Hillslope Morphology

In humid temperate climates where hills are mantled in soil, hillslope profiles typically have an upper slope that is convex up and a lower slope that is concave up. On the hilltop, the slope is gentle. Precipitation easily infiltrates the soil and runoff is relatively small. The upper slope has its convex shape because mass wasting (mostly creep) is the principle transport mechanism for the soil. As the slope becomes steeper down the hillside, infiltration is reduced and runoff increases. Runoff becomes the principle means of soil transport. In particular, slope wash collects into systems of small rills. As rills merge into larger rills down the slope these larger rills carry more water for only slightly more friction. They therefore can flow more efficiently. They don't need as steep a slope to carry their sedimentary load. Just as in networks of streams to be discussed later, networks of rills on a wash slope develop a characteristic concave up profile.

There may also be a middle straight segment or scarp, perhaps initially caused by faulting or stream undercutting. The initial slope of a scarp produced by faulting or undercutting is normally steeper than the angle of repose if the slope is unconsolidated material. Oversteepened scarps will rapidly (months to years) become less steep by mass wasting from the upper slope thereby depositing at the base of the slope. Eventually the slope will come to some equilibrium profile depending on the relative contributions of weathering, creep, and runoff (erosion).

Hillslope Evolution

A longstanding controversey: Over long periods of erosion, how do the shapes of hills change?

The classic view, held by the majority, is that they gradually become lower and broader with decreasing slopes.

A minority but persistent view is that hillslopes establish an equilibrium shape (slope) dependent on the nature of the underlying geology, the dominant processes, and the climate. The slopes then migrate backwards over time while maintaining the same shape.

Noses and Hollows

The fronts of highland areas are not perfectly straight or planar. There are convexities and concavities, or noses and hollows. On a nose that sticks out from a highland area, water flowing down the slope is spread. In Hollows or embayments in a highland front, water is concentrated as it flows down the slope. The result is that there is more water available in the hollows to promote weathering, mass wasting, and runoff (erosion). The slopes of hollows are locations for springs, gullies, and creeks. the slopes of noses are dry. Over time, the hollows recede much faster than the noses, gradually disecting highland areas.