The Geology of the Two Lights and Crescent Beach State Parks Area, Cape Elizabeth, Maine

Glossary of Geologic Terms

Amphibolite: A dark metamorphic rock composed mostly of amphibole, an iron-magnesium bearing mineral.

Anticline: An upwardly convex fold of layered rocks.

Avalonia: An ancient land mass that formed the eastern edge of the Iapetus Ocean during Cambrian, Ordovician, Silurian, and Early Devonian time.

Axial plane: In folded rocks, an imaginary plane that passes through all points of maximum curvature of beds in an anticline or syncline.

Axial plane cleavage: Closely-spaced rock fractures oriented parallel to the axial planes of anticlines and synclines.

Axis: In folded rocks, an imaginary line extending along the points of maximum curvature of a bed in the crest of an anticline or the trough of a syncline.

Basalt: A dark fine-grained igneous rock containing little quartz. May be extruded at the surface as a lava flow or solidify beneath the surface as thin dikes or sills.

Bed: An individual layer of a sedimentary rock.

Bedding: The layering characteristic of sedimentary rocks and metamorphic rocks derived from sedimentary rocks.

Bedrock: The solid rock that underlies unconsolidated surficial materials.

Biotite: A common, iron-rich black mica found in a wide variety of rocks.

Chlorite: A pale-green, iron-magnesium-aluminum rich mineral with a platy form similar to mica.

Cleavage: The tendency of certain metamorphic rocks to break along a set of closely-spaced parallel fractures. Particularly characteristic of slate and phyllite.

Convection current: A circulation pattern in which warmer material rises and colder material sinks. Thought to occur in the mobile part of the earth's mantle due to heat produced by disintegration of radioactive elements deep in the earth's interior.

Dike: A thin tabular igneous rock body that cross-cuts the bedding of the rock that it intrudes.

Eustatic: Referring to sea level fluctuations due to glacial buildup or melting. Glacial buildup subtracts water from the ocean and results in lowering of sea level; glacial melting results in the return of water to the ocean causing a rise of sea level.

Fault: A fracture of the earth's crust along which movement occurs. Earthquakes are most commonly caused by such movements.

Feldspar: The most abundant mineral in the earth's crust. Composed of aluminum, silica, and sodium, potassium, or calcium.

Formation: A regionally distinctive sequence of sedimentary or metamorphic rocks that is extensive enough to be mapped.

Garnet: A red or red-brown aluminum-rich iron-magnesium silicate mineral.

Glacial erratic: A boulder picked up and transported by glacial ice. The rock type of the boulder differs from that of the underlying bedrock.

Gneiss: A metamorphic rock whose minerals tend to be segregated into bands.

Graded bedding: Sedimentary layering which displays a gradual change in particle size, usually from coarse particles at the base of the bed to fine particles at the top. At the top of each bed there is an abrupt change back to the coarse particles of the next bed.

Granite: An intrusive igneous rock of medium grain size composed of quartz and feldspar.

Graphite: A crystalline form of the element carbon. Characterized by a gray-black color and soft, greasy texture.

Hinge: The point of maximum curvature or bending of a fold.

Iapetus: An ocean that once extended through the New England area from Late Cambrian to Early Devonian time. Subduction of ocean lithosphere closed the ocean by Early Devonian time.

Igneous rock: A rock that has solidified from molten rock material or magma.

Intrusive rock: An igneous rock that solidified from molten rock material that was injected into older rocks below the earth's surface.

Island arc: A chain of volcanic islands adjacent to a deep ocean trench where subduction is occurring. Melting of descending oceanic lithosphere provides the molten rock material to build the volcanic islands.

Joints: Planar fracture surfaces in rock along which there has been no movement.

Limb: The sloping portion of a fold between the axial planes of a syncline and anticline.

Limestone: A sedimentary rock consisting chiefly of calcium carbonate. Formed in marine waters by organic or inorganic processes.

Lithosphere: The solid outer shell of the earth consisting of the earth's crust and solid upper mantle.

Lodgement till: Compact unstratified, unsorted glacial sediments deposited at the base of moving glacial ice.

Magma: Molten rock material.

Mantle: The shell of the earth between the crust and the core. The upper part of the mantle is rigid, whereas an intermediate zone is quite mobile and contains convection currents. The lowest part of the mantle is solid.

Metalimestone: Metamorphosed limestone.

Metamorphic rock: A rock whose original mineral composition or texture has been changed by recrystallization at high temperatures and pressures, but has not melted.

Metamorphism: A process where rocks are recrystallized by heat and pressure in the earth's crust. Metamorphism is a process that usually accompanies orogeny.

Mica: A group of silicate minerals characterized by a plate-like form. Two common varieties are biotite and muscovite.

Mineral: A naturally occurring inorganic element or compound, having a regular internal crystalline structure.

Muscovite: A common, iron-free white mica found in a wide variety of metamorphic and some igneous rocks.

Orogeny: The process whereby the deformation structures - folds, thrust, faults, etc. - of mountain belts are formed.

Outcrop: An exposure of bedrock at the earth's surface.

Outwash: Stratified sand and gravel deposited by meltwater streams in front of a glacier.

Pangea: A hypothetical supercontinent thought to have existed during Permian time and to have included all continental masses. Pangea broke up in Jurassic time, forming the present continents which reached their present positions as a result of sea-floor spreading.

Pegmatite: A very coarse-grained igneous rock commonly of granitic composition. Frequently contains semi-precious minerals such as topaz, beryl, and tourmaline.

Phyllite: A soft, fine-grained metamorphic rock composed of mica and characterized by a lustrous and crinkled cleavage.

Plate Tectonics: The theory and study of the formation, movement, and interaction of the rigid plates that make up the earth's crust and upper mantle.

Pyrite: A common iron sulfide mineral sometimes known as "fool's gold" because of its metallic brass-yellow color.

Quartz: One of the most common minerals. Composed of silicon and oxygen, and typically clear or slightly milky or smoky.

Quartzite: A very hard metamorphic rock composed almost entirely of the mineral quartz. Before metamorphism quartzites were quartz-rich sandstones.

Recumbent fold: A fold with a horizontal axial plane. A recumbent fold is one which might be characterized as "lying on its side."

Rift basin: A basin formed when a continental plate breaks apart.

Rock flour: A glacial sediment composed of finely ground rock formed by abrasion at the base of the glacier.

Sandstone: A sedimentary rock composed of sand-sized particles, commonly quartz or feldspar.

Schist: A coarse-grained mica-rich metamorphic rock characterized by aligned mica flakes.

Sea-floor spreading: A hypothesis that oceanic lithosphere forms at mid-ocean ridges and spreads laterally away from these ridges. It is thought to be the principal means of continental movement.

Sedimentary rock: A rock formed by the accumulation and cementation of mineral grains transported by water (and less commonly by wind or ice), or by the precipitation of minerals from water.

Shale: A sedimentary rock composed of silt and clay sized particles.

Silicified zone: A narrow zone, usually along faults or joints, where quartz has been extensively injected.

Strata: Layers of sedimentary rocks.

Striae, striations: Parallel grooves on bedrock surfaces produced by the abrasive action of pebbles frozen into the base of an actively moving glacier.

Subduction: The process whereby one lithosphere block descends diagonally under another block. A result of sea-floor spreading.

Surficial sediments: Unconsolidated sedimentary material such as gravel, silt, sand, clay or till.

Syncline: A downwardly convex fold of layered rocks.

Tectonic: Relating to large-scale structural features of the earth and their origin.

Till: Unsorted and unstratified glacial sediments.

Turbidity currents: A sediment-laden current that moves rapidly down the surface of a submarine slope.

Volcanic vent: The opening of a volcano through which lava, gas, ash and dust are emitted.

Weathering: The physical or chemical breakdown of rock.

Preface   Introduction   Rocks   Geologic History   Glaciation   Conclusion   Glossary

Last updated on January 16, 2008