Chapter 15: Mass Wasting

Terms in this set (31)

The shearing action produced by the moving soil causes weak, partly weathered, bedrock strata beneath the soil to bend in a downslope direction. As most trees grow, their trunks and rootballs develop in straight line continuity at right angles to a horizontal plane, not at right angles to the slope of the land surface. If a tree sprouts in an area of active soil creep, a bend develops between the base of the trunk and the rootball because the deeper roots are anchored in stationary regolith while the shallow roots are moving slowly downhill with the surface soil layer. Thus the tree seems to be growing laterally (sideways) out from the slope. For similar reasons, manmade features such as posts and utility poles, gradually rotate in the downslope direction from their original, vertical orientations; and linear structures, such as walls and fences built at right angles to the slope direction, may show bending and displacement from their original, straight line construction due to different soil creep rates at different locations along the slope. These relations are well illustrated in Figure 9.16.

Wetting and swelling of clay minerals and frost heaving (when water in open pores in the soil expands as it freezes) push overlying soil particles upward and raise the elevation of the soil surface very slightly. As the soil dries, or the ice melts, the soil shrinks, and the particles move a slight distance downslope, under the pull of gravity. Repetitions of these processes (Fig. 9.15) over long periods of time can produce slow, downhill movement of the entire soil layer. In response to gravity, moist, clay-rich soils deform internally and slowly move downhill over the deeper regolith or bedrock.