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arroyo - A steep-sided and flat-bottomed gully in an arid region that is occupied by a stream only intermittently, after rains.artesian well - A well that penetrates an impermeable layer of rock to reach an aquifer containing water under pressure.bankfull stage - The height of water in a stream that just corresponds to the level of the surrounding floodplain.bar - A unit of pressure equal to 10 to the sixth dynes/square centimeter; approximately one atmosphere.bar (stream) - An accumulation of sediment, usually sandy, which forms at the borders or in the channels of streams or offshore from a beach.barchan - A crescent-shaped sand dune moving across a clean surface with its convex face upwind and its concave slip face downwind.aeolian process - A geological modification, such as sand dunes or erosion, caused by wind activity. algal mat - A layered communal growth of algae observed in fossils and in present day tidal zones associated with carbonate sedimentation.alkali metal - A strongly basic metal like potassium or sodium.
A-horizon - The uppermost layer of a soil, containing organic material and leached minerals.
anthracite - The most highly metamorphosed form of coal, containing 92 to 98 percent of fixed carbon. anticline - A fold of rock layers with a convex shape pointing upwards. aphanitic - An igneous rock in which the matrix grains are too small to be seen with the unaided eye. aquifer - A permeable formation that stores and transmits groundwater in sufficient quantity to supply wells.
archipelago - A group of islands; an expanse of water with scattered islands.
banded iron formation - Rock consisting of alternating light and dark layers of iron-rich chert (hematite and limonite) formed approximately 3,800 million years ago.
bank - The land alongside or sloping down to a river or lake.
Thus water in the well rises above the surrounding water table.