Glossary of Terms Used in the Geologic Classification

The following listing defines the terms that appear in the geologic classification system shown in Table 1. The codes used in the classification system are shown in parentheses. When the classification number given contains an "X" (e.g., 1XX) it is implied that the definition is valid for all subsets of the given geologic feature. This list defines only those rock types mentioned within Table 1 and should not be construed as a comprehensive set of geologic definitions.

IGNEOUS ROCK (1XX) - Rock that has crystallized from a silicate melt at high temperatures (i.e., 900 to 1600°C).

VOLCANIC (EXTRUSIVE) ROCK (OLD=11X) (NEW=4XX) - Igneous rock that has reached the Earth's surface as a result of eruptive processes in a molten or partially molten state. Since these rocks tend to cool rapidly they are usually fine-grained.

ANDESITE (110) - Grayish fine-grained volcanic rock composed of oligoclase/andesine (plagioclase feldspar), with lesser amounts of hornblende, biotite, or pyroxene. Potassium feldspar and quartz compose less than 10% of the total mineral content (plutonic equivalent is quartz diorite).

BASALT (110) - Dark fine-grained volcanic rock consisting of labradorite (plagioclase feldspar) and augite (pyroxene), with minor olivine (plutonic equivalent is gabbro).

RHYOLITE (110) - Light fine-grained volcanic rock composed essentially of alkali feldspar and quartz, with minor biotite occasionally present (plutonic equivalent is granite).

PLUTONIC (INTRUSIVE) ROCK (13X) - Igneous rock which has crystallized from molten material (magma) at depth and has reached the Earth's surface through uplift and erosion. Because cooling is generally slower, these rocks are coarser-grained than their volcanic equivalents.

METAMORPHIC ROCK (15X) - Rock derived from preexisting materials (either igneous, sedimentary, or metamorphic) when recrystallization occurs under higher temperatures, pressures, and shear stresses than normally exist at the Earth's surface.

GNEISS (150) - Metamorphic rock that exhibits alternating bands of lighter minerals (quartz, feldspars) and darker minerals (biotite, hornblende, pyroxene).

QUARTZITE (150) - Metamorphic rock composed essentially of quartz. It results from high-grade metamorphism of a quartz-rich sandstone in which recrystallization of silica has produced a tough, hard rock with interlocking quartz grains.

SCHIST (150) - Metamorphic rock characterized by a layered or foliated appearance (schistosity) cause by the planar alignment of platy minerals, such as mica together with quartz, and minor amounts of other minerals, like garnet.

SERPENTINITE (150) - Green to greenish-yellow rock composed chiefly of the mineral serpentine, derived from metamorphism of iron-magnesium-rich igneous rocks.

SEDIMENTARY ROCK (2XX) - Rock consisting of weathered or eroded fragments of preexisting rocks that have been cemented together as a result of chemical cementation, compression, or precipitation.

SHALE (210) - Sedimentary rock consisting of very fine-grained particles (<0.004 mm) composed chiefly of clay minerals. It is distinguished from mudstone, by its ability to split into thin layers.

SILTSTONE (220) - Sedimentary rock consisting of fine-grained particles in the size range of 0.004 to 0.062 mm. Composed chiefly of clays and fine-grained quartz with mica.

SANDSTONE (230) - Fine to medium-grained sedimentary rock with particles in the size range between 0.062 to 2.0 mm. Typically composed of quartz, feldspars, and rock fragments, which are cemented together by silica, calcite, iron oxide, or clay. The hardness or strength of this rock depends largely on the nature and extent of the cement.

CONGLOMERATE (240) - Coarse-grained sedimentary rock composed of boulders to granule-sized particles (>2.0 mm), which are cemented together by silica, calcite, iron oxide, or clay. The hardness or strength of this rock depends largely on nature of the cement.

LIMESTONE (250) - Carbonate rock that can consist either of fragmental material, including fossils, pellets, etc., or a chemical precipitate.

EOLIANITE (260) - Layer of wind-blown beach sand often cemented by deposition of calcium carbonate. Tends to occur above the mean tide level in warm climates.

UNCONSOLIDATED SEDIMENTS (3XX) - Fragmented materials that are derived from the chemical and mechanical weathering process or from chemical precipitation and that have not yet undergone cementation and induration into a consolidated rock.

MUD, CLAY (310) - Very fine-grained particles (<0.004 mm) of clay and quartz.

SILT (320) - Fine-grained particles (<0.062 mm) of clay, quartz, and mica.

SAND (330) - Fine- to medium-grained particles (2.0 to 0.062 mm) of quartz, feldspar, other heavy minerals, and rock fragments.

GRAVELS, CONGLOMERATES (340) - Coarse-grained rock fragments (>2.0 mm), usually rounded to some degree, depending on the amount of transportation before the fragments came to rest.

GLACIAL TILL (350) - Unsorted materials, ranging in size from fine-grained "rock flour" to large boulders, deposited by glaciers (also known as glacial drift).

CALCAREOUS SEDIMENT (360) - Very fine-grained to fine-grained carbonate sediment, which can be fragmental or chemically precipitated.

LAVA (410) - Geologically recent volcanic rock that has formed by extrusion of molten magma to the Earth's surface as a sheet or flow.

ASH, TEPHRA (420) - Tephra is the general term for all fragmental volcanic materials ejected through a surface-reaching vent. Ash is unconsolidated, fine-grained ejected material (coarser-grained fragments are called bombs, scoria, pumice, etc.).

CORAL REEF (500) - Mass of calcareous material consisting of the skeletal structures of corals, growing in situ, as well as coralline debris and chemically precipitated material. Reefs are generally built of coral, but calcareous algae and shells contribute to the reef structure in many areas.

Glossary of Terms Used in the Geomorphic Classification

The following list defines landforms and gives their associated classification values (shown in Table 2). The terms are defined on the basis of the descriptions found in Bird (1984), Pethick (1984), Ritter (1986), Schwartz (1982), and Shepard and Wanless (1971). When the actual classification number contains an "X" (e.g., 222X) in the last digit, it is implied that the description is valid for all subsets of the given feature.

ALLUVIAL PLAIN SHORELINE (221X) - Intersection of broad alluvial slope, located at the base of a mountain range, with the ocean. These alluvial plains may also occur on delta coasts (222X) or outwash plains (231X).

BARRIER COASTS (212X) - In its most general sense, a barrier refers to accumulations of sand or gravel lying above high tide along a coast. These barriers may be partially or fully detached from the mainland. A barrier beach (2121) is a narrow strip of beach with a single ridge and often foredunes. A barrier island (2122) is completely surrounded by water and usually has multiple ridges, dunes, and salt marshes on the landward side of the island. It usually encloses a body of water known as a lagoon. Although barrier islands are the most common feature off the U.S. East and Gulf Coasts, they constitute 10% - 15% of the rest of the world's shorelines. A bay barrier (2123) is a beach barrier built across an embayment and is found in areas with low tide ranges, and high to moderate wave energies.

BEACH (21XX) - A beach is generally made up of sand, cobbles, or boulders and is defined as the portion of the coastal area that is directly affected by wave action and that is terminated inland by a sea cliff, a dune field, or the presence of permanent vegetation and seaward at the breaker/plunge point (the active portion of this zone varies based on wave and tide conditions).

BEACH ROCK (2112) - Cementation of beach sand by CaCO3 in intertidal zones. Confined to warm climates.

CLIFFED COASTS (11XX) - Coasts with cliffs and other abrupt changes in slope at the ocean land interface. Cliffs indicate marine erosion and imply that the sediment supply of the given coastal segment is low. The cliffs height depends upon the topography of the hinterland, lithology of the area, and climate.

COASTAL PLAIN (211X) - Sedimentary deposits formed on a trailing-edge coast. Trailing-edge coasts are often associated with barrier beach systems and are commonly subject to subsidence.

CORAL REEF COASTS (241X, 242X) - Shoal water area built up by secretions of CaCO3 by coral, marine algae, and other marine organisms. Reefs may form either fringing reefs that surround the shore or barrier reefs that grow at some distance from the coast and protect the coast from large waves.

CUSPATE FORELAND (2126) - Seaward projection of accumulated unconsolidated marine sand or gravel, bounded on both sides by wave-dominated coasts (indicates convergence of currents in a low-tide environment).

DELTA (222X) - Accumulations of fine-grained sedimentary deposits at the mouth of a river. The sediment is accumulating faster than wave erosion and subsidence can remove it. These are associated with mud flats (2224) and salt marshes (2225).

DROWNED KARST (1500) - Terrain with distinctive characteristics of relief and drainage arising from a high degree of rock solubility that was submerged at the end of the Wisconsin glaciation period (i.e., geologic substrate that is made of highly soluble, usually carbonate, rock).

ESTUARY COAST (133X) - Tidal mouth of a river or submerged river valley. Often defined to include any semi-enclosed coastal body of water diluted by freshwater, thus includes most bays. The estuaries are subjected to tidal influences with sedimentation rates and tidal ranges such that deltaicaccumulations are absent. Also, estuaries are associated with relatively low-lying hinterlands, mud flats (1334), and salt marshes (1335).

FJORD (122X) - Narrow steep-walled, U-shaped, partially submerged glacial valley.

FIARD (123X) - Glacially eroded inlet located on low-lying rocky coasts (other terms used include sea inlets, fjardur, and firth).

ICE COAST (1400) - Coast bordered by glaciers.

LAGOON (225X) - A shallow water body separated from the open sea by sand islands (e.g., barrier islands) or coral reefs.

MANGROVE SWAMP (245X) - Coasts with tree vegetation of subtropical/tropical origin located on muddy, peaty substrates. Occur in coastal regions with low wave energies that are located in tropical and subtropical climates (occupies same ecological niche as salt marsh in temperate zones).

MUD FLATS - Located in areas with fine-grained sediments at low ends of the intertidal zone and are exposed at low tide. Found in estuaries (1334), deltaic environments (2224), and areas with marine/fluvial deposits (2254).

OUTWASH PLAIN (231X) - A river deposition coast. Deposits are derived from meltwater from the front of a glacier. Grades from gravel near the glacier edge to sand farther away. Other types of glacial deposits include moraines (2320), composed of poorly sorted till, and drumlins (2330), hills sculpted by glaciers, that are composed of well-sorted till.

SALT MARSH - Salt-tolerant vegetation that colonizes the intertidal zones of estuaries (1335), deltas (2225), and lagoons (2255). Located on slightly higher elevations than mud flats, and vegetation zonation reflects subtle changes in elevation.

SPIT (2127) - Curved or hooked depositional feature formed by longshore drift. Often has salt marshes on landward side and beach ridges marking former positions of the shoreline. Very mobile landform.

VOLCANIC COASTS (25XX) - Coasts dominated by volcanic landforms. The coasts may be built up of lava flows (251X), ash flows (252X), peninsular and island volcanoes, or calderas (253X). Often may be flanked by coral reefs (241X) if the volcano has become submerged.


Bird, E. C. F. 1984. Coasts. Basil Blackwell Publishing, New York, New York.

Pethick, J. 1984. An Introduction to Coastal Geomorphology. Edward Arnold Publishers, London, England.

Ritter, D. F. 1986. Process Geomorphology. William Brown Publishers, Dubuque, Iowa.

Schwartz, M. L. (ed.). 1982. The Encyclopedia of Beaches and Coastal Environments. Hutchinson & Ross Publishing, Stroudsburg, Pennsylvania.

Shepard, F. P. and H. R. Wanless. 1971. Our Changing Coastline. McGraw-Hill Book Company, New York, New York.