accordant (ac-cord'-ant). Said of topographic features that have the same or nearly the same elevation; e.g. an accordant valley whose stream enters the main stream at the same elevation as that of the main stream. Ant: discordant [geomorph].

Alluvial (Al-lu'-vi-al). A name, now obsolete, applied by Jameson (1808) from the teachings of A.G. Werner in the 1790's to the group or series of rocks consisting of unconsolidated or poorly consolidated gravels, sands, clays, and peat that were believed to have been formed after the withdrawal of the ocean from the continents. It constituted the fourth (following the Floetz ) of the divisions in which Werner placed the rocks of the geologic column. Syn: Tertiary.

spondylium duplex . A spondylium formed by convergence of dental plates and supported by variably developed median septum arising from the floor of the pedicle valve.

isofract (i'-so-fract). A graphic representation of the locus of all compositions in a system having a given value for the index of refraction.

tongue [ice] . A projection of the ice edge up to several kilometers in length, caused by wind and current.

gyrocone (gy'-ro-cone). A loosely coiled cephalopod shell in which the successive whorls are not in contact with each other, or in which only a single whorl is approximately completed. Also called: gyroceracone.

plug [drill] . n. A watertight or gastight seal, such as an interval of cement or a bridge plug, installed in a borehole or well to prevent movement of fluids. v. To set a plug in a borehole; to fill in or seal off fractures, cavities, or other pores in the walls of a borehole.

peak-ring crater . A complex impact crater with a central mountainous ring, instead of a central peak, surrounding its center. Peak-ring craters are generally larger than central-peak craters on a given planet or satellite. The radial profile of the ring is roughly symmetric, as opposed to that of a multi-ring crater.

azonal soil . In the 1938 soil-classification system, one of the three soil orders. Azonal soils lack distinct genetic horizons and resemble the parent material (USDA, 1938). Not used in current U.S. system of soil taxonomy. Cf: intrazonal soil; zonal soil; Entisols. Syn: immature soil.

underconsolidation (un''-der-con-sol''-i-da'-tion). Consolidation (of sedimentary material) less than that normal for the existing overburden; e.g. consolidation resulting from deposition that is too rapid to give time for complete settling. Ant: overconsolidation.