The Geology of Mount Desert Island

A Visitor's Guide to the Geology of Acadia National Park

Glossary of Geologic Terms

Abrasion: The grinding or wearing away of rock surfaces caused by the scraping action of rock fragments frozen into the base of a glacier.

Albite: Sodium-rich (Na) feldspar, NaAlSi3O8.

Alpine glaciers: A medium to small glacier that forms in a mountain range and flows down valleys.

Basal till: A compact mixture of sediment, ranging in size from clay to boulders, deposited directly from the bottom of a glacier.

Basalt: A dark, fine-grained igneous rock that is erupted onto the surface as a lava flow.

Beds: Individual layers of a sedimentary rock.

Cauldron subsidence: The process in which a more or less cylindrical block of rock above a magma chamber collapses into the space left as the magma moves toward the surface.

Continental drift: The theory suggesting that rigid continental and oceanic plates move over the surface of the globe, producing many of the large scale geologic features (such as mountain ranges) we see today.

Continental glaciers: A glacial ice sheet of considerable thickness which covers a sizable portion of a continent and obscures most of the underlying terrain. The modern Antarctic ice sheet is a good example.

Correlative: A term applied to two or more rock units of similar age and possibly similar origin.

Country rock: The rock into which a mass of igneous rock was intruded.

Crust: The outermost, least dense layer of the earth. Includes continental crust and oceanic crust.

Deglaciation: The disappearance of an ice sheet through melting or by breaking off of icebergs.

Delta: A deposit of sand and gravel, often triangular and fan shaped, formed at the mouth of a river or stream where the current becomes too slow to carry the sediment.

Diabase: A dark gray to black rock with the composition of basalt that solidifies beneath the surface as thin dikes or sills.

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

Diorite: A coarse-grained intrusive igneous rock intermediate between granite and gabbro in silica and iron and magnesium content.

Drift: A general term applied to all rock material transported by glacial ice and deposited either directly from a glacier or by meltwater streams.

Downdraw: The process by which the ice flow of a glacier accelerates and the surface of the ice drops rapidly.

End moraine: Ridges of till and/or sand and gravel formed at the margin of a glacier.

Erratic: A glacially-transported rock that has been deposited some distance away from its point of origin and now rests on bedrock of a different type.

Exfoliation: The process by which horizontal to near-horizontal fractures cut rock into sheets, due to pressure release caused by erosion of overlying rock material. Very common in granites.

Extrusive: Term applied to igneous rocks that erupt onto the surface as flows or into the air as ash.

Fault: A fracture in rock along which there has been substantial movement, either horizontally or vertically.

Feldspar: A family of common silica and alumina rich rock-forming minerals. Includes plagioclase (calcium-feldspar), albite (sodium-feldspar), K-feldspar (potassium-feldspar), and perthite.

Felsite: A fine-grained, generally light-colored intrusive or extrusive igneous rock forming dikes, sills, or flows.

Foliation: A general term for the sub-parallel, planar arrangement of platy minerals such as micas. Typically best developed in metamorphic rocks such as the Ellsworth Schist.

Gabbro: Silica-poor, iron and magnesium rich intrusive igneous rock. Generally coarse-grained and dark in color.

Geologic map: A map which shows the distribution of rocks or surficial materials and geologic structures of a region as if all soil and vegetation were stripped away.

Geologic time: The time extending from the beginning of the earth as a separate planetary body to the beginning of written history; implies extremely long duration or remoteness in the past: approximately 4.5 billion years.

Glacier: A large mass of ice formed by the compaction and recrystallization of snow and showing evidence of past or continuing flow.

Graded bedding: Sedimentary layering which displays a gradual change in particle size 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: A silica-rich, iron and magnesium poor intrusive igneous rock of medium grain size composed primarily of quartz and feldspar. Generally light in color.

Igneous rock: Rock that solidified from molten rock material or magma.

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

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

Joint sets: Sub-parallel joints oriented in clearly definable directions.

K-feldspar: A type of feldspar containing potassium (K), an essential mineral in granites.

Magma: A mixture of molten rock and mineral grains which may be erupted as volcanic material or cool slowly at depth to produce intrusive rocks (such as granite).

Mantle: The middle layer of the earth, lying below the crust. Denser than the crust; rigid plates of oceanic and continental crust move over a partially molten zone in the mantle. See plate tectonics.

Marine clay: A deposit of very fine rock and mineral particles that accumulated on the ocean floor.

Matrix: The finer-grained part of an igneous rock in which larger crystals (phenocrysts) occur. Also the light colored rock that encloses the dark blocks in the shatter zone.

Meltwater channels: Gulley cut into bedrock by water flowing beneath or from a glacier.

Metamorphic rocks: Rocks whose original mineral composition or texture has been changed by recrystallization at high temperatures and pressures, but have 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.

Orogeny: The process whereby mountain belts are formed.

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

Peat: A deposit of partially decomposed plant remains that accumulates in a wetland environment such as a marsh or heath.

Perthite: A mineral made up of intergrown albite (sodium-feldspar) and K-feldspar (potassium-feldspar). A common constituent of granites.

Phenocrysts: Large, conspicuous crystals surrounded by fine-grained material in a volcanic rock or dike.

Plagioclase: Calcium (Ca) feldspar, CaAl2Si2O8.

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.

Plucking: A glacial erosion process in which fragments or blocks of bedrock are first loosened by the expansion of freezing water in fractures and joints and then picked up and removed as the overlying glacial ice advances.

Quarrying: Same as plucking.

Rebound: The rise of the earth's crust in response to the removal of substantial thicknesses (and weight) of ice.

Roche moutonnée: A small elongate bedrock knob with a distinctive asymmetric profile, usually a few meters in height, length and breadth. The up-ice surface (which faced into the flowing ice) is gently inclined and rounded, and the down-ice side is steep and rough.

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

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

Sea stack: A pillar-shaped bedrock pinnacle resulting from wave erosion along the ocean shore.

Sediment: Unconsolidated deposits of sand, silt, clay, etc., deposited and transported by water, ice, or wind.

Sedimentary rocks: Rocks 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.

Sills: Thin, tabular sheets of igneous rock parallel or sub-parallel to the bedding in the sedimentary rocks they intrude.

Stratified: A term describing rocks formed or arranged in layers or beds.

Striations: Parallel grooves on bedrock surfaces produced by the abrasive action of rock fragments frozen in the base of an actively moving glacier.

Talus: Angular rock fragments of any size (usually boulders) lying at the base of a steep rock cliff.

Till: A glacial deposit consisting of a mixture of clay, silt, sand, gravel, and boulders that was deposited directly from glacial ice.

Unconformity: A gap in the geologic record, resulting from a period of erosion or non-deposition between an older rock unit and the younger, overlying rock strata.

U-shaped valleys: A valley whose cross-sectional profile is similar to the letter "U". It has steep sides and a relatively flat bottom. This type of profile is typical of glacially carved valleys where the long axis of the valley is parallel to the former direction of ice flow.

Volcanic tuff: A term describing rocks composed of volcanic ejecta, such as broken pieces of volcanic glass, phenocrysts, rock fragments, etc.

Whaleback: A large mound or hill with a distinctive asymmetric profile. The up-ice surface (which faced into the flowing ice) is gently inclined and rounded, and the down-ice side is steep and rough.

Introduction   Bedrock   Stratified Rocks   Igneous Rocks   Structure   Schoodic   Isle au Haut   Bedrock History   Glacial   Erosion   Retreat   Glacial History   Processes   Conclusion   Reading   Glossary   Maps

Last updated on January 11, 2008