Science / Geology Glossary

Bar-Finger Sand: An elongated lens of sand deposited during the growth of a distributary in a delta. The bar at the distributary mouth is the growing segment of the bar finger.

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 an . . . View Full Definition

Barrier Island: A long, narrow island parallel to the shore, composed of sand and built by wave action.

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 . . . View Full Definition

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 oldest rocks recognized in a given area, a complex of metamorphic and igneous rocks that underlies all the sedimentary formations. Usually Precambrian or Paleozoic in age.

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 great irregular mass of coarse-grained igneous rock with an exposed surface of more than 100 square kilometers, which has either intruded the country rock or been derived from it through m . . . View Full Definition

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.

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 i . . . View Full Definition

Bed-Load: The sediment that a stream moves along the bottom of its channel by rolling and bouncing.

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 botto . . . View Full Definition

Bedding Plane: A distinct surface of contact between two sedimentary rock layers.

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.

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