types of soil and rock, physical properties, types of motion, amount of water involved, and the speed of movement.

Gravity is the main influence on mass wasting. It is always pulling things down. When rocks are piled on a steep mountain slope, there is a high amount of friction that holds them to the slope. However, gravity is pulling the rock downward. The downslope pull of gravity that causes mass wasting is known as shearing stress . The steeper the slope, the greater the shearing stress. Figure 15-5 illustrates the difference of slope angle on shearing stress.

Weathering and Topography Mass Wasting

Fig. 15-5. When a mass is the same on two slopes, the greater the slope angle, the greater the shearing stress.

Shearing stress is linked to the mass of the rock being pulled downward by gravity and to the slope’s angle.

The counteracting force that works against shearing stress is friction or with a large body of rock, it is called shear strength . When the amount of shear stress is higher than the shear strength, something has got to give. A quick movement like an earthquake acts as a trigger. It provides just enough energy to overcome the last bit of friction and allow gravity to pull everything down.

There are several different mass wasting rock movements.

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