Processes acting on slopes

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).   Although 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