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.

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seismic pulse . The signal generated by an impulsive seismic energy source (explosive, thumper, air gun, sparker, etc.); may include correlated vibroseis sweep signal. "Wavelet" is sometimes used as a synonym.

accordant junction . The joining of two streams or two valleys whose surfaces are at the same level at the place of junction. See also: Playfair's law. Ant: discordant junction. Syn: concordant junction.

histic epipedon . A thin organic soil horizon that is saturated with water at some period of the year unless artificially drained and that is at or near the surface of a mineral soil. The histic epipedon has a maximum thickness depending on the kind of materials in the horizon and the lower limit of organic carbon is the upper limit for the mollic epipedon.

pellodite (pel'-lo-dite). A term used by Schuchert (1924, p.441) for a water-laid sandy varved clay, and by Pettijohn (1957, p.273) for the lithified equivalent of a varved clay. It is apparently an equivalent or variant spelling of pelodite.

eukrite (eu'-krite). Obsolete spelling of eucrite [meteorite].

exocyclic (ex-o-cy'-clic). Said of an irregular echinoid in which the periproct is located outside of the oculogenital ring. Ant: endocyclic.

homogeneous structure . An aggregation of more or less equidimensional, irregularly shaped crystallites lacking clear first-order structural arrangement except for possible accretion banding. This excludes fine complex crossed lamellar and crossed acicular structures, which may appear structurally featureless at the optical level but are easily differentiable by SEM. This also excludes diffuse crossed lamellar structure, which shows the general organization of a crossed lamellar structure but may appear similar to homogeneous structure in fractures under the SEM. The present definition of homogeneous structure carries no implication as to mineralogy or crystallographic orientation of the constituent crystallites. Crystallites in fine - and coarse-grained homogeneous structures are generally less than or greater than (respectively) 5 microns in diameter (Carter 1980a,b). Abbrev: HOM.

water system . (a) river system. (b) water supply.

flat [geog] . (a) A tract of low-lying, level wetland; e.g. a marsh or swamp in a river valley. (b) A term used in northern and central U.S. as a syn. of bottom, or low-lying land along a watercourse.

oscillation ripple . A symmetric ripple with a sharp, narrow, relatively straight crest between broadly rounded troughs, formed by the orbital or to-and-fro motion of water agitated by oscillatory waves. Cf: current ripple. Syn: oscillatory ripple; wave ripple.


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