age (a) The formal geochronologic unit of lowest rank, below epoch, during which the rocks of the corresponding stage were formed. (b) A term used informally to designate a length of geologic time during which the rocks of any stratigraphic unit were formed. (c) A division of time of unspecified duration in the history of the Earth, characterized by a dominant or important type of life form; e.g. the "age of mammals." (d) The time during which a particular geologic event or series of events occurred or was marked by special physical conditions; e.g. the "Ice Age." (e) The position of anything in the geologic time scale; e.g. "the rocks of Miocene age."   It is often expressed in years before present. 

ammonite The flat, spiral, or coiled fossil shell of an extinct mollusk of the cephalopod group: ammonites are related to the living nautilus.

archaeology The study of human cultures through the recovery and analysis of their material relics. Also spelled: archeology.

Bedrock The solid rock that underlies soil or other unconsolidated, unlithified superficial material.

bentonite  A soft, plastic, porous, light-colored rock composed essentially of clay minerals of the montmorillonite (smectite) group plus colloidal silica, and produced by devitrification and accompanying chemical alteration of a glassy igneous material, usually a tuff or volcanic ash.

biology The science that deals with the origin, history, physical characteristics, habits, etc. of plants and animals: it includes botany, zoology, and their subdivisions.

calcareous An adjective applied to rocks containing calcium carbonate.

community A group of animals or plants living together in the same environment.

contact  A plane or irregular surface between two types or ages of rock.

correlate To show a definite correspondence in character and stratigraphic position between geologic formations in two or more separated areas.

cross section A diagram or drawing that shows the sequence of rocks and sediment layers as they occur in a vertical plane; commonly drawn from the ground surface down to some selected depth, such as the bedrock surface.

delta A deposit of sediment, usually triangular in shape, at the mouth of a river.

drumlin  A low, smoothly rounded, elongate oval hill, mound, or ridge of compact glacial till or, less commonly, other kinds of sediment of glacial origin (sandy till, banded clay), built under the margin of the ice and shaped by its flow, or carved from an older moraine by readvancing ice; its longer axis is parallel to the direction of movement of the ice. It usually has a blunt nose pointing  in the direction from which the ice approached, and a gentler slope tapering in the other direction.  In North Dakota, many of the drumlins have extreme length to width ratios, and are  much longer than drumlins typical in other areas.

erosion (a) The general process or group of processes whereby the materials of the Earth's crust are loosened, dissolved, or worn away, and simultaneously moved from one place to another by natural agencies, which include weathering, solution, corrosion, and transportation; the mechanical destruction of the land and the removal of material (such as soil) by running water (including rainfall), waves and currents, moving ice, or wind.

escarpment  A long, more or less continuous cliff or relatively steep slope facing in one general direction, breaking the continuity of the land by separating two level or gently sloping surfaces, and produced by erosion or by faulting. The term is often used synonymously with scarp,  although escarpment is more often applied to a cliff formed by differential erosion.

erratics  Boulders and other rock fragments transported by glacial ice from their place of origin to an area where the bedrock is different.

esker  A long, narrow, sinuous ridge of sand and gravel deposited by a meltwater stream flowing upon, within, or beneath a glacier that is melting away.

exposure  Part of a glacial deposit or bedrock that is visible at the Earth's surface.

formation The basic geologic unit of lithostratigraphic (rock-strata) classification. A formation must be identifiable on the basis of easily recognized physical properties and widespread enough to be mapped at a regional scale. A formation can be divided into smaller units (called members) where these, too, are recognizable and mappable. Formations may be combined into groups when useful and appropriate.

fossil  Any remains, trace, or imprint of a plant or animal that has been preserved in the Earth's crust since some past geologic or prehistoric time; loosely, any evidence of past life. Said of any object that existed in the geologic past and of which there is still evidence.

geologic age (a) The age of a fossil organism or of a particular geologic event or feature referred to the geologic time scale and expressed in terms either of years or centuries (absolute age) or of comparison with the immediate surroundings (relative age); an age datable by geologic methods. (b) The term is also used to emphasize the long-past periods of time in geologic history, as distinct from present-day or historic times.

geologic map  A diagram or drawing on a horizontal plane of part of the Earth's surface showing by means of lines, colors, symbols, and orientation the distribution of selected features such as particular surface or subsurface units of rocks or sediments.  A map on which is recorded geologic information, such as the distribution, nature, and age relationships of rock units (surficial deposits may or may not be mapped separately), and the occurrence of structural features (folds, faults, joints), mineral deposits, and fossil localities.

glacial  adj. (a) Of or relating to the presence and activities of ice or glaciers, as glacial erosion. (b) Pertaining to distinctive features and materials produced by or derived from glaciers and ice sheets, as glacial lakes. (c) Pertaining to an ice age or region of glaciation. n.  A glacial age,  or glacial stage,  of a glacial epoch, especially of the Pleistocene Epoch; e.g. the Wisconsinan glacial age.

glacial lake  (a) A lake that derives much or all of its water from the melting of glacier ice, e.g. fed by meltwater, or lying on glacier ice and due to differential melting. (b) A lake occupying a basin produced by glacial deposition, as one held in by a morainal dam.

glacial lake plain  A large flat area underlain by fine-grained sediment that was deposited in a lake formed by ponded meltwater from a glacier.

glaciation  (a) The formation, movement, and recession of glaciers or ice sheets. (b) The covering of large land areas by glaciers or ice sheets. (c) The geographic distribution of glaciers and ice sheets. (d) A collective term for the geologic processes of glacial activity, including erosion and deposition, and the resulting effects of such action on the Earth's surface. (e) Any of several minor parts of geologic time during which glaciers were more extensive than at present; a glacial epoch, or a glacial stage. A climatic episode during which extensive glaciers developed, attained a maximum extent, and receded.

gypsum  A mineral consisting of hydrated calcium sulfate.

ice age A loosely used synonym of glacial epoch, or time of extensive glacial activity; specifically the latest of the glacial epochs, also known as the Pleistocene Epoch.

invertebrate fossil  Fossil of an animal without a backbone.

lake plain (a) The nearly level surface marking the floor of an extinct lake, filled in by well-sorted deposits from inflowing streams. (b) A flat lowland or a former lake bed bordering an existing lake.

landform   Any physical, recognizable form or feature of the Earth's surface, having a characteristic shape, and produced by natural causes; it includes major forms such as plain, plateau, and mountain, and minor forms such as hill, valley, slope, esker, and dune. Taken together, the landforms make up the surface configuration of the Earth.

Marine Of the sea or ocean; inhabiting, found in, or formed by the sea.

marl  Clay containing calcium carbonate.

paleontology The study of life in past geologic time, based on the phylogeny of fossil plants and animals, their relationships to existing plants, animals, and environments, and the chronology of the Earth's history.

predator  An animal that hunts or kills other animals for food.

reptile Any of a group of cold-blooded vertebrates that crawl on their bellies, as snakes, or creep on short stubby legs, as lizards, crocodiles, alligators, and turtles.

siliceous An adjective applied to rocks that contain silica, an oxide of silicon such as quartz.

sedimentary  One of the three basic categories into which rocks can be classified, of which the other two are igneous and metamorphic. Sedimentary rocks result from the consolidation of loose sediment that has accumulated in layers; they are formed by the compaction and cementation of sediment or by the precipitation of dissolved minerals from salt or fresh water. Most sediments were deposited from water, but they may be deposited by wind, glaciers, or other agents.

shale  A fine-grained detrital sedimentary rock, formed by the consolidation, especially by compression, of clay, silt, or mud. It is characterized by a finely laminated  structure, which imparts a fissility typically parallel to the bedding.

still stand  A period of time when lake level stayed fairly constant.

vertebrate fossil Fossil of an animal with a backbone.

volcanic ash  The fine, dust-like, fragmentary rock material violently blown from volcanoes during explosive eruptions.

zoology A division of biology that deals with the classification of animals and the study of animal life.