Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated that have been deposited at the base of hillslopes by either rainwash, Sheet erosion, slow continuous downslope creep, or a variable combination of these processes. Colluvium is typically composed of a heterogeneous range of rock types and sediments ranging from silt to rock fragments of various sizes. This term is also used to specifically refer to sediment deposited at the base of a hillslope by unconcentrated surface runoff or sheet erosion.
Location
Colluviation refers to the buildup of colluvium at the base of a hillslope.
[Jackson, JA, J Mehl, and K. Neuendorf (2005) Glossary of Geology American Geological Institute, Alexandria, Virginia. 800 pp. ][Goodie, AS (2003) Colluvium in A. S. Goodie, ed., pp. 173, Encyclopedia of Geomorphology Volume 1, A–I. Routledge, New York, New York. 1200 pp.] Colluvium is typically loosely consolidated angular material located at the base of a steep hill slope or cliff. Colluvium accumulates as gently sloping aprons or fans, either at the base of or within gullies and hollows within hillslopes. These accumulations of colluvium can be several meters in thickness and often contain buried soils (
), crude bedding, and cut and fill sequences.
Importance
Thick accumulations of colluvium may preserve a rich record of long term
paleoclimatic change based on the paleosols and the remains of plants and animals, invertebrate and vertebrates that they often contain.
These fossils indicate previous geologic and environmental settings. Thick accumulations of colluvium often contain well-preserved and sometimes deeply buried archaeological deposits as excavated at the Cherokee Sewer Site, Cherokee County, Iowa, and the
Koster Site, Greene County, Illinois.
[Anderson, D, and HA Semken (1980) The Cherokee Excavations: Holocene Ecology and Human Adaptations in Northwestern Iowa. Academic Press, New York.][Angel JR (1990) Koster site archaeology I: stratigraphy and landscape evolution. Research Series. vol. 8. Center for American Archeology, Kampsville, Illinois.] Colluvium can also be rocks that have been transported downward from
and so can indicate past stages of cooler and/or wetter weather. Deposits of detrital colluvium can reveal the soil composition and signify processes of chemical weathering.
Compared to alluvium
The definitions of colluvium and
alluvium are interdependent and reliant on one another. Distinctions between the two are important in order to properly define the
Geomorphology processes that have occurred in a specific geological setting. Alluvium is sand, clay, or other similar detrital material deposited by running water.
The distinction between colluvium and alluvium relates to the involvement of running water. Alluvium specifically refers to the geomorphic processes involved with flowing water and so alluvium is generally fine-grained clay and silt material that has the capacity to be entrained in water currents and eventually deposited. For these same reasons, alluvium is also generally well sorted material while colluvium is not.
See also
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