DEEP: Glossary of Geologic Features/Terms

Glossary of Geologic Features/Terms

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).

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.

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

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.

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.

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.

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.

Vein: A thin, sheetlike igneous intrusion.