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Illinois State Geological Survey - Geological terms
Category: Sciences > Geological Terms
Date & country: 25/07/2013, US
Words: 169


MONOLITH
(a) A piece of unfractured bedrock, generally more than a few meters across. (b) A large upstanding mass of rock

MORAINE
A mound, ridge, or other distinct accumulation of glacial drift, predominantly till, deposited in a variety of topographic landforms that are independent of control by the surface on which the drift lies. (see also End Moraine)

MORPHOLOGY
The scientific study of form, and of the structures and development that influence form; term used in most sciences.

MTSIMON
Cambrian sandstone with with a few thin red shale beds.

N-GAMMA
One of several kinds of measurements of rock characteristics taken by lowering instruments into cased or uncased, air- or water-filled boreholes. Elevated natural gamma radiation levels in a rock generally indicate the presence of clay minerals.

NEBRASKAN
Pertaining to the first classical glacial stage of the Pleistocene Epoch in North America, followed by the Aftonian interglacial stage.

NICKPOINT
A place with an abrupt inflection in a stream profile, generally formed by the presence of a rock layer resistant to erosion; also, a sharp angle cut by currents at base of a cliff.

NONCONFORMITY
An unconformity resulting from deposition of sedimentary strata on massive crystalline rock.

ONEOTAD
The Ordovician age Oneota Dolomite underlies all of Illinois except the northmost part of the state. It ranges in thickness from 100 feet in the north to over 500 feet in the south. This formation consists of fine- to coarse-grained, light gray to brownish gray, cherty dolomite that contains minor amounts of sand and, at its base, thin shaly beds.

ORDOVICIAN
The second earliest period of the Paleozoic era (after the Cambrian and before the Silurian), thought to have covered the span of time between 490 and 443 million years ago; also, the corresponding system of rocks. It is named after a Celtic tribe called the Ordovices. In the older literature the Ordovician is sometimes know as the Lower Silurian.

ORTHOIMAGERY
An aerial photograph or satellite scene that has been transformed by the orthogonal projection, yielding a map that is free of most significant geometric distortions.

OUTWASH
Stratified glacially derived sediment (clay, silt, sand, gravel) deposited by meltwater streams in channels, deltas, outwash plains, on flood plains, and in glacial lakes.

OUTWASHPLAIN
The surface of a broad body of outwash formed in front of a glacier.

OVERBURDEN
The loose soil, silt, sand, gravel or other unconsolidated material overlying bedrock, either transported or formed in place.

OXBOW
A crescent-shaped lake in an abandoned bend of a river channel. A precursor of a meander scar.

PALEOZOIC
An era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic, or from about 543 to about 248 million years ago.

PANGEA
The supercontinent that existed from 300 to 200 million years ago. It combined most of the continental crust of the Earth, from which the present continents were derived by fragmentation and movement away from each other by means of plate tectonics. During an intermediate stage of the fragmentation, between the existence of Pangea and that of the p...

PED
A naturally formed unit of soil structure, (for example, granule, block, crumb, or aggregate).

PENEPLAIN
A land surface of regional scope worn down by erosion to a nearly flat or broadly undulating plain.

PENNSYLVANIAN
A period of the Paleozoic era (after the Mississippian and before the Permian), thought to have covered the span of time between 323 and 290 million years ago; also, the corresponding system of rocks. It is named after the state of Pennsylvania in which rocks of this age are widespread and yield much coal. It is the approximate equivalent of the Up...

PERIOD
An interval of geologic time; a division of an era (for example, Cambrian, Jurassic, Tertiary).

PERMIAN
The last period of the Paleozoic era (after the Pennsylvanian), thought to have covered the span of time between 290 and 248 million years ago; also, the corresponding system of rocks. The Permian is sometimes considered part to the Carboniferous, or is divided between the Carboniferous and Triassic. It is named after the province of Perm, Russia, ...

PHYSIO-PROV
(a) A region, all parts of which are similar in geologic structure and climate and which has consequently had a unified geologic history. (b) A region whose pattern of relief features or landforms differs significantly from that of adjacent regions.

PHYSIOGRAPHY
The study and classification of the surface features of Earth on the basis of similarities in geologic structure and the history of geologic changes.

PLEISTOCENE
An epoch of the Quaternary period, after the Pliocene of the Tertiary and before the Holocene; also, the corresponding worldwide series of rocks. It began one to two million years ago and lasted until the start of the Holocene, some 10,000 years ago. When the Quaternary is designated as an era, the Pleistocene is considered to be a period.

POINTBAR
A low arcuate ridge of sand and gravel developed on the inside of a stream meander by accumulation of sediment as the stream channel migrates toward the outer bank.

PRAIRIEDUCHIEN
Ordovician age group of dolomites and sandstones underlying the Glenwood-St.Peter formations, but is missing in parts of northern Illinois. It thickens considerably to the south.

PRECAMBRIAN
All geologic time, and its corresponding rocks, before the beginning of the Paleozoic; it is equivalent to about 90% of geologic time. Precambrian time has been divided according to several different systems, all of which use the presence or absence of evidence of life as a criterion.

QUATERNARY
The second period of the Cenozoic era, following the Tertiary; also, the corresponding system of rocks. It began two to three million years ago and extends to the present. It consists of two grossly unequal epochs: The Pleistocene, up to about 8,000 years ago, and the Holocene since that time. The Quaternary was originally designated an era rather ...

RADIO-LOGS
Any of several types of geophysical measurements taken in bore holes using either the natural radioactivity in the rocks, or the effects of radiation on the rocks to determine the lithology or other characteristics of the rocks in the walls of the borehole. (Examples: Natural gamma radiation log; neutron density log).

RELIEF
(a) A term used loosely for the actual physical shape, configuration, or general unevenness of a part of Earth's surface, considered with reference to variations of height and slope or to irregularities of the land surface; the elevations or differences in elevation, considered collectively, of a land surface (frequently confused with topography). ...

RIFT
A long narrow trough, generally on a continent, bounded by normal faults, a graben with regional extent. Formed in places where the forces of plate tectonics are beginning to split a continent. (Example: East African Rift Valley).

SANDSTONE
A medium-grained sedimentary rock composed of abundant rounded or angular fragments of sand size set in a fine-grained matrix (silt or clay) and more or less firmly united by a cementing material (commonly silica, iron oxide, or calcium carbonate); the consolidated equivalent of sand, intermediate in texture between conglomerate and shale. The sand...

SEDIMENT
Solid fragmental matter, either inorganic or organic, that originates from weathering of rocks and is transported and deposited by air, water, or ice, or that is accumulated by other natural agents, such as chemical precipitation from solution or secretion from organisms. When deposited, it generally forms layers of loose, unconsolidated material (...

SEDIMENTARY
A rock resulting from the consolidation of loose sediment that has accumulated in layers (e.g., sandstone, siltstone, limestone).

SHALE
A fine-grained detrital sedimentary rock, formed by the consolidation (esp. by compression) of clay, silt, or mud. It is characterized by finely laminated structure, which imparts a fissility approximately parallel to the bedding, along which the rock breaks readily into thin layers and that is commonly most conspicuous on weathered surfaces, and b...

SHOALING
Said of an ocean or lake bottom that becomes progressively shallower as a shoreline is approached. The shoaling of the ocean bottom causes waves to rise in height and break as they approach the shore.

SILTSTONE
An indurated silt having the texture and composition of silt but lacking its fine lamination or fissility; a massive mudstone in which silt predominates over clay; a non-fissile silt shale.

SILURIAN
A period of the Paleozoic, thought to have covered the span of time between 443 and 417 million years ago; also, the corresponding system of rocks. The Silurian follows the Ordovician and precedes the Devonian; in the older literature, it was sometimes considered to include the Ordovician. It is named after the Silures, a Celtic tribe.

SINKHOLES
Any closed depression in the land surface formed as a result of the collapse of the underlying soil or bedrock into a cavity. Sinkholes are common in areas where bedrock is near the surface and susceptible to dissolution by infiltrating surface water. Sinkhole is synonymous with doline, a term used extensively in Europe. The essential component of ...

SLIP-OFF
Long, low, gentle slope on the inside of a stream meander. The slope on which the sand that forms point bars is deposited.

SOILPIP
The movement and entrainment of soil along an initially small pathway in the soil. As water moves along the pathway, the pathway enlarges and the velocity of the flow may increase proportionally, thus, entraining more soil. The result is the formation of an ever enlarging cavity along the flow path. At some point, structural support may be lost and...

STAGE
Geologic time-rock units; the strata formed during an age or subage, respectively. Generally applied to glacial episodes (for example, to the Woodfordian Substage of the Wisconsinan Stage).

STLOUIS
The St. Louis Limestone is named for St. Louis, Missouri, where it is extensively exposed. No type section was designated. The St. Louis is typically exposed in Illinois in the Mississippi River bluffs at Alton, Madison County. It is also well exposed in the Mississippi and Illinois Valleys in western and southern Illinois and along the Ohio River ...

STPETER
The Glenwood formation is a characteristically poorly sorted sandstone, impure dolomite, and green shale of Ordovician age. It overlies the Ordovician-age St. Peter Sandstone in northern Illinois. The St. Peter consists of well sorted, frosted, friable quartz sand.

STRAT-UNIT
A stratum or body of strata recognized as a unit in the classification of the rocks of Earth's crust with respect to any specific rock character, property, or attribute or for any purpose such as description, mapping, and correlation.

STRATIGRAPHY
The study, definition, and description of major and minor natural divisions of rocks, particularly the study of their form, arrangement, geographic distribution, chronologic succession, classification, correlation, and mutual relationships of rock strata.

STRATUM
A tabular or sheet-like mass, or a single, distinct layer of material of any thickness, separable from other layers above and below by a discrete change in character of the material or by a sharp physical break, or by both. The term is generally applied to sedimentary rocks, but could be applied to any tabular body of rock. (see also Bed)

SUBAGE
A small interval of geologic time; a division of an age.

SYNCLINE
A convex-downward fold in which the strata have been bent to form a trough; the strata on either side of the core of the trough are inclined in opposite directions toward the axis of the fold; the core area of the fold contains the youngest rocks. (see also Anticline).

SYSTEM
A fundamental geologic time-rock unit of worldwide significance; the strata of a system are those deposited during a period of geologic time (for example, rocks formed during the Pennsylvanian Period are included in the Pennsylvanian System).

TECTONIC
Pertaining to the global forces that cause folding and faulting of the Earth's crust. Also used to classify or describe features or structures formed by the action of those forces.

TECTONICS
The branch of geology dealing with the broad architecture of the upper (outer) part of Earth; that is, the major structural or deformational features, their origins, historical evolution, and relations to each other. It is similar to structural geology, but generally deals with larger features such as whole mountain ranges, or continents.

TEMP-REST
A borehole log, run only in water-filled boreholes, that measures the water temperature and the quality of groundwater in the well.

TERRACE
An abandoned flood plain formed when a stream flowed at a level above the level of its present channel and flood plain.

TERRIGEN
Sediment eroded from the land, or a continent, and deposited in water (generally in a marine environment).

TILL
Unconsolidated, nonsorted, unstratified drift deposited by and underneath a glacier and consisting of a heterogenous mixture of different sizes and kinds of rock fragments.

TILLPLAIN
The undulating surface of low relief in the area underlain by ground moraine.

TOPOGRAPHY
The natural or physical surface features of a region, considered collectively as to form; the features revealed by the contour lines of a map.

TRIASSIC
The first period of the Mesozoic era (after the Permian of the Paleozoic era, and before the Jurassic), thought to have covered the span of time between 225 and 190 million years ago; also, the corresponding system of rocks. The Triassic is so named because of its threefold division in the rocks of Germany. Syn: Trias.

UNCONFORMABLE
Said of strata that do not succeed the underlying rocks in immediate order of age or in parallel position. A general term applied to any strata deposited directly upon older rocks after an interruption in sedimentation, with or without any deformation and/or erosion of the older rocks.

UNCONFORMITY
A surface of erosion or nondeposition that separates younger strata from older strata; most unconformites indicate intervals of time when former areas of the sea bottom were temporarily raised above sea level.

UNCONSOLIDATED
Nonlithified sediment that has no mineral cement or matrix binding its grains.

VALLEY-TRAINS
The accumulations of outwash deposited by rivers in their valleys downstream from a glacier.

VALMEYER
The Valmeyeran Series is named for Valmeyer, Monroe County, near which much of the series is exposed. It is the middle series of the Mississippian System and includes formations assigned to two series (Osagian and Meramecan) in other areas. The Valmeyeran Series underlies most of central and southern Illinois and includes strata from the top of the...

WARSAW
The Warsaw Shale is named for Warsaw, Hancock County, and the exposure in Geode Glen at Warsaw has become the type section. The Warsaw is widely present in the bluffs of the Mississippi and Illinois Valleys in western and southwestern Illinois. It consists of as much as 300 feet of siltstone in west-central Illinois, but it thins to less than 100 f...

WATER-TABLE
The point in a well or opening in the Earth where groundwater begins. It generally marks the top of the zone where the pores in the surrounding rocks are fully saturated with water.

WEATHERING
The group of processes, chemical and physical, whereby rocks on exposure to the weather, change in character, decay, and finally crumble into soil.

WISCONSIN
Pertaining to the classical fourth glacial stage (and the last definitely ascertained, although there appear to be others) of the Pleistocene Epoch in North America, following the Sangamonian interglacial stage and preceding the Holocene.