Geomorphology of Slopes
- Review Mass Wasting Lecture
- Dalrymple, J.B., et al, 1968, An hypothetical nine unit landsurface
- McCullagh, Patrick, 198, Slopes, in FitzGerald, B.P. (ed.),
Modern Concepts in Geomorphology
||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
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.
- 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
Processes acting on slopes
- Mass Wasting (see lecture outline for
- 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,
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
- 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
- convex upward profiles along divides
- subsurface flow (downward percolation, throughflow and groundwater
- 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).
1. Geology: Slope composition and structure
controls the detachability of slope material by a particular process
- Rock slopes: Slope is controlled by rock strength
- 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
- 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,
|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.
- 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:
- Slope form is controlled by processes acting on regolith:
slopes tend to be transport limited (fig.
- Lack of vegetation increases the efficiency of water and
- slope form is controlled by bedrock strength and characteristics:
slopes tend to be weathering limited (fig.
3. Local activity: Rates of mass-wasting
are promoted by:
- proximity to stream, shoreline, etc.
- activity of man
- rate of uplift and incision; relief
- Slope angle. See Table
- 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.
55° and greater
Table 1. Terminology used to describe slope angle.
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)
9-Unit Slope Model
9-Unit Slope Model of Dalrymple
et al, 1968
- interfluve: divide area characterized
by largely vertical subsurface water and soil movement
- seepage slope: gently dipping
portion dominated by downward percolation
- convex creep slope: upper convex
zone characterized by creep and terracette formation
- fall face: Cliff face characterized
by rapid detachment of material or bedrock (weathering
- transportational mid-slope: Active
region characterized by mass movement, terracette formation,
slope wash and subsurface water action
- colluvial foot slope: Depositional
region. Material is further transported down slope by creep,
slopewash and subsurface flow.
- Alluvial toe-slope: region
of alluvial deposition (e.g. levee deposits)
- Channel wall: removal by corrasion,
slumping, fall etc.
- Channel bed: Down stream transport
Figure 4. 9-unit slope model.
|Examples of slopes classified using Dalrymples
hypothetical nine-unit model are shown below in figures 5 and
that not all slope units need be present in a particular landscape.
Figure 5. Transport-limited slope in the California
Coastal Range underlain by poorly consolidated
sedimentary rocks and melange. These slopes have a thick active
Figure 6. Merrimack Butte Utah. This arid climate produces
weathering-limited slopes that typically lack unit 3.
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:
- parallel: straight
- diverging: ridge,
- 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.
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.
(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.
|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
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Chandler, R.J., 1977, The Application of soil mechanics methods
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*Dalrymple, J.B., et al, 1968, An hypothetical nine unit landsurface
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of Geological Sciences/Salem