Processes acting on slopes
- Mass Wasting
- creep, flow, fall, etc
- Action of water
- raindrop impact (aids in the
suspension of sediment)
- slope wash (Horton overland
flow, sheetflow)
- channelized flow (rills)
- subsurface flow (elluviation and
solute transport, sapping, and throughflow
mass movement and morphology
- creep leads to the development of
convex upward slope segments
- solifluction, slumps, and flows
commonly result in concave upward profiles at their heads and convex toes
of colluvium
- rock fall forms a talus (scree
slope)beneath a free face (cliff)
- slope of talus is governed by:
- angularity of sediment
- Rate of rock fall vs. rate of
weathering and erosion of talus
- Pediment surfaces that lack
significant debris beneath the free face develops because talus is
weathered and removed faster than it is produced
Effects of water
- surface flow (Horton overland
flow, or slopewash, and channel flow):
- aids the development of concave
upward profiles in valleys and
- convex upward profiles along
divides
- subsurface flow (downward
percolation, throughflow and groundwater flow)
- aids in elluviation (minor?mechanism
of slope decline)
- aids in the formation of
earthflows and solifluction
- may lead to surface channel
formation by piping (sapping).
Other factors influencing slope
morphology
1. Geology: Slope composition and structure controls the
detachability of slope material by a particular process
- Rock slopes: Slope is controlled
by rock strength and structure.
- rock strength: high strength
promotes the development of a free face low strength promotes flatter
slopes (fig. 1)
- structure: orientation, type and
abundance of planes of weakness (e.g. bedding planes & joints)
- fall faces typically occur where
- there is an active geologic
agent oversteepening the slope
- previously oversteepened slope
has not yet been deeply weathered or consumed by colluvium
- Change in base level exhumes
buried topography
- Soil slopes: Shape controlled
more by processes
- Erosion by water is influenced
by permeability and erodibility of slope materials and vegetative cover
- sharp divides typically develop
on poorly vegetated, impermeable and easily eroded slopes (fig. 2)
- Mass wasting is influenced by
sediment characteristics (cohesiveness, grain size, sorting and
angularity), degree of consolidation, and structure.
Figure 1. Slope developed on horizontal sedimentary rock, Grand Canyon , AZ. Variations in lithology strongly
influence the rock slopes that flank the canyon. Cliffs of limestone and
sandstone alternate with gentle slopes composed of shale.
Figure 2. Slopes developed in playa sediments (Furnace Creek Formation
near Zabrinskie Point, Death Valley ,
CA Death Valley receives less than 2 inch/year of
precipitation runoff is the dominant process shaping the slopes. Photo by
Paul Stone,
2. Climate
- controls intensity of chemical
vs. mechanical weathering
- controls vegetation and water
content
- In arid landscapes lacking
vegetation, such as those shown in figures 1 and 2, fluvial erosion
is quite effective.
Generalizations regarding the effects of climate:
- Humid
- Slope form is controlled by
processes acting on regolith: slopes tend to be transport limited
- Arid/semiarid
- Lack of vegetation increases
the efficiency of water and wind
- slope form is controlled by
bedrock strength and characteristics: slopes tend to be weathering
limited
3. Local activity: Rates of mass-wasting are promoted by:
- proximity to stream, shoreline,
etc.
- activity of man
- rate of uplift and incision;
relief