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Glossary of Geologic Features/Terms

Applitic: Similar to applite, a light-colored igneous rock characterized by a fine-grained granular texture.

Basalt: A dark, fine-grained igneous rock.

Biotite: Black mica that is a common rock-forming mineral.

Cave: A natural rock shelter typically formed by plucking action of glacial ice. May also be formed by dissolution of soft minerals comprising rocks like limestone (though not common in Connecticut).

Cataclastic: Rock texture produced by severe mechanical stress applied during metamorphism.  Cataclastic rocks are often composed of angular fragments cemented in a matrix of fine rock powder.  Both the fragments and powder are created as a result of bending, breaking, and granulation of the initial minerals present.

Chemical Weathering: The process of weathering by which chemical reactions (hydrolysis, oxidation, carbonation, ion exchange, and solution) transform rocks and minerals into new chemical combinations that are stable under conditions prevailing at or near the Earth's surface.

Competence: A term used to compare the resistance to deformation among different rock types.  Less competent rock types weather more quickly.

Cliffed Headland:  A headland characterized by a cliff, such as one formed by erosion during development of an embayed coast.

Differential Weathering: Weathering that occurs at different rates because of variations in composition.

Dike: A sheet-like body of igneous rock that cuts across layering or contacts in the rock into which it intrudes.

Drumlin: An elongate, elliptically shaped hill composed of till, formed by glacial deposition.

Eddy: A circular current.  Generally found downstream of marine currents, but also occurs in terrestrial streams at smaller scales.

Erosion: The general process by which the materials of Earth's crust are loosened, dissolved, or worn away, and simultaneously moved from one place to another by natural agencies.  These agencies generally include weathering, dissolution, corrosion, and transportation.  The term is sometimes limited to only the removal of earth materials, excluding further transportation.

Escarpment: A long cliff or relatively steep slope facing one general direction, produced by faulting or erosion, and separating two levels or gently sloped surfaces into tiers.  Sometimes marking the outcrop of a more competent layer between softer strata.

Esker: A long, narrow ridge composed of stratified sand and gravel, which was deposited by a glacial stream beneath or within a glacier.

Faulting: The process of initial fracturing, slip along the fracture plane, and displacement accumulated over multiple events, that produces a fault.

Fold: Usually the result of structural deformation, a fold is a curve or bend of a planar structure.

Garnet: The Connecticut state mineral. Garnets are usually red and have a glassy luster.

Geologic Time Scale: Used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of the Earth.

Geology: The science and study of the Earth, its composition, structure, physical properties, history, and the processes that shape it.

Glacial Erratic: A rock fragment carried by glacial ice. Erratics are generally deposited on bedrock of differing lithology.

Glacial Plucking: The movement of rock fragments caused by the freezing of water along joints.

Glacial Polishing: A high luster rock surface caused by the movement of glaciers.

Gneiss: (pronounced "nice") A high-grade metamorphic rock subjected to intense heat and pressure during formation. Gneiss is easily identifiable by the segregation of light and dark minerals giving it a banded texture. Gneiss usually consists of mostly elongated and granular, as opposed to platy, minerals.

Headland (coastal):  A steep area of considerable height jutting out from the coast into a large body of water.

Igneous Rocks: One of the three rock types (see Metamorphic Rocks and Sedimentary Rocks). Produced by the solidification of molten magma from the mantle. Igneous rocks can be intrusive, solidifying beneath the Earth’s surface, or extrusive, solidifying at the Earth's surface.

Karst: A type of topography that is formed primarily by dissolution of rock, often limestone and other soluble rock types.  Karst is characterized by sinkholes, caves, and underground drainage.

Metamorphic Rocks: One of the three rock types (see Igneous Rocks and Sedimentary Rocks). Metamorphic rocks are formed when existing rock is chemically or physically modified by intense heat or pressure.

Mine: A subterranean excavation of mineral deposits.

Mineral: A naturally occurring inorganic solid that has a crystalline structure, a distinct chemical composition, crystal form, and set of physical properties.

Moraine: A glacially generated ridge like landform of unsorted deposits formed along glacial margins or at the ice front upon melting.

Muscovite: A type of mineral. Muscovite is a white mica found in many metamorphic rocks.

Pegmatite: A type of igneous rock. Pegmatite forms from molten rock buried deep below the surface of the Earth. Since the molten rock is well insulated beneath the surface of the Earth, it cools very slowly, allowing the crystals to grow very large. Pegmatite intrusions usually have the same composition as granite, only coarser grained.

Physical Weathering: The process by which frost action, salt-crystal growth, absorption of water, and other physical processes break a rock into fragments, involving no chemical change.

Pothole: A smooth, bowl shaped or cylindrical hollow, generally deeper than wide, formed in the rocky bed or a stream by the grinding action of stones whirled around and kept in motion by eddies or the force of the stream current in a given spot.

Quarry: A surficial excavation of mineral or sediment deposits.

Quartz: A common rock-forming mineral. Quartz has a glassy luster and can be an array of colors.

Recessional Moraine:  An unsorted ridge like glacial landform depositng during a significant period of glacial retreat.

Rock: A solid substance that occurs naturally because of the effects of three basic geological processes: magma solidification, sedimentation of weathered rock debris, and metamorphism. There are three different rock types: igneous, metamorphic, and sedimentary.

Rockshelter: A natural overhang on the side of a rocky slope or cliff that forms a protected shelter. Rockshelters are typically created by long-term geological forces such as water and wind erosion or glacial action.

Schist: A type of metamorphic rock that has undergone intense heat, pressure, and hot fluids. By definition, schist contains more than 50% platy and elongate minerals such as mica and amphibole. This high percentage of platy minerals allows schist to be easily split into thin flakes or slabs.

Sedimentary Rocks: One of the three rock types (see Igneous Rocks and Metamorphic Rocks). Sedimentary rocks are formed by burial, compression, and chemical modification of deposited weathered rock debris or sediments at the Earth's surface.

Sill: A sheet-like igneous intrusion that forms parallel to the orientation of the host rock.

Stratigraphy: The study and interpretation of strata, or discrete layers of rock.  It is concerned with succession and age relations of strata, and also form, composition, fossil content, geochemistry, spatial relationships, and all other properties of strata.  All classes of rocks and unconsolidated sediment are considered within the general scope of stratigraphy.

Stratum: A sheet-like body or layer of sedimentary rock, visually separable from other layers above and below.  Can be used to describe a layer varying from centimeters to meters thick.  Plural: strata.

Stream Gradient: The angle between the water surface or the channel floor, and the horizontal measured in the direction of the flow.  The slope of a stream.

Tectonics: Geology subdiscipline dealing with the architecture of the Earth's surface, such as regional assembly of structural and deformational features, their mutual relations, origins, and historical evolution.  Related to structural geology with larger scale features.

Till: Sediment deposited by a glacier, till is a mixture of clay, silt, sand, gravel, and boulders ranging greatly in size and shape.

Transgressing Sea: The advance of the sea over land areas.

Turbulent Flow: Water flow in which flow lines are confused and heterogeneously mixed.  It is typical of flow in surface-water bodies.  Opposite would be laminar flow, in which all currents are parallel to the flow direction.

Vein: A thin, sheetlike igneous intrusion.

Content last updated June 30, 2020