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Geological Terms Beginning With "B"



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Backwash:

The seaward rush of water down a beach that occurs with a receding wave.

Banded Iron Ore:

A rock that consists of alternating layers of chert and iron oxide mineral (usually hematite) with the iron oxide in high enough concentration to be of economic value.

Bankfull Stage:

A height of water in a stream that completely fills the natural channel. If the water rises any higher a flood will occur.

Bank Storage:

Water that seeps into the ground along the banks of a stream during a time of high flow. This loss of water into the ground slightly reduces the height that the stream will attain and then slowly seeps into the stream as the high water level subsides - hence the term "bank storage".

Bar:

An underwater ridge, usually of sand and/or gravel, that forms from the deposition and reworking of sediments by currents and/or waves. Bars occur in rivers, river mouths and in offshore waters.

Barchan:

A sand dune that is crescent-shaped in map view. Barchan dunes form in areas of limited sand supply. They move across the desert floor with their gently sloping convex sides facing upwind and their steeply sloping concave sides facing downwind.

Barrier Island:

A long, narrow island that parallels a shoreline.

Basalt:

A dark-colored fine-grained extrusive igneous rock composed largely of plagioclase feldspar and pyroxene. Similar in composition to gabbro. Basalt is thought to be one of the main components of oceanic crust. Picture of Basalt.

Base Flow:

Water that seeps into a stream through a permeable rock or sediment unit that outcrops in the bottom or banks of the stream.

Base Level:

The lower limit of erosion by a stream. Sea level is the ultimate base level. However, lakes can serve as a temporary base level in upstream areas.

Basement:

The igneous and metamorphic rocks that exist below the oldest sedimentary cover. In some areas such as shields the basement rocks may be exposed at the surface.

Basic Rock:

An igneous rock that has a relatively low silica content. Examples are gabbro and basalt. Also see entries for acidic, intermediate and ultrabasic rocks.

Basin:

In tectonics, a circular, syncline-like depression of strata. In sedimentology, the site of accumulation of a large thickness of sediments.

Batholith:

A very large intrusive igneous rock mass that has been exposed by erosion and with an exposed surface area of over 100 square kilometers. A batholith has no known floor.

Bathymetry:

The measurement of ocean depths and the preparation of topographic maps of the ocean floor.

Bauxite:

The principal ore of aluminum. A mixture of aluminum oxides and hydroxides that forms from intense chemical weathering of a soil in tropical environments.

Bedding:

The characteristic structure of sedimentary rocks in which layers of different composition, grain size or arrangement are stacked one on top of another in a sequence with oldest at the bottom and youngest at the top.

Bedding Plane:

A distinct surface of contact between two sedimentary rock layers.

Bed Load:

The larger, heavier particles that are being transported by a stream. Instead of being dissolved or suspended, these are being rolled or bounced along, spending at least part of their time in contact with the stream bottom. See also: load, suspended load, dissolved load.

Bedrock:

Solid rock present beneath any soil, sediment or other surface cover. In some locations it may be exposed at Earth's surface.

Beta-Particle:

An electron emitted with high energy and velocity from the nucleus of an atom during radioactive decay.

B-horizon:

A layer in the soil, below the A-horizon, where materials leached from above accumulate. Typically enriched in clay and oxides.

Biochemical Rocks:

A sedimentary rock that forms from the chemical activities of organisms. Organic (reef and fossiliferous) limestones and bacterial iron ores are examples.

Bioturbated:

An adjective used in reference to a sediment or sedimentary rock. Bioturbated sediments have been disturbed by animals (such as burrowing worms or shell fish) or plant roots. These have penetrated the sediment and disturbed any or all original sedimentary laminations and structures. Bioturbated rocks were disturbed in this way while still in the soft sediment phase of their formation.

Bituminous Coal:

A rank of coal that falls between anthracite and semi-bituminous. The most abundant rank of coal. Frequently referred to by the layman as "soft coal".

Block Fault Mountain:

A linear mountain that is bounded on both sides by normal faults.

Blowout:

A shallow, round or trough-shaped depression in sand or dry soil that is formed by wind erosion. The material removed by the wind may also be referred to as "blowout".

Breccia:

A clastic sedimentary rock that is composed of large (over two millimeter diameter) angular fragments. The spaces between the large fragments can be filled with a matrix of smaller particles or a mineral cement which binds the rock together. Picture of Breccia..

Butte:

A conspicuous hill with steep sides and a flat top. The top is usually a cap-rock of resistant material. This structure is frequently an erosional remnant in an area of flat-lying sedimentary rocks.

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