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Guidebook to the Geology of Travis County

Glossary of Technical Terms

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Agglutinated - refers to those foraminiferans of which which the tests are composed Of minute pieces of substrate or other material cemented by an animal secretion.
Aquiclude - a formation so impermeable that it will not transmit enough water for a well or a spring.
Aquifer - a permeable layer of rock under the surface of the ground through which water can move. Usually, aquifers are sand-stone or limestone, but some may be mixed sand and gravel, or even fractured rock. Claystones, shales, and most igneous and metamorphic rocks are aquicludes.
Artesian water - water that is under pressure, naturally. When tapped by a well (or spring) it isable to flow upward. If it flows to the surface it is flowing artesian.
Basanite - an extrusive rock composed of calcic plagioclase and iron-magnesium silicates (augite and/or olivine), and a feld-spathoid (nepheline, leucite, or analcime).
Biolithite - limestone that is bound together by a framework of shells of organisms; a boundstone; reef rock that is held together by an organic framework.
Biomicrite - a limestone consisting of a variable proportion of fossil skeletal debris contained in a carbonate mud; when used, the major organism should be specified: crinoid biomicrite.
Biosparite - a sparite, (see same), containing fragments of,the shells of fossils.
Bivalve - a common term for the pelecypods (see same).
Calcarenite - a sandstone composed of cemented, sandsize grains of calcium carbonate.
Carbonate - salt of carbonic acid; a compound containing the C03 radical; in most uses carbonate refers to calcium carbonate -CaCO3-
Clastic - consisting of fragments of rocks or of the hard parts of organisms.
Creep - the almost imperceptibly slow movement of soil and surface debris down the slope propelled by gravity.
Disconformity - a buried surface of erosion that represents in succession (1) deposition of strata, (2) erosion of some of the strata, and (3) deposition of additional strata.
Fault - a break in the earth's crust accompanied by a displacement of one side of the fracture with respect to the other, and in a direction parallel to the fracture; the fracture does not have to be straight.
Floodplain - that part of a stream valley that is covered with water when the river or stream overflows its banks; it is usually that part of the even valley floor that has been formed under the present regime.
Flood-stage - any part of the floodplain that is covered by floodwater for any designated description; usually in terms of return to that magnitude again, as in 20-year, 40-year, or 100-year flood.
Fold - a bend, flexure, or wrinkle in rock strata, which was produced when the rock was bent by internal earth forces.
Foraminiferan - an individual of the order Foraminifera (Class Sarcodina); one of many small, unicellular animals with a protective test of harder calcium carbonate or agglutinate.
Geosyncline - a large linear trough that received stratified sediments for a long period of time, subsiding slowly enough that the sediments were usually terrestrial or deposited in relatively shallow water.
Glauconite - a green mineral commonly occurring in sedimentary rocks; it is a hydrous potassium iron silicate that is closely related to micas and clay minerals.
Graben - a depressed segment of the earth's crust bounded on at least two sides by faults.
Gradient - the slope of a streambed, measured in feet per mile or meters per kilometer.
Grainstone - a limestone in which there is no mud and in which the allochems (usually shell fragments or oolites) are in contact and are self-supporting; a biosparite.
Inert gas - one of any of those gases in the zero group of the periodic table; its outer electron shell is saturated and it does not actively participate in chemical reactions. Inert gases included helium, neon, argon, krypton, xenon, and radon.
Intertidal- - used to describe that area between high and low tide (sometimes including stor-Tr. tides) and the sediments deposited in that area and the rocks resulting from those sediments.
Isotopic - an adjective referring to Isotopes; i-sotopes are elements having an identical number of protons in their nuclei, but differing in the number of electrons as oxygen-16 vs Oxygen-18.
Lava - the molten silicates or the congealed rock that issues onto the surface of the earth.
Mafic - refers to rocks or magmas that are formed largely of iron and magnesium silicates.
Magma - naturally occurring fluid that congeals to form rock, either within the earth or upon the surface of the earth.
Mantle - the earth is composed of three layers. This is the layer of the earth's interior between the core and the crust.
Melilite - a group of minerals containing (sodium or calcium) plus (magnesium or aluminum) plus silicates.
Metamorphism - the process of altering rock by pressure and/or heat; alteration may be in texture, composition, or internal structure of the constituent minerals to produce new minerals.
Micrite - a limestone composed of clay-sized fragments; a claystone of calcium carbonate; mudstone.
Mollusk - one of the animals belonging to the Phylum Mollusca, which includes cephalopods, gastropods (including snails), pelecypods (bivalves), and some smaller groups.
Mudflow - a flow of heterogeneous debris or sediment lubricated by a large amount of water.
Nepheline - a mineral composed of (sodium or potassium) aluminum silicate.
Olivine - a greenish, translucent mineral series composed of varying amounts of magnesium silicate and iron silicate.
Oolite - a spherical or ellipsoid body, less than 2 mm in diameter, that has concentric internal structure.
Outcrop - an outcrop is any appearance of rock at the surface of the earth.
Packstone - a packstone is a limestone composed of grains with the intervening spaces filled with mud; a packed biomicrite.
Pelecypod - a bivalve; a member of the Mollusca in which the hard parts are composed of two sections fitting together to enclose a space that contains the soft parts of the organism.
Permeability - the degree or rate at which a liquid will pass through rock or other earth material.
Phenocryst - one of the large, conspicuous crystals of a mineral in a rock, which because of its greater size is set off from the rest of the rock.
Plagioclase - the name for a series of minerals ranging from sodium aluminum silicate through various combinations of sodium and calcium silicate to calcium silicate.
Plastic - any permanent change in shape or volume deformation that does not include failure by rupture; and that, once started, continues without increase in the deforming force.
Pyroclastic - rock that is formed of detrital volcanic material deposited as sediment from air or water.
Pyroxene - one of a number of minerals composed of (magnesium, iron, calcium, sodium) plus (magnesium and/or iron or aluminum) silicate.
Relief - the general difference in elevation between the highest and lowest parts of an area.
Runoff - the amount of water discharged through surface streams.
Sabkah - an area of deposition of salt in surface or near-surface sediments.
Sediment -a solid material in suspension in air or water; the material that has settled out of such a suspension.
Septarian - a roughly rounded concretion cut into sections by cracks that have later been filled with some mineral.
Shear - a cutting or breaking of a solid body such as a rock by adjacent forces acting in opposite directions.
Shrink-swell - refers to those clays or soils that clay alternately expand when wetted and shrink when dried.
Slickensides - polished grooves on a fault surface resulting from abrasion along the fault plane.
Slump - (1) material that has slid down a slope; (2) the en masse movement of such material.
Soil - rock material that has been altered by physical, chemical, and biological agents to produce a medium that will support plant life.
Sparite - a limestone composed of grains without mud, but cemented with calcite.
Spinel - a mafic rock without plagioclase and peridotite containing spinel, a (magnesium-iron) aluminum silicate material.
Spring - a place where the water table reaches the surface and water flows more or less continuously.
Stability - in reference to soil or rock, stability refers to those that do not easily move downslope when saturated with water or shook by earthquakes.
Storm sewer - an open or enclosed conduit to rapidly remove surface runoff from rains or melting snows; most carry the runoff to the nearest natural draw, gully, or creek.
Strain - the change of dimensions of matter in response to stress.
Striation - a scratch or small groove caused by movement along a fault.
Strength - (of rocks) is the stress at which rupture occurs or plastic deformation begins.
Stress - a force applied to a material that tends to change the dimensions of the material.
Subtidal - refers to the area below low tide or to sediments deposited in that area.
Supertidal - refers to that area above the tidal range, or to sediments deposited in that area. Such sediments are usually storm deposits, if limestone, or reworked storm deposits, or deposits resulting from evaporation of water.
Tension - a system of forces tending to draw apart the parts of a body. Opposite of compression.
Trap-rock - a non-technical term that frequently refers to any dark colored rock intersecting other rock.
Ultramafic - an igneous rock containing no quartz or feldspar and less than 45% silica.
Underground - groundwater; the water that is confined in the rocks beneath the soil.
Volcanic ash - pyroclastic sediment with fragments of less than 4 mm in diameter.
Wackestone - in limestone composed of mud, but also continaing grains scattered through the mud; a biomicrite.
Water table - the upper surface of the zone that is continually and completely filled with underground water, except where that surface is formed by an impermeable stratum.

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