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. . . 4G~... GLOSSARY OF TERMS Acre-foot - The quantity of water (43,560 cubic feet) that would cover 1 acre to a depth of 1 foot. Adit - An approach or entrance; an almost horizontal passageway into a tunnel. Afterbay - A channel, short stretch of stream, or small reservoir con- ducting water away from a water turbine or into which a hydropowerplant discharges. Aggregate - The sand or gravel added to cement in making concrete or mortar. Agronomic - Pertaining to agronomy, which is the science of soil management and the production of field crops. A Horizon - The stratum of soil consisting of one or more of the following layers: Ao horizon, partly decomposed or matted plant remains lying on top of the mineral soil (duff); Aoo horizon, the relatively fresh leaves and other plant debris, generally of the previous year lying on Ao horizon (litter); Al horizon, the surface mineral layer, relatively high in organic matter, usually dark in color; A2 horizon, below the Al horizon, in places the surface layer, usually lighter in color than the underlying horizon, in which. leaching of solutes and suspended materials occur; A3 horizon, transitional to the B horizon, more like A than B, sometimes absent. Alluvial fan - A gradually sloping mass of sand, clay, etc., that widens out like a fan from the place where a stream slows down little by little as it enters a plain. Anhydrite - Anhydrous calcium sulfate. It represents gypsum without its water of crystallization. Apishapa focus - A population distinguished by the construction of small villages with distinctive walls and rock based houses. The people of this culture subsisted on crops they grew and from hunting and gathering activities. Aquifer - An underground layer of porous rock, sand, etc., containing water. Autecology - The study of a species or an individual plant or animal in its reactions to environment and more especially to habitat.