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.


extensional overstep . An overstep that has to be accommodated by extension, e.g., a left-step between left-lateral faults. This type of overstep occurs in cross section along normal faults, with the lower fault overstepping or projecting into the footwall of the upper fault (Peacock et al., 2000). Cf: contractional overstep.

tourelle . A little tower; applied to small flat-topped buttes of limestone in karst areas. Contrasted with pitons, which have pointed tops, and with coupoles, which have rounded tops. Etymol: French.

annular tracheid . In plants, commonly the first tracheid to mature, characterized by the deposition of secondary wall material in ringlike form. Cf: helical tracheid; protoxylem.

mollic epipedon . A surface horizon of mineral soil that is dark colored and relatively thick, contains at least 5.8 g kg-1 organic carbon, is not massive and hard or very hard when dry, has a base saturation of >50% when measured at pH 7, has <110 mg P kg-1 soluble in 0.05 M citric acid, and is dominantly saturated with divalent cation.

nonsorted polygon . A form of patterned ground "whose mesh is dominantly polygonal and has a nonsorted appearance due to the absence of a border of stones" (Washburn, 1956, p.831-832); never developed singly. Its borders commonly, but not invariably, are marked by wedge-shaped fissures narrowing downward; it typically results from infilling of these fissures. Diameter: a few centimeters to tens of meters. See also: fissure polygon; mud polygon; ice-wedge polygon; vegetation polygon; sand-wedge polygon; frost-crack polygon; desiccation polygon.

interstream (in'-ter-stream). Said of an area, divide, or topographic feature situated or lying between streams, such as an interfluve. See also: interfluvial.

alkali-feldspar granite . In the IUGS classification, a plutonic rock with Q between 20 and 60, and P/(A+P) less than 10.

vermicular . Microscopic igneous texture involving a wormy intergrowth of optically continuous quartz bodies with the outer fringes of K-feldspar crystals in contact with grains of plagioclase.

Westphal balance (West'-phal). In mineral analysis, a balance used to determine specific gravity of a heavy liquid. Syn: beam balance.

glaciated (gla'-ci-at''-ed). Said of a formerly glacier-covered land surface, esp. one that has been modified by the action of a glacier or an ice sheet, as a glaciated rock knob. Cf: glacier-covered.

Pages

Subscribe to GeoWord of the Day