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Sell creep

Steep rock walls of gorges and high mountains shed countless rock particles under the attack of physical weathering processes. These accumulate in distinctive landforms, the talus cone. A talus slope, or scree slope, has a remarkably constant slope angle of about 34o or 35 o. So long as the talus slope is freshly formed and contains little very fine material mixed in with the coarse, the angle is constant within one or two degrees of variation, regardless of the rock type or the shape of the blocks.

Most cliffs are notched by narrow ravines, which funnel the fragments into individual tracks, so as to produce cone like talus bodies arranged side by side along the cliff. Where a large range of sizes of particles is supplied, the larger pieces, by reason of their greater momentum and ease of rolling, travel to the base of the cone, whereas the tiny grains lodge in the apex. Most fresh talus slopes are unstable, so that the disturbance created by walking across the slope, or dropping of a large rock fragment from the cliff above, will easily set off a sliding of the surface layer of particles.

Sell creep. On almost any moderately steep, soil-covered slope, some evidence may be found of extremely slow downslope movement of soil and overburden, a process called soil creep. In some layered rocks such as shales or slates, edges of the strata seem to bend in the downhill direction.

This is not true plastic bending , but is the result of slight movement on many small joint cracks. Fence posts and telephone poles lean downslope and even shift measurably out of line.

Heating and cooling of the soil, growth of frost needles, alternate drying and wetting of soil trampling, and burrowing by animals, and shaking by earthquakes all produce some disturbance of the soil and mantle. Because gravity exerts a downhill pull on every such rearrangement that takes place, the particles are urged progressively downslope.

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