Colluvial
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Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
s that have been deposited at the base of hillslopes by either rainwash, sheetwash, 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 Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel ...
to
rock Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wales ...
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 Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
.


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 (
paleosol In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science. In geolo ...
s), 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 Cherokee County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 11,658. The county seat is Cherokee. The county was formed on January 15, 1851, from open territory. It was named after the Cherokee I ...
, and the
Koster Site The Koster Site is a prehistoric archaeological site located south of Eldred, Illinois. The site covers more than 3 acres and extends 30 feet down into the alluvial deposits of the Illinois River valley. Over the course of its excavation betw ...
,
Greene County, Illinois Greene County is a county located in the U.S. state of Illinois. According to the 2020 United States Census, it has a population of 11,843. Its county seat is Carrollton. A notable archaeological area, the Koster Site, has produced evidence o ...
.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
glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...
s 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 Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. ...
are interdependent and reliant on one another. Distinctions between the two are important in order to properly define the
geomorphic Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or ...
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

*
Colluvium-filled bedrock hollow There have been known various classifications of landslides. Broad definitions include forms of mass movement (geology), mass movement that narrower definitions exclude. For example, the ''McGraw-Hill Encyclopedia of Science and Technology'' distin ...
*
Diluvium Diluvium is an archaic term applied during the 1800s to widespread surficial deposits of sediments that could not be explained by the historic action of rivers and seas. Diluvium was initially argued to have been deposited by the action of extra ...
*
Eluvium In geology, eluvium or eluvial deposits are those geological deposits and soils that are derived by ''in situ'' weathering or weathering plus gravitational movement or accumulation. The process of removal of materials from geological or soil horiz ...
*
Erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
*
Illuvium Illuvium is material displaced across a soil profile, from one layer to another one, by the action of rainwater. The removal of material from a soil layer is called eluviation. The transport of the material may be either mechanical or chemical. Th ...
*
Scree Scree is a collection of broken rock fragments at the base of a cliff or other steep rocky mass that has accumulated through periodic rockfall. Landforms associated with these materials are often called talus deposits. Talus deposits typically ...


References

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External links

*Anonymous (2007
''Field Analysis:Is this a colluvial deposit''Soil Analysis Support System for Archaeology
Natural Environment Research Council, United Kingdom. Geomorphology Sedimentology