Geomorphology of Slopes

Assignments

  1. Review Mass Wasting Lecture
  2. Dalrymple, J.B., et al, 1968, An hypothetical nine unit landsurface model
  3. McCullagh, Patrick, 198, Slopes, in FitzGerald, B.P. (ed.), Modern Concepts in Geomorphology
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Terms: primary and secondary slope, elluviation, piping, creep, pediment, solifluction, talus, horton overland flow, angle of repose, weathering-limited slope, transport-limited slope, diverging, converging and parallel, slope decline, slope replacement, parallel retreat

Slope formation

Of all landforms slopes are clearly the most common and often the most overlooked. Understanding slope processes is of particular interest to land use planners, and because slopes often reflect changes in lithology they are of particular interest to bedrock mappers.

Slopes can be genetically categorizes into primary slopes, formed by processes that tend to promote relief, and secondary slopes, formed by processes tending to decrease relief. Secondary slopes evolve from the erosion and modification of primary slopes. The distinction is not always clear because primary and secondary processes do not operate independently. However, its important to understand to what degree a slope is the result of primary and secondary processes. Many slopes are paleoslopes formed under a different climatic regime.   This is especially true in New England where slopes occupy the flanks of relict glacial features, such as drumlins, moraines, glacial troughs, and meltwater valleys. A slope's shape is governed by its internal structure and external processes, such as slope wash, creep and other mechanisms of sediment transport.  Material deposited while in transit down the slope is termed colluvium--an unsorted mixture of  rock and sediment derived from the slope face.

Origins of primary slopes

  • Tectonic (fault scarps)
  • Depositional (volcanoes, glacial moraines, drumlins(?), dunes, alluvial fans, delta foreset, etc.)
  • Erosional (glacial and riverine valleys, etc.)
  • Human activity (blasted rock slopes, hydraulic mining, tailings piles, etc)

Processes acting on slopes

  • Mass Wasting (see lecture outline for review)
    • 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

How process affects slope morphology

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.
    Grand Canyon DV badlands
    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. Click to enlarge. 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, USGS, URL: http://wrgis.wr.usgs.gov/docs/parks/deva/galzab.html.

 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 (fig. 5)
    • 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 (fig. 6)

3. Local activity: Rates of mass-wasting are promoted by:

  • proximity to stream, shoreline, etc.
  • activity of man
  • rate of uplift and incision; relief 

Terminology used to describe slopes

  • Slope angle. See Table 1.
  • Transport limited: Rate of transport is lower than regolith formation: Weathering and soil formation rates are faster than rates of removal. Slope form is greatly controlled by creep, solifluction and similar mass movement processes, and slope wash.
  • Weathering limited: Rates of regolith formation is slower than transport: Erosional processes, such as mass-wasting, slope wash, fluvial activity, etc., are faster than weathering (soil-forming) processes. Slopes are steep and have little to no soil (sensu stricto). Structure and lithology control the shape of the slope.

0°-0°

plain

0°-30'

slightly sloping

2°-5°

gently inclined

5°-15°

strongly inclined

15°-25°

steep

25°-35°

very steep

35°-55°

precipitous

55° and greater

vertical

 

Table 1. Terminology used to describe slope angle.

Profile Shapes

Slope profiles can have several manifestations depending on the factors discuss above. Below are two popular examples of slope models.

Four Unit Slope Model

Four unit slope model (Wood, 1942)

The four unit slope is best developed on a high initial (primary) slope composed of strong rock and the absence of local undercutting. As the steep fall face retreats the base is covered by a straight talus slope.

 

Figure 3. Four Unit slope model modified from Wood (1942)

Wood slope model

9-Unit Slope Model

9-Unit Slope Model of Dalrymple et al, 1968

  1. interfluve: divide area characterized by largely vertical subsurface water and soil movement
  2. seepage slope: gently dipping portion dominated by downward percolation
  3. convex creep slope: upper convex zone characterized by creep and terracette formation
  4. fall face: Cliff face characterized by rapid detachment of material or bedrock (weathering limited) exposure.
  5. transportational mid-slope: Active region characterized by mass movement, terracette formation, slope wash and subsurface water action
  6. colluvial foot slope: Depositional region. Material is further transported down slope by creep, slopewash and subsurface flow.
  7. Alluvial toe-slope: region of alluvial deposition (e.g. levee deposits)
  8. Channel wall: removal by corrasion, slumping, fall etc.
  9. Channel bed: Down stream transport of material

 

slope model Dalrymple

Figure 4. 9-unit slope model.

Examples of slopes classified using Dalrymples hypothetical nine-unit model are shown below in figures 5 and 6.  Note that not all slope units need be present in a particular landscape.

Coastal Plain slopes

Figure 5. Transport-limited slope in the California Coastal Range underlain by poorly consolidated sedimentary rocks and melange. These slopes have a thick active regolith.

Merrimack Butte

Figure 6. Merrimack Butte Utah. This arid climate produces weathering-limited slopes that typically lack unit 3.

Terminology related to the plan view of the hillslope:

Slopes can be divided laterally into sections based on surface flow. Imagine drawing flowlines from the highest contour to the base of the slope. Diverging flowlines define diverging sections, parallel flowlines parallel sections, and converging flowlines converging sections. These are further outlined below:

  1. parallel: straight section
  2. diverging: ridge, interfluve, etc.
  3. converging: valley, hillside depression or embayment 

When defining a model for a slope profile it's best to do so along a straight section.

Figure 7. Plan view of a slope.

Styles of slope retreat

Early geomorphologist, such as W.M. Davis, L. King and W. Penck each devised different conceptual models for slope retreat for which they strongly advocated. However, their models are not necessarily contradictory, but reflect the different climates, tectonic settings, lithologies and processes affecting the slopes where they studied.  William Morris Davis studied slopes in New England where the climate is humid and temperate and the tectonic setting is stable.  Lester King worked in the-semi arid climate of South Africa and Walther Penck in the tectonically active Andes.

Slope decline (W.M. Davis - New England)

  • upper slope weathers and erodes at at faster rate so there is progressive decline of slope angle occurs.  Hillslopes have a thick mantle of regolith (e.g. fig. 5).

 Parallel retreat (W. Penck)

  • slope angle and lengths remain uniform as the slope retreats parallel to itself. (See figures 6 and 9)   Hillslopes are esentially free of sediment. Parallel retreat occurs where underlying strata are protected by a resistant cap rock, such as a layer of sandstone, limestone, or lava.  Failure of the caprock occurs only when erosion has removed the weaker rock supporting it. Parallel retreat is responsible for the classic stepped topography of the Colorado Plateau, and the formation of flat-topped buttes, mesas, and pinnacles.

 Slope replacement (King)

  • steep slope is progressively replaced by shallower lower slope deposits. The upper slope retreats parallel to itself while replacement of the lower slope froms a pediment. 
Figure 8. Styles of slope retreat.
monument Valley Bryce slopes
Figure 9.  Monument Valley, Utah exhibits some of the best examples of slope retreat.  Slopes are weathering limited and reflect the angle of repose of the various lithologies. Click image to enlarge.  Figure 10. Bryce Canyon Utah exhibits slope retreat where weak rocks are protected by resistant caprock.  Once removed the slope undergoe rapid decline.  Click image to enlarge

 

Bibliography


Bloom, Arthur. 1998, Geomorphology, A systematic analysis of Late Cenozoic landforms, (3rd edition): Prentice Hall, Upper Saddle River, N.J., 482 p.

Chandler, R.J., 1977, The Application of soil mechanics methods to the study of slopes: in Hails, J.R., (ed.), Applied Geomorphology, Elsevier, Amsterdam, p. 157-181.

Chorley, R.J., Schumm, S.A., Sugden, D.E., 1984, Geomorphology: Methuen and Co. Ltd., London, 605 p.

*Clark, M.J., and Small, R.J., 1982, Slopes and weathering: Cambridge University Press, Cambridge, England, 112 p.

*Dalrymple, J.B., et al, 1968, An hypothetical nine unit landsurface model

Easterbrook, Donald J., 1993, Surface Processes and Landforms: Macmillan Pub. Co., 520 p.Howard, A.D., 1967, Drainage analysis in geologic interpretation: a summation: The Amer. Assoc. of Petr. Geol., v. 51, n. 11, p. 2246-2259.

Mayer, Larry, 1990, Introduction to Quantitative Geomorphology: Prentice Hall, Englewood Cliffs, NJ, 380 p.

McCullagh, Patrick, 198, Slopes, in FitzGerald, B.P. (ed.), Modern Concepts in GeomorphologyRitter, D.F., Kochel, C.R., and Miller, J.R., Process Geomorphology (3rd Edition): Wm.C. Brown Publishers, Dubuque, IA, 544 p.

Crozier, Michael, 2004, Slope Evolution, in Goudie, A.S., ed., Encyclopedia of Geomorphology, Volumn 2, Routledge, New York, NY, pp. 963-970.

Summerfield, M.A., 1991, Global Geomorphology. John Wiley and Sons, New York, NY, 536 p. 


Lindley Hanson/Department of Geological Sciences/Salem State College/Geomorphology/