Page 191 - Fundamentals of Geomorphology

174 PROCESS AND FORM

In transport-limited processes, the rate of soil and rock Stage 1: Fault controlled
transport limits the delivery of sediment to streams. 5°
In other words, the supply of sediment exceeds the capac-
Fault scarp
ity to remove it, and transport processes and their spatial 60°
variation dictate hillslope form. Soil creep, gelifluc-
tion, through-wash, rainflow, rainsplash, and rillwash are
5°
all hillslope processes limited by transporting capacity.
On supply-limited (or weathering-limited) hillslopes, Stage 2: Gravity and debris controlled
the rate of sediment production by weathering and ero- Free face (slope
sional detachment (through overland flow and mass replacement begins)
movement) limits the delivery of sediment to streams. Debris slope
In other words, weathering and erosional processes dic-
tate hillslope form. Leaching of solutes, landsliding, Wash slope
debris avalanches, debris flows, and rockfall are all Stage 3: Debris controlled
hillslope processes limited by sediment supply.
Thedistinctionbetweentransport-limitedandsupply-
limited process is often blurred. Nonetheless, it is an 35°
important distinction because it affects the long-term
evolution of hillslopes. Hillslopes and landscapes dom-
inated by transport-limited removal typically carry a
thick soil layer supporting vegetation, and slope gradi- Stage 4: Debris and wash controlled
ents tend to reduce with time. Hillslopes and landscapes
dominated by supply-limited removal often bear thin 35°
soils with little vegetation cover, and characteristically
steep slopes tend to retreat maintaining a sharp gradi-
ent. Mathematical models of hillslope evolution support
these findings, suggesting that the wearing back or wear-
Stage 5: Wash controlled
ing down of the mid-slope depends upon the processes
in operation. As a generalization, surface wash processes Slope
decline
lead to a back-wearing of slopes, whereas creep pro- 8–25° operative
cesses lead to a down-wearing of slopes (e.g. Nash 1981).
Nonetheless, the pattern of slope retreat and slope decline 5–10°
iscruciallydependentonconditionsattheslopebase,and
especially on the transport capacity of streams.
A study of young fault scarps formed in alluvium Figure 7.5 Proposed sequence of change on a fault scarp
in north-central Nevada, USA, showed that hillslope developed in alluvium, Nevada, USA. The changes are
processes change as the scarps age (Wallace 1977) incremental, the dashed line shown at each stage
(Figure 7.5). The original fault scarps stand at 50 ◦ representing the hillslope profile at the previous stage.
to 70 . At this stage, mass wasting is the dominant Source: Adapted from Wallace (1977)
◦
process, a free face develops at the scarp top, which
retreats through debris fall, and material accumulates Hillslope development
lower down. Later, the scarp slope adopts the angle of
repose of the debris, which is about 35 . At this gentler Slope processes fashion hillsides over hundreds of thou-
◦
gradient, wash erosion dominates hillslope development sands to millions of years. It is therefore impossible
and further slope decline occurs. to study hillslope evolution directly. Location–time