Abrasion—erosion of rock material by friction of solid particles moved by water, ice, wind, or gravity.
Absolute time—geologic time measured in years. Compare with relative time.
Amphibians—cold-blooded four-footed animals which have gills in youth and lungs in maturity (e.g., frog).
Anhydrite—the mineral calcium sulfate, CaSO4. See Gypsum.
Anticline—an arch-like fold in the rocks, with the beds dipping in opposite directions on the two sides.
Aquifer—a water-bearing layer of porous and permeable rock.
Aragonite—a form of calcium carbonate (CaCO3).
Archeozoic—the oldest known geological era; early Precambrian.
Bedding plane—the plane of demarcation between two individual rock layers or strata.
Calcite—a mineral composed of calcium carbonate, CaCO3.
Caliche—an accumulation of calcium carbonate, commonly white in color, in the soil profile.
Cenozoic—the latest era of geologic time, containing the Tertiary and Quaternary Periods and continuing to the present time.
Chert—dense, hard rock of very fine-grained silica, usually in nodular form. This material is also called flint.
Concretion—a concentration, usually spherical, of mineral matter in sedimentary rocks, produced by deposits from solution; it is harder than the surrounding rock.
Conglomerate—a sedimentary rock composed of rounded, water-worn gravel, usually mixed with sand, and cemented together by another mineral substance.
Coprolite—the fossilized excrement of animals.
Eolian—pertaining to the erosion and the deposits resulting from wind action and to sedimentary rocks composed of wind-transported material.
Epoch—a subdivision of a geologic period, such as the Pliocene Epoch of the Tertiary Period.
Era—a major division of geologic time. All geologic time is divided into five eras: the Archeozoic, Proterozoic, Paleozoic, Mesozoic, and Cenozoic Eras.
Fluorescence—luminescence of a mineral during exposure to radiation (such as from ultraviolet or X-rays).
Fluvial deposit—sediment deposited by streams.
Formation—a rock unit useful for mapping and distinguished primarily on the basis of lithologic character.
Fossil—any remains or traces of plants or animals preserved in deposits of a past geologic age.
Geode—a hollow stone, usually lined or filled with mineral matter.
Geologic age—the age of an object as stated in terms of geologic time (e.g., a Pennsylvanian fern, Cretaceous dinosaur).
Geologic time—all time which has elapsed since the first known rocks were formed and continuing until recent, or modern, times.
Geologic time scale—record of the divisions of earth history.
Gypsum—a mineral, hydrated calcium sulfate (CaSO4·2H2O). See Anhydrite.
Hoodoo—a form produced by erosion of rock.
Ice age—the Pleistocene Epoch of the Quaternary Period, Cenozoic Era; a time of extensive glaciation.
Igneous rock—rocks which have solidified from lava or molten rock called magma.
Joint—a fracture in a rock along which there has been no displacement on opposite sides of the break.
Joint System—a series of two or more sets of joints passing through a rock mass and separating it into blocks of more or less regular pattern.
Mass-wasting—erosion caused chiefly by gravity.
Mesozoic—the geologic era between the Paleozoic and Cenozoic Eras; the “Age of Reptiles.”
Metamorphic rock—rock formed from igneous or sedimentary rocks that have been subjected to great changes in temperature, pressure, or chemical environment.
Metamorphism—the process whereby rocks are changed physically by heat, pressure, or chemical environment into different kinds.
Mineral—a naturally occurring inorganic substance possessing definite chemical and physical properties.
Nodule—rounded lump of rock or mineral.
Outcrop—the area where a particular rock formation comes to the surface.
Paleontology—the science which deals with the study of fossils.
Paleozoic—that era of geologic time following the Proterozoic and preceding the Mesozoic.
Period—a basic unit of the geologic time scale into which the eras are divided, such as the Pennsylvanian Period of the Paleozoic Era.
Permian—the seventh and last period of the Paleozoic Era.
Pleistocene—the first of the two epochs of the Quaternary Period, and that which precedes modern time, known as the Great Ice Age.
Pliocene—last and youngest epoch of the Tertiary Period of the Cenozoic Era.
Proterozoic—youngest era of the Precambrian; follows the Archeozoic Era and precedes the Cambrian Period of the Paleozoic Era.
Red beds—a general term for red sandstone, shales, etc., which appear to characterize arid periods in the past.
Ripple marks—wave-like corrugations produced in unconsolidated materials by wind or water.
Rock—any natural aggregate of mineral matter, usually consisting of a mixture of two or more minerals.
Sandstone—sedimentary rock composed of cemented sand grains, usually quartz.
Sediment—material that has been deposited by settling from a transportation agent such as water or air.
Sedimentary rock—rocks formed by the accumulation of sediments.
Shale—a sedimentary rock formed by the hardening of mud and clay and usually tending to split into thin sheets or layers.
Silica—an oxide of silicon (SiO2).
Siliceous—containing or pertaining to silica.
Silt—fine muddy sediment consisting of particles intermediate in size between clay particles and sand grains.
Siltstone—a very fine-grained sedimentary rock composed of silt grains, and intermediate between shale and sandstone.
Stratified rocks—sedimentary rocks; those formed in beds, layers, or strata.
Stratum—an individual layer of rock formation. (Plural, strata.)
Superposition, law of—in an undisturbed sequence of rocks younger beds overlie older beds.
Syncline—a trough-like fold in the rocks, with the beds dipping inward on either side. See Anticline.
Talus—a mass of rock debris commonly on slopes or at the base of a steep mountain or cliff.
Topography—the configuration of a land surface.
Unconformity—a break in the sequence of rock formations which separates younger strata from older ones; caused primarily by removal of older rocks by erosion before those of a later sequence were laid down.
Weathering—any natural process, mechanical or chemical, whereby rocks are disintegrated or decomposed into smaller particles and ultimately into clay and soil.
