South Africa-Eastern Cape-Gamtoos Valley-Alluvial Gravel01

Alluvium (from

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

''alluvius'', from ''alluere'' 'to wash against') is loose

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

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 ...

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...

, or gravel that has been deposited by running water in a stream bed, on a

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

, in an

alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...

or beach, or in similar settings. Alluvium is also sometimes called alluvial deposit. Alluvium is typically geologically young and is not consolidated into solid rock. Sediments deposited underwater, in seas, estuaries, lakes, or ponds, are not described as alluvium. Floodplain alluvium can be highly fertile, and supported some of the earliest human civilizations.

Definitions

The present consensus is that "alluvium" refers to loose sediments of all types deposited by running water in floodplains or in

alluvial fans An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...

or related landforms. However, the meaning of the term has varied considerably since it was first defined in the French dictionary of Antoine Furetière, posthumously published in 1690. Drawing upon concepts from

Roman law Roman law is the legal system of ancient Rome, including the legal developments spanning over a thousand years of jurisprudence, from the Twelve Tables (c. 449 BC), to the '' Corpus Juris Civilis'' (AD 529) ordered by Eastern Roman emperor Ju ...

, Furetière defined ''

alluvion Alluvion, is a Roman law method of acquisition of heritable property (land). The typical cause is sediment (alluvium) deposited by a river. This sediment, legally termed ''the accessory, accreses'' (i.e., merges with) a piece of land, the principal, ...

'' (the French term for alluvium) as new land formed by deposition of sediments along rivers and seas. By the 19th century, the term had come to mean recent sediments deposited by rivers on top of older ''

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 ...

'', which was similar in character but interpreted as sediments deposited by

Noah's flood The Genesis flood narrative (chapters 6–9 of the Book of Genesis) is the Hebrew version of the universal flood myth. It tells of God's decision to return the universe to its pre- creation state of watery chaos and remake it through the micro ...

. With the rejection by geologists of the concept of a primordial universal flood, the term "diluvium" fell into disfavor and was replaced with "older alluvium". At the same time, the term "alluvium" came to mean all sediment deposits due to running water on plains. The definition gradually expanded to include deposits in estuaries and coasts and young rock of both marine and fluvial origin. Alluvium and diluvium were grouped together as '' colluvium'' in the late 19th century. However, "colluvium" is now generally understood to mean sediments produced by gravity-driven transport on steep slopes, while the definition of "alluvium" has switched back to an emphasis on sediments deposited by river action. There continues to be disagreement over what other sediment deposits should be included under the term "alluvium."

Age

Most alluvium is Quaternary in age and is often referred to as "cover" because these sediments obscure the underlying bedrock. Most sedimentary material that fills a basin ("basin fill") that is not

lithified Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificatio ...

is typically lumped together as "alluvial". Alluvium of

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

age occurs, for example, in the valley of the

San Joaquin River The San Joaquin River (; es, Río San Joaquín) is the longest river of Central California. The long river starts in the high Sierra Nevada, and flows through the rich agricultural region of the northern San Joaquin Valley before reaching Suis ...

, California.

References

{{authority control Sedimentology

Definitions

Age

See also

References