GeoWord of the Day

The GeoWord of the Day is a free service of the American Geosciences Institute. All of the terms and definitions are from the Glossary of Geology, 5th Edition Revised.

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pelagic deposit . Marine sediment that has formed by settling of material (either biogenic or terrigenous) through the deep water column. Generally consists of fine-grained terrigenous and sand-sized biogenic material. Cf: terrigenous deposit; hemipelagic deposit; deep-sea deposit.

strath [geomorph] . (a) A terrace remnant of a broad, flat, former bedrock valley floor that now stands above the present drainage as a result of downcutting following uplift, change in base level, or climate change; e.g., a river terrace along a valley wall, interrupted in its development during the mature stage of a former erosion cycle. Bucher (1932) preferred the term "strath terrace" for this feature. Bascom (1931) proposed that "strath" be replaced by "berm". (b) A broad, flat valley bottom formed in bedrock and resulting from degradation, "first by lateral stream cutting and later by whatever additional processes of degradation may be involved" (Bucher, 1932, p.131); a level valley floor representing a local base level. It is usually covered by a veneer of alluvium, and is wider and flatter than a glen. Etymol: Scottish/Gaelic, "valley". Syn: incipient peneplain.

argyrodite (ar-gyr'-o-dite). A steel-gray orthorhombic mineral: Ag8GeS6 . It is isomorphous with canfieldite.

aerosiderolite (aer-o-sid'-er-o-lite''). An obsolete syn. of stony-iron meteorite.

disaggregation (dis''-ag-gre-ga'-tion). Separation or reduction of an aggregate into its component parts; specif. mechanical weathering.

suite [ign] . (a) A set of apparently comagmatic igneous rocks. (b) A collection of rock specimens from a single area, generally representing related igneous rocks. (c) A collection of rock specimens of a single kind, e.g. granites from all over the world.

fossil geochronometry . The measurement of growth lines on fossil shells as a means of estimating the length of days and lunar months in geologic time. Cf: geochronometry; lichenometry.

nautiloid (nau'-ti-loid). Any cephalopod belonging to one of the subclasses Nautiloidea, Endoceratoidea, or Actinoceratoidea, characterized by a centrally located siphuncle and by a straight, curved, or coiled chambered external shell with less elaborate sutural flexures than in ammonoids. Nautiloids, known today only from the genus Nautilus, reached their peak in the Ordovician and Silurian. Range, Upper Cambrian to Holocene. adj. Pertaining to Nautiloidea.

flagellar field (fla-gel'-lar). The area around the flagella of a coccolithophore; e.g. covered flagellar field and naked flagellar field.

allingite (al'-ling-ite). A fossil resin (retinite) containing no succinic acid but considerable sulfur, found at Allinges in Haute-Savoie, France.

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