Colluvial Deposit
Susan W. S. Millar*
Department of Geography, Syracuse University, Syracuse, NY, USA
Synonyms
Colluvial depositional system; Colluvial mantle; Colluvial soil; Colluvium; Debris slope; Hillslope
(or hillside) deposits; Scree (UK); Slope mantle; Slope-waste deposits; Talus (US)
Definition
A sedimentary deposit composed of surface mantle that has accumulated toward the base of a slope
as a result of transport by gravity and non-channelized flow.
Description
The International Geomorphological Association recognizes colluvium as a hillslope deposit
resulting from two general nonexclusive processes (Goudie 2004). Rainwash, sheetwash, or creep
can generate sediment accumulations at the base of gentle slopes; or non-channelized flow can
initiate sheet erosion and toe-slope sediment accumulation. The term “colluvium” is frequently
applied broadly to include mass wasting deposits in a variety of topographic and climatic settings.
For example, Blikra and Nemec (1998) describe colluvium as any “clastic slope-waste material,
typically coarse grained and immature, deposited in the lower part and foot zone of a mountain slope
or other topographic escarpment, and brought there chiefly by sediment-gravity processes.” Lang
and Honscheidt (1999) describe colluvium as “slope wash and tillage sediments, resulting from soil
erosion. . ..” The composition of a colluvial deposit can therefore be coarse-grained, eroded bedrock,
with an open-work structure and several meters thick (Blikra and Nemec 1998), to fine-grained soil,
ranging from a few millimeters to meters in thickness (e.g., Lang and Hönscheidt 1999). Some
deposits may exhibit distinct macro- and micro-fabric development, bedding structures, and evidence
of distinct periods of accumulation (e.g., Bertran et al. 1997; Pederson et al. 2000; Millar and
Nelson 2001). Morphological expression can include gentle, curvilinear slope profiles, steep
rectilinear slopes, or incipient gullying and lobe forms, dependent on rate of movement, sediment
type, water content, post-depositional modification, and time since deposition (see Coe et al. 2008
and articles therein). The scope of formative mechanisms results in the presence of colluvial deposits
in all climatic environments from tropical (Hiruma et al. 2012) to high latitude (Blikra and Nemec
1998).
*Email: swmillar@maxwell.syr.edu
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Subtypes
Colluvial deposits have no diagnostic morphometric characteristics, but can illustrate certain
features and forms indicative of the dominant type of mass wasting processes responsible for
their movement. (See descriptions under ▶ Mass Wasting Processes, ▶ Rock Glacier, ▶ Protalus
Features, ▶ Mass wasting, ▶ Denivation.) Several compound terms are used to describe colluvial
slope deposits, primarily based on the dimension and extent of the deposit. Slope angle is dependent
on a number of factors including sorting, grain-size, and the presence of interstitial ice (e.g., Graham
1984).
(1) A talus cone is a cone-shaped landform at the base of a cliff or escarpment that is sourced in
a relatively small declivity or ravine (NSSH 2008; Figs. 1 and 2). The ravine is also called rock
fall funnel (Rapp 1960), which is “a half-funnel-shaped excavation in a rock wall, wide above,
Fig. 1 Talus cone formed at the base of a bedrock gully, Lofoten, Norway (Photo by Susan W. S. Millar)
Fig. 2 (a) Talus cones on Mars, Mojave Crater (Williams and Malin 2008). CTX P01_001481_1874_XI_07N033W.
Scale bar 1 km. (NASA/JPL/MSSS). (b) The walls of one of the depressions of the eastern Noctis Labyrinthus.
Colluvium probably consists primarily of rocky debris. The dark rayed crater is on the upland plain above the
depression. MOC2-1411 near: 10
S, 265
E. Scale bar ca. 500 m. South is up (NASA/JPL/Malin Space Science
Systems)
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narrow below, through which debris moves by rock-falls.” The narrow mouth of the funnel is
called gorge (Rapp 1960).
(2) Talus fans (colluvial fan) are common on mountain slopes (slope fan), often accumulating at the
base (Fig. 3). Typically they are composed of immature gravel; the coarsest debris is situated in
the lower (toe) zone. They are produced by rock fall, debris flow, or snow flow, with minor
contributions from waterflows from gullies. Their gradient is 35–45
near the apex, 15–20
near
the toe. On Earth they are typically <0.5 km large (Blikra and Nemec 1998) (cf. ▶ Alluvial Fan).
(3) A talus slope (debris slope, scree slope) is a hillslope mantled by talus (NSSH 2008; Figs. 4 and
5) consisting of an accumulation of “[r]ock fragments of any size or shape (usually coarse and
angular) derived from and lying at the base of a cliff or very steep, rocky slope” (Bates and
Jackson 1980, p. 638). If it is stratified, it is called a bedded colluvium. Minor albedo variations
of added rockfall material can cause slight streaking of talus slopes on Earth. On Mars,
significant albedo variations are seen in talus ▶ Slope Streaks (Williams 1991).
(4) A debris apron (talus apron, colluvial apron) is a thick wedge-shaped deposit of colluvium
and/or slope alluvium (NSSH 2008). It is composed of coalesced talus fans that may surround
a hill, ▶ Plateau, Mesa, Butte; or comprise a (▶ Blockfield, ▶ Protalus Features, ▶ Rock
Glacier). Debris apron is a term predominantly applied today in the planetary geomorphology
Fig. 3 Talus fan produced by rock-slide, Banff, Canada (Photo by Susan W. S. Millar)
Fig. 4 Talus slope, Maroon Bells, Colorado, USA (Photo by Susan W. S. Millar)
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literature (Fig. 6); similar terrestrial features are usually termed talus or colluvial fans, although
“apron” has been used to describe colluvial slope morphology (e.g., Coe et al. 2008). Stewart
and Crown (1997) have described subtypes of debris aprons in the eastern Hellas region of Mars,
as lobate or arcuate, morphologies dependent upon their landform associations.
Fig. 5 Cliff-forming debris (talus) slopes at the southwestern margin of Tvashtar plateau on Io (Moore et al. 2004,
Fig. 3). Galileo 05273457.01 at 61
N, 120
W (NASA/JPL)
Fig. 6 Mesa on Mars with debris apron in West Candor Chasma. Scale bar 500 m. (MOC r2100326) (NASA/JPL/
MSSS)
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Interpretation
Macro-fabrics (Mills 1991; Millar 2005) and micro-fabrics (Bertran et al. 1997) have some utility in
differentiating specific types of mass movement processes responsible for the emplacement of
colluvial deposits, as well as identifying the influence of freezing and thawing on sediments
(Millar and Nelson 2001). Grain-size characteristics and the degree of soil development are useful
indicators of depositional processes, time since deposition and the stability of the surface (e.g.,
Schaetzl and Anderson 2005). The presence of surface features such as gullies, levees, and lobes
may indicate the mechanism of downslope movement or the post-depositional modification of
a previous colluvial surface. Micromorphological features observed in soils, such as silt caps and
deformed sediment, can indicate the operation of ice, and freezing and thawing (e.g., Bockheim and
Tarnocai 1998).
Formation
Regolith or in situ weathered bedrock can be mobilized by a variety of gravity transfer processes
commonly referred to as ▶ mass wasting processes, and can include ▶ flows, ▶ slides, and ▶ creep.
Associated with gravity transfer due to stress-strain relations and pore-water pressures in the
colluvial mass implicit in flow, slide and creep, are a variety of possible triggers for transport
initiation. These can include seismic activity (Keefer 1984), land-use change and agricultural
activities (e.g., Lang and Hönscheidt 1999; Leopold and Völkel 2007), climatic change (Blikra
and Nemec 1998; Pederson et al. 2000; Geertsema et al. 2006), and avalanching (e.g., Blikra and
Nemec 1998; Decaulne and Saemundsson 2006).
Age
Terrestrial colluvial deposits can range from present-day to Cenozoic and older, often interbedded
and with pedogenic development.
Degradation
Buried colluvial horizons retain little significant morphological expression, but may have remnant
fabric signatures (e.g., Mills 1991; Pederson et al. 2000; Millar and Nelson 2001) and/or textural
composition (e.g., Blikra and Nemec 1998; Nemec and Kazanci 1999) that can help identify specific
emplacement mechanisms.
Surface/Structural Units
In some cases discrete morphological features may be present that are indicative of specific and/or
localized mass-wasting processes. These may include rockfall boulders, protalus ramparts, slump
blocks, ▶ Periglacial Landforms, ▶ rock glaciers, etc.
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Composition
Colluvial deposits are typically heterogeneous, open-work sedimentary masses, but can vary in
grain-size characteristics from fine silts and clays to large, freshly weathered bedrock (Fig. 7). The
degree of sorting can range from high, as in surface wash deposits, to very poor sorting, as in rock
glaciers or debris flows.
Distribution and Studied Locations
Colluvial deposits occur on any hillslope, thus are evident in all climatic environments. Terrestrial
examples are ubiquitous. Some examples include:
(1) Unglaciated Eastern United States (Cremeens et al. 2003; Millar and Nelson 2001; Todd and
Kite 2010; Fig. 8)
(2) Western Norway (Blikra and Nemec 1998; Blikra and Selvik 1998)
(3) Central Europe (Lang and Hönscheidt 1999; Lang 2003; Leopold and Völkel 2007)
Significance
Colluvial deposits are important stores of sediments. The presence of debris aprons and debris flow
deposits on Mars are suggestive of ice-rich surficial material that has thawed and flowed during
periods of warmer climatic conditions (Lanza et al. 2010).
Fig. 7 Simulated astronauts’ work on a colluvial deposit. Mars Desert Research Station, Utah, USA (Photo by
H. Hargitai)
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Astrobiological Significance
The presence of flowing material suggests that liquid water may have existed thus providing
a foundation for life-forms to evolve.
Planetary Analogs
Colluvial deposits on Mars have mean inclinations as low as 20
that suggests formation by
gravitational creep driven by repeated deposition and sublimation of ground ice. Impact-triggered
mass wasting processes or thermal expansion may have only minor contribution to debris slope
sediment transport (Perron et al. 2003).
Distribution
On Mars, extensive colluvial deposits occur at the base of bedrock escarpments in impact craters,
outflow channels, chasmae (canyons), fossae (graben), etc., over much of the Martian surface but are
most abundant at intermediate southern latitudes (Perron et al. 2003). The eastern Hellas region has
prominent debris aprons with superimposed lobate forms and gullies that Stewart and Crown (1997)
interpret as degradational forms associated with slope instability on upland massifs and on inner
crater rims. Malin and Edgett (2000) describe areas of seepage on the Martian surface that exhibit
a source area, transportation slopes and channels, and a debris apron at the toe (▶ Gully). HiRISE
images enabled a morphometric analysis to establish area-source relations and morphologies
commonly found in terrestrial analogs of shallow surface debris flows.
Age
Martian colluvial deposits are associated with the Amazonian starting approximately 1.8 billion
years ago and a general cooling of the planet.
Fig. 8 Open-work, blocky slope mantle of periglacial colluvium (▶ Blockfield), W. Virginia, USA (Photo by Susan
W. S. Millar)
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History of Investigation
Although not specified as colluvial, the effects of creep on the slope waste mantle were investigated
and debated by Geikie (1877) and Thomson (1877) in the late nineteenth century. Geomorphologists
such as W.M. Davis (1899) and G.K. Gilbert (1909) understood the importance of wash and creep
and graded waste slopes in their discussions of hillslope evolution. Emerson (1941) used the term
“colluvial soils” to describe loam soils that accumulate at the base of a slope, leaving a stonier lag
deposit mid-slope, as a result of creep and surface runoff. Early application of the term to relict
periglacial deposits by Denny (1951) replaced the more commonly used waste deposits or “warp” of
Bryan (1946). By the 1970s colluvium was entrenched in the literature to encapsulate both fine- and
coarse-grained slope materials that have undergone some downslope movement. This is in contrast
to ▶ regolith, which assumes that the waste material is in situ weathered bedrock. Non-terrestrial
slope deposits are rarely designated as “colluvial,” the preference being to define them more
specifically on the basis of the possible mechanisms that were responsible for their transport and
deposition.
Origin of Term
The term derives from Latin “colluvies,” meaning “offscourings” (OED 2012). It is used to
distinguish between slope, non-channelized deposits, and alluvium. Original usage was presented
by Johnson (1880): “Colluvial soils . . . while consisting in part of drift or alluvium, also contain
sharp angular fragments of the rock from which they mainly originated.”
See Also
▶ Blockfield
▶ Gully
▶ Lobate Debris Apron
▶ Sand Ramp
▶ Slope
▶ Slope Streak
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