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  • 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.
  • 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.
  • 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.
  • (Plural: strata) 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).
  • (Plural: striae) Also called striation Geology any of the parallel scratches or grooves on the surface of a rock caused by abrasion resulting from the passage of a glacier, motion on a fault surface, etc.
  • Formed, or occurring beneath a glacier or other body of ice