Glossary of Geologic Terms
The following definitions are from several sources in total or in part, but the main reference is: Bates, R.L., and J.A. Jackson, editors, 1987, Glossary of Geology: American Geological Institute, Alexandria, VA, 3rd edition, 788 p. © 2003 American Geological Institute and reprinted with their permission. For more information, log on to www.agiweb.org. Stratigraphic definitions come from: Handbook of Illinois Stratigraphy, H.B. Willman, Elwood Atherton, T.C. Buschbach, Charles Collinson, J.C. Frye, M.E. Hopkins, J.A. Lineback, J.A. Simon, 1975. 261 p.
Separation and removal of rock material and formation of deposits, especially by wind action or the washing away of loose and soluble materials.
An interval of geologic time; a division of an epoch.
One that is actively depositing sediment in its channel or floodplain because it is being supplied with more load than it can transport.
One that has been at least partially filled with sand, silt, and mud by flowing water.
A general term for clay, silt, sand, gravel, or similar unconsolidated sorted or semi-sorted sediment deposited during comparatively recent time by a stream or other body of running water.
A convex upward rock fold in which strata have been bent into an arch; the strata on each side of the core of the arch are inclined in opposite directions away from the axis or crest; the core contains older rocks than does the perimeter of the structure.
A geologic formation that is water-bearing and which transmits water from one point to another.
Said of rock or sediment that contains, or is composed of, clay-sized particles or clay minerals.
A relatively clean quartz sandstone that is well sorted and contains less than 10% argillaceous material.
Formed or generated in place; specif. said of rock constituents and minerals that have not been transported or that crystallized locally at the spot where they are now found, and of minerals that came into existence at the same time as, or subsequently to, the formation of the rock of which they constitute a part. The term, as used, often refers to a mineral (such as quartz or feldspar) formed after deposition of the original sediment.
The Aux Vases Sandstone is named for the Aux Vases River in Ste. Genevieve County, Missouri, and the type section consists of outcrops in the Mississippi River bluffs at the mouth of the Aux Vases River. The Aux Vases consists of sandstone, siltstone, and minor amounts of shale and, locally, dolomite and limestone. It occurs in much of the area of the Chesterian Series. It crops out along the Mississippi River Valley in St. Clair, Monroe, and Randolph Counties, being particularly well exposed in the bluffs 2-3 miles southeast of Prairie du Rocher, Randolph County. In southern Illinois it crops out principally in Union, Johnson, and Hardin Counties.
Lower limit of erosion of the land's surface by running water. Controlled locally and temporarily by the water level of stream mouths emptying into lakes, or more generally and semi-permanently by the level of the ocean (mean sea level).
The suite of mostly crystalline igneous and/or metamorphic rocks that generally underlies the sedimentary rock sequence.
A topographic or structural low area that generally receives thicker deposits of sediments than adjacent areas; the low areas tend to sink more readily, partly because of the weight of the thicker sediments; the term also denotes an area of relatively deep water adjacent to shallow-water shelf areas.
A naturally occurring layer of earth material of relatively greater horizontal than vertical extent that is characterized by physical properties different from those of overlying and underlying materials. It also is the ground upon which any body of water rests or has rested, or the land covered by the waters of a stream, lake, or ocean; the bottom of a stream channel.
The solid rock (sedimentary, igneous, or metamorphic) that underlies the unconsolidated (non-indurated) surface materials (for example, soil, sand, gravel, glacial till, etc.).
A drainageway eroded into the solid bedrock beneath the surface materials. It may be completely filled with unconsolidated (non-indurated) materials and hidden from view.
A calcarenite containing abundant fossils or fossil fragments.
A low-gradient, low-volume stream flowing through an intricate network of interlacing shallow channels that repeatedly merge and divide, and are separated from each other by branch islands or channel bars. Such a stream may be incapable of carrying all of its load. Most streams that receive more sediment load than they can carry become braided.
The Burlington Limestone is named for the city of Burlington, Des Moines County, Iowa, where the formation is well exposed and about 70 feet thick. In Illinois, the Burlington extends from Henderson County in the northwest across a roughly triangular area southward to Jackson County and eastward to Iroquois County. Good outcrops are found in the Mississippi River bluffs from Quincy, Adams County, to near Alton, Madison County. In the vicinity of Quincy the lower 25 feet is relatively pure and is quarried as the "Quincy Lime." In the southern area the Burlington and the overlying Keokuk can be distinguished only by their fossils and are generally referred to as the Burlington-Keokuk Limestone.
Describes a limestone composed of more or less worn fragments of shells or pieces of older limestone. The particles are generally sand-sized.
Said of a rock containing as much as 50% of calcium carbonate (CaCO3), but also composed of something else (synonym: limy).
The heating of calcite or limestone to its temperature of dissociation so that it loses its carbon dioxide; also applied to the heating of gypsum to drive off its water of crystallization to make plaster of paris.
A common rock-forming mineral consisting of CaCO3; it may be white, colorless, or pale shades of gray, yellow, and blue; it has perfect rhombohedral cleavage, appears vitreous, and has a hardness of 3 on the Mohs' scale; it effervesces (fizzes) readily in cold dilute hydrochloric acid. It is the principal constituent of limestone.
The earliest period of the Paleozoic, thought to have covered the span of time between 543 and 490 million years ago; also, the corresponding system of rocks. It is named after Cambria, the Roman name for Wales, where rocks of this age were first studied.
A cavity in the earth large enough for a human to enter. Caves can form as a result of physical and chemical weathering of rock. Physical weathering usually produces shelter-type caves that extend into the rock for only a few feet. Chemical weathering of rock can produce caves (solution channels along fractures and bedding planes) that extend for many miles into the rock.
An era of geologic time, from the beginning of the Tertiary period to the present. (Some authors do not include the Quaternary, considering it a separate era.) It is characterized paleontologically by the evolution and abundance of mammals, advanced mollusks, and birds; paleobotanically, by angiosperms. The Cenozoic is considered to have begun about 65 million years ago.
Silicon dioxide (SiO2); a compact, massive rock composed of minute particles of quartz and/or chalcedony; it is similar to flint but lighter in color.
The Chesterian Series, the uppermost series of the Mississippian System, is named for Chester, Randolph County, where it is well exposed in the bluffs of the Mississippi River. The Chesterian Series consists of limestone-shale formations alternating with sandstone-shale formations. It extends from the major unconformity at the base of the Pennsylvanian System (sub-Absaroka unconformity) down to the base of the Shetlerville Member of the Renault Formation, which is the top of the Valmeyeran Series. Many of the Chesterian limestones and shales are abundantly fossiliferous. The sandstones are much less fossiliferous, but can contain some plant fossils. Coals, a few inches thick at most, occur in some of the Chesterian sandstones. Many of the limestones contain abundant crinoidal fragments, and the series is characterized by the presence of Talarocrinus which distinguishes it from the underlying Valmeyeran containing Platycrinites penicillus. Blastoids and the corkscrew bryozoan, Archimedes, are common and characteristic.
Chitin: A resistant organic compound with the same basic carbohydrate structure as cellulose, but nitrogenous because some hydroxyl groups are replaced by ascetamide groups. It is a common constituent of various invertebrate skeletons such as insect exoskeletons and foraminiferal inner test, and also occurs in hyphae and spores of fungi.
Said of rocks composed of particles of other rocks or minerals, including broken organic hard parts as well as rock substances of any sort, transported and deposited by wind, water, ice, or gravity.
A low, roughly concave topographic feature in a landscape. Rain falling within the boundaries of the depression would be channeled toward its lowest part (usually near its center).
The difference in altitude between the crest of a dome or anticline and the lowest structural or elevation contour that completely surrounds it.
A graphic representation, in the form of one or more vertical column(s), of the vertical succession and stratigraphic relations of rock units in a region.
(a) A hard, compact mass or aggregate of mineral matter, normally subspherical, but commonly oblate, disk-shaped, or irregular with odd or fantastic outlines; formed by precipitation from aqueous solution about a nucleus or center, such as a leaf, shell, bone, or fossil, in the pores of a sedimentary or fragmental volcanic rock, and usually of a composition widely different from that of the rock in which it is found and from which it is rather sharply separated. It represents a concentration of some minor constituent of the enclosing rock or cementing material, such as silica (chert), calcite, delimite, iron oxide, pyrite, or gypsum, and it ranges in size from a small pellet-like object to a great spheroidal body as much as 3 m in diameter. Most concretions were formed during diagenesis, and many (especially in limestone and shale) shortly after sediment deposition. (b) A collective term applied loosely to various primary and secondary mineral segregations of diverse origin, including irregular nodules, spherulities, crystalline aggregates, geodes, septaria, and related bodies.
Said of strata deposited one upon another without interruption in accumulation of sediment; beds parallel.
The final period of the Mesozoic era (after the Jurassic and before the Tertiary period of the Cenozoic era), thought to have covered the span of time between 144 and 65 million years ago; also, the corresponding system of rocks. It is named after the Latin word for chalk (creta) because of the English chalk beds of this age.
A low, nearly flat, alluvial land form deposited at or near the mouth of a river where it enters a body of standing water; commonly a triangular or fan-shaped plain extending beyond the general trend of a coastline.
Pertaining to or formed from detritus; said esp. of rocks, minerals, and sediments. The term may indicate a source outside the depositional basin or a source within it.
Loose rock and mineral material produced by mechanical disintegration and removed from its place of origin by wind, water, gravity, or ice; also, find particles of organic matter, such as plant debris.
A period of the Paleozoic era (after the Silurian and before the Mississippian), thought to have covered the span of time between 400 and 345 million years ago; also, the corresponding system of rocks. It is named after Devonshire, England, where rocks of this age were first studied.
An unconformity marked by a distinct erosion-produced, irregular, uneven surface of appreciable relief between parallel strata below and above the break; sometimes represents a considerable interval of nondeposition.
A mineral, calcium-magnesium carbonate (Ca,Mg[CO3]2); also the name applied to sedimentary rocks composed largely of the mineral. It is white, colorless, or tinged yellow, brown, pink, or gray; has perfect rhombohedral cleavage; appears pearly to vitreous; and effervesces feebly in cold dilute hydrochloric acid.
All rock material transported by a glacier and deposited either directly by the ice or reworked and deposited by meltwater streams and/or the wind.
A 10,000-square-mile area in northeastern Iowa, southwestern Wisconsin, and northwestern Illinois where the absence of glacial drift suggests that the area may not have been glaciated.
An adjective describing geologic features that are in an overlapping or staggered arrangement. Each is relatively short but collectively they form a linear zone.
A ridge or series of ridges formed by accumulations of drift built along the outer margin of an actively flowing glacier at any given time; a moraine that has been deposited at the lower or outer end of a glacier.
An interval of geologic time; a division of a period. (Example: Pleistocene Epoch).
A unit of geologic time that is next in magnitude beneath an eon; consists of two or more periods. (Example: Paleozoic Era).
A long, more or less continuous cliff or steep slope facing in one general direction; it generally marks the outcrop of a resistant layer of rocks, or the exposed plane of a fault that has moved recently.
A fracture surface or zone of fractures in Earth materials along which there has been vertical and/or horizontal displacement or movement of the strata on opposite sides relative to one another.
A mnemonic adjective derived from feldspar + lenad (feldspathoid) + silica + c, and applied to an igneous rock having abundant light-colored minerals in its mode; also, applied to those minerals (quartz, feldspars, feldspathoids, muscovite) as a group. It is the complement of mafic.
Containing iron and magnesium; applied to mafic minerals.
[sed] Any sediment deposited by an agent so as to fill or partly fill a valley, a sink or other depression.
A general term for the property possessed by some rocks of splitting easily into thin layers along closely spaced, roughly planar, and approximately parallel surfaces, such as bedding planes in shale or cleavage planes in schist; its presence distinguishes shale from mudstone. The term includes such phenomena as bedding fissility and fracture cleavage.
A relatively wide planar opening in bedrock that originated as a fracture or fault. The opening may be partially or totally filled with soil or, if open, can act as a conduit for flowing water.
Said of rock that tends to split into layers of suitable thickness for use as flagstone.
The surface or strip of relatively smooth land adjacent to a stream channel produced by the stream's erosion and deposition actions; the area covered with water when the stream overflows its banks at times of high water; it is built of alluvium carried by the stream during floods and deposited in the sluggish water beyond the influence of the swiftest current.
Of or pertaining to a river or rivers.
(a) A very common mineral of the apatite group: Ca5(PO4) 3F. It is a common accessory mineral in igneous rocks. Syn: apatite. (b) An apatite mineral in which fluorine predominates over chlorine and hydroxyl.
Sedimentary deposits formed by a combination of fluvial (river) and lacustrine (lake) conditions.
The basic rock unit distinctive enough to be readily recognizable in the field and widespread and thick enough to be plotted on a map. It describes the strata, such as limestone, sandstone, shale, or combinations of these and other rock types. Formations have formal names, such as shale, or combinations of these and other rock types formations have formal names, such as Joliet Formation or St. Louis Limestone (Formation), generally derived from the geographic localities where the unit was first recognized and described.
Any remains or traces of a once-living plant or animal preserved in rocks (arbitrarily excludes Recent remains); any evidence of ancient life. Also used to refer to any object that existed in the geologic past and for which evidence remains (for example, a fossil waterfall).
Said of a rock or mineral that crumbles naturally or is easily broken, pulverized, or reduced to powder, such as a soft and poorly cemented sandstone.
Ordovician age groups of rock which are largely dolomite with a shaly zone near the middle and some limestone beds in the lower portion.
Computer software used to input, store, retrieve, manipulate, analyze and output geographically referenced data or geospatial data, often in the form of maps, in order to support decision making for planning and management of land use, natural resources, environment, urban facilities, transportation, and other administrative records.
The study of the planet Earth that is concerned with its origin, composition, and form; its evolution and history; and the processes that acted (and act) upon it to control its historic and present forms.
Study of the Earth with quantitative physical methods. Application of the principles of physics to the study of the earth, especially its interior.
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.
A large, slow-moving mass of ice formed on land by the accumulation, compaction, and recrystallization of snow.
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.
A part of a surface feature of the Earth that slopes upward or downward; the angle of slope, as of a stream channel or of a land surface, generally expressed by a ratio of height versus distance, a percentage or an angular measure from the horizontal.
A plutonic rock in which quartz constitutes 10 to 50 percent of the felsic components and in which the alkali feldspar/total feldspar ratio is generally restricted to the range of 65 to 90 percent.
Water present below the water table in small, often microscopic, interconnected pore spaces between grains of soil, sand and/or gravel, and in open fractures and/or solution channels in rock.
Said of a rock or mineral that solidified from molten or partly molten material, (that is from magma).
Pertaining to the classical third glacial stage of the Pleistocene Epoch in North America, between the Yarmouthian and Sangamonian interglacial stages.
Said of a compact rock or soil hardened by the action of pressure, cementation, and especially, heat.
Cambrian age sandstone formations occurring under northern and central Illinois which are clean, white, coarse to fine grained. Neither outcrop in Illinois. Thin dolomite beds may occur in upper part of the Ironton.
A fracture or crack in rocks along which there has been no movement of the opposing sides (see also Fault).
Collective term for the land forms and subterranean features found in areas with relatively thin soils underlain by limestone or other soluble rocks; characterized by many sinkholes separated by steep ridges or irregular hills. Tunnels and caves formed by dissolution of the bedrock by groundwater honeycomb the subsurface. Named for the region around Karst in the Dinaric Alps of Croatia where such features were first recognized and described.
Pertaining to the classical second glacial stage of the Pleistocene Epoch in North America, after the Aftonian interglacial stage and before the Yarmouthian.
An aquifer whose porosity and permeability is dominated by connected conduits (for example, joints, fractures, caves, tubes) that were enlarged by dissolution of rock. Karst aquifers have extremely rapid recharge and relatively large hydraulic conductivities (greater than 10-4 cm/s) and a turbulent groundwater flow regime (as opposed to laminar flow).
An area or region of the surface of the earth whose landscape is characterized by sinkholes, caves, springs, disrupted land drainage, and an underground drainage system. Karst terrains form in areas with carbonate rock (limestone and dolomite), and areas underlain by other types of soluble rock (for example, salt or gypsum).
The Keokuk Limestone is named for Keokuk, Lee County, Iowa. At the type locality about 70 feet of Keokuk Limestone overlying the Burlington Limestone is well exposed along Soap Creek and in the Mississippi River bluff near the mouth of the creek. Like the Burlington, the Keokuk is primarily a biocalcarenite. In the type region the lower 30 feet is very cherty and is differentiated as the Montrose Chert Member. The part of the Keokuk above the Montrose is composed of beds of fossiliferous, crinoidal limestone interbedded with fine-grained limestone, argillaceous dolomite, and calcareous gray shale. The limestone is light gray, speckled with darker gray, brown, or black, and contains beds and nodules of chert. It is generally thinner bedded and darker than limestone of the Burlington, and the shale partings are more numerous. In contrast to the crinoidal limestone of the Burlington, the Keokuk shows a great heterogeneity of skeletal remains, with abundant bryozoans, corals and brachiopods.
Produced by or belonging to a lake.
A protocontinent of the Northern Hemisphere, corresponding to Gondwana in the Southern Hemisphere, from which the present continents of the Northern Hemisphere have been derived by separation and continental displacement. The supercontinent from which both were derived is Pangea. Laurasia included most of North America, Greenland, and most of Eurasia, excluding India. The main zone of separation was in the North Atlantic, with a branch in Hudson Bay; geologic features on opposite sides of these zones are very similar.
The LaSalle Anticlinal Belt is the most prominent anticlinal feature in the Illinois Basin. It is actually a complex structure of en echelon folds, asymmetrical anticlines, and monoclines stretching from Stephenson County in northern Illinois through Lawrence County in southeastern Illinois.
Molten, fluid rock that is extruded onto the surface of the Earth through a volcano or fissure. Also the solid rock formed when the lava has cooled.
A sedimentary rock consisting primarily of calcium carbonate (the mineral, calcite). Limestone is generally formed by accumulation, mostly in place or with only short transport, of the shells of marine animals, but it may also form by direct chemical precipitation from solution in hot springs or caves and, in some instances, in the ocean.
To change to stone, or to petrify; especially to consolidate from a loose sediment to a solid rock.
The description of rocks on the basis of color, structure, mineral composition, and grain size; the physical character of a rock.
The vertical difference in elevation between the highest and lowest points of a land surface within a specified horizontal distance or in a limited area.
A homogeneous, unstratified accumulation of silt-sized material deposited by the wind.
Said of an igneous rock composed chiefly of one or more ferromagnesian, dark-colored minerals in its mode; also, said of those minerals. The term was proposed by Cross, et al. to replace the term femag, which they did not consider to be euphonious. Etymol: a mnemonic term derived from magnesium + ferric + ic. It is the complement of felsic.
Naturally occurring molten rock material generated within Earth and capable of intrusion into surrounding rocks or extrusion onto the Earth's surface. When extruded on the surface it is called lava. The material from which igneous rocks form through cooling, crystallization, and related processes.
The Maquoketa Shale Group is Ordovician in age, with green to blue shale with limestone and dolomite beds in lower portion. The group has been eroded over part of northern Illinois, but underlies most of the rest of the state.
One of a series of somewhat regular, sharp, sinuous curves, bends, loops, or turns produced by a stream, particularly in its lower course where it swings from side to side across its valley bottom.
Crescent-shaped, swales and gentle ridges along a river's flood plain that mark the positions of abandoned part of a meandering river's channel. They are generally filled in with sediments and vegetation and are most easily seen in aerial photographs.
An era of geologic time, from the end of the Paleozoic to the beginning of the Cenozoic, or from about 248 to about 65 million years ago.
Any rock derived from pre-existing rocks by mineralogical, chemical, and structural changes, essentially in the solid state, in response to marked changes in temperature, pressure, shearing stress, and chemical environment at depth in Earth's crust (for example, gneisses, schists, marbles, quartzites, etc.).
A naturally formed chemical element or compound having a definite chemical composition, an ordered internal arrangement of its atoms, and characteristic crystal form and physical properties.
A period of the Paleozoic era (after the Devonian and before the Pennsylvanian), thought to have covered the span of time between 354 and 323 million years ago; also, the corresponding system of rocks. It is named after the Mississippi River valley, where rocks of this age are well exposed. It is the approximate equivalent of the Lower Carboniferous of European usage.
A local steeping of an otherwise uniform gentle dip.
(a) A piece of unfractured bedrock, generally more than a few meters across. (b) A large upstanding mass of rock
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)
The scientific study of form, and of the structures and development that influence form; term used in most sciences.
Cambrian sandstone with with a few thin red shale beds.
Pertaining to the first classical glacial stage of the Pleistocene Epoch in North America, followed by the Aftonian interglacial stage.
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.
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.
An unconformity resulting from deposition of sedimentary strata on massive crystalline rock.
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.
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.
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.
Stratified glacially derived sediment (clay, silt, sand, gravel) deposited by meltwater streams in channels, deltas, outwash plains, on flood plains, and in glacial lakes.
The surface of a broad body of outwash formed in front of a glacier.
The loose soil, silt, sand, gravel or other unconsolidated material overlying bedrock, either transported or formed in place.
A crescent-shaped lake in an abandoned bend of a river channel. A precursor of a meander scar.
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.
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 present widely separated continents, Pangea was split into two large fragments, Laurasia on the north and Gondwana in the southern hemisphere.
A naturally formed unit of soil structure, (for example, granule, block, crumb, or aggregate).
A land surface of regional scope worn down by erosion to a nearly flat or broadly undulating plain.
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 Upper Carboniferous of European usage.
An interval of geologic time; a division of an era (for example, Cambrian, Jurassic, Tertiary).
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, where rocks of this age were first studied.
The study and classification of the surface features of Earth on the basis of similarities in geologic structure and the history of geologic changes.
(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.
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.
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.
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.
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.
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 than a period, with the epochs considered to be periods, and it is still sometimes used as such in the geologic literature. The Quaternary may also be incorporated into the Neogene, when the Neogene is designated as a period of the Tertiary era.
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).
(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). (b) The vertical difference in elevation between the hilltops or mountain summits and the lowlands or valleys of a given region; high relief has great variation; low relief has little variation.
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).
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 particles usually consist of quartz, and the term sandstone, when used without qualification, indicates a rock containing about 85-90% quartz (Krynine, 1940). The rock varies in color, may be deposited by water or wind, and contains numerous primary features (sedimentary structures and fossils). Sandstones may be classified according to composition of particles, mineralogic or textural maturity, fluidity index, diastrophism, primary structures, and type of cement (Klein, 1963). (b) A field term for any clastic rock containing individual particles that are visible to the unaided eye or slightly larger.
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 (for example, sand, gravel, silt, mud, till, loess, alluvium).
A rock resulting from the consolidation of loose sediment that has accumulated in layers (e.g., sandstone, siltstone, limestone).
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 by an appreciable content of clay minerals and detrital quartz; a thinly laminated or fissile claystone, siltstone, or mudstone. It normally contains at least 50% silt, with 35% clay or fine mica fraction and 15% chemical or authigenic materials. Shale is generally soft but sufficiently indurated so that it will not fall apart on wetting; it is less firm than argillite and slate, commonly has a splintery fracture and a smooth feel, and is easily scratched. Its color may be red, brown, black, or gray.
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.
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.
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 a hydrologically active sinkhole is a drain that allows any water that flows into the sinkhole to flow out the bottom into an underground conduit.
Long, low, gentle slope on the inside of a stream meander. The slope on which the sand that forms point bars is deposited.
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 the ground surface or structures on the surface may collapse into the cavity.
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 in Hardin County. The St. Louis is 500 feet thick in southeastern Illinois and thins north-westward to less than 200 feet before being truncated by pre-Pennsylvanian erosion. The St. Louis Limestone in Illinois is characterized by fine-grained, micritic to lithographic, cherty limestone, but it contains beds of dolomite, crystalline limestone, fossiliferous limestone, and evaporites.
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).
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.
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.
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)
A small interval of geologic time; a division of an age.
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).
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).
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.
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.
A borehole log, run only in water-filled boreholes, that measures the water temperature and the quality of groundwater in the well.
An abandoned flood plain formed when a stream flowed at a level above the level of its present channel and flood plain.
Sediment eroded from the land, or a continent, and deposited in water (generally in a marine environment).
Unconsolidated, nonsorted, unstratified drift deposited by and underneath a glacier and consisting of a heterogenous mixture of different sizes and kinds of rock fragments.
The undulating surface of low relief in the area underlain by ground moraine.
The natural or physical surface features of a region, considered collectively as to form; the features revealed by the contour lines of a map.
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.
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.
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.
Nonlithified sediment that has no mineral cement or matrix binding its grains.
The accumulations of outwash deposited by rivers in their valleys downstream from a glacier.
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 Chouteau Limestone upward to the base of the Shetlerville Member of the Renault Limestone. The series is thickest, over 1800 feet, in southeastern Illinois, and it thins to 600 feet or less before being truncated by erosion in northern Illinois.
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 feet in the outcrop area, where it consists of gray shale containing beds of argillaceous limestone. Quartz geodes are common and locally abundant; some are replacements of fossils. Some contain petroleum. The Warsaw is fossiliferous, with brachiopods, bryozoans, and crinoids especially common.
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.
The group of processes, chemical and physical, whereby rocks on exposure to the weather, change in character, decay, and finally crumble into soil.
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.
Updated 05/27/2011 SLD