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Padre Island National Seashore: A Guide to the Geology, Natural Environments, and History of a Texas Barrier Island


The glossary below defines terms as used in this guide. Many of the definitions are modifications of those listed in the American Geological Institute's Glossary of Geology, edited by M. Gary, R. McAfee, Jr., and C. L. Wolf (1977), and in Land Resources of Texas by R. S. Kier, L. E. Garner, and L. F. Brown, Jr. (1977).

Accretion — The gradual extension of land by the deposition of sediment.

Active processes — Natural phenomena, commonly involving movements of water or the wind, that produce continuous or periodic changes in physical and biological environments.

Algal mat — A thin, matlike, and occasionally leathery layer of algae growing on a moist surface that is alternately emergent and submergent, such as a tidal flat.

Angle of repose — The maximum angle or steepness that can be naturally maintained by loose sand or other sediment on a stable slope.

Backbeach — The horizontal or gently landward-sloping part of the beach that is affected only by storm waves and extremely high tides and not by normal waves and tides. The backbeach includes the storm berm and lies between the crest of the berm and the point landward where there is a definite change in physiographic form, such as at a fore-island ridge.

Back-island dune field — A large area of nonvegetated, migrating sand dunes of various types, mostly large elongate dunes, found in the back, or lagoonward, side of a barrier island.

Back-island sandflat — A sandflat with scattered, small, migrating dunes, found on the lagoonward side of a barrier island and representing an environment transitional in characteristics between a large, active dune field and a tidal flat.

Backwash — The seaward return of water running down the forebeach following an uprush of waves.

Bar — An elongate, submerged, offshore ridge, bank, or mound of sediment, built up by the action of waves or currents.

Barchan dune — A moving, isolated, crescent- or horseshoe-shaped sand dune situated perpendicularly to the direction of the prevailing wind, with the horns (ends) of the crescent pointing downwind.

Barometric pressure — The pressure (force per unit area) caused by the Earth's atmosphere. Normal pressure at sea level is defined as 14.66 pounds per square inch or 29.92 inches (76.0 cm) of mercury.

Barrier flat — A relatively flat area, at least partially vegetated, located between the foredunes of a barrier island and the lagoon. Barrier flats generally originate as wind-eroded plains or wind-deflation flats.

Barrier island — A long, low, narrow island separated from the mainland by a lagoon and commonly having dunes, vegetated zones, and marshy environments extending lagoonward from the beach.

Barrier system — The group of geologic environments, primarily subaerial, that compose a barrier island.

Beach — The narrow strip of land immediately bordering a body of water; specifically the zone between the low-water line and the point landward where there is a definite change in physiographic form, such as at a fore-island dune ridge. The Gulf beach, like most beaches, is subdivided into a fore-beach and a backbeach.

Beach cusp — Any of a series of low, crescent-shaped mounds or ridges of beach sediment built by wave action and separated by smoothly curved, shallow depressions, spaced generally at regular intervals along and at right angles to the shoreline.

Beach tar — A black, gummy asphalt formed from offshore natural oil seepage or from various hydrocarbon-based substances introduced into the ocean by man and subsequently washed ashore.

Berm — A low, nearly horizontal or landward-sloping bench or ridge on the backbeach, formed of sand and shell washed up and deposited by storm waves.

Berm crest — The gulfward edge of a berm, usually coinciding with the highest point of the berm and marking the break in slope between the berm and forebeach.

Blowout — A saucer- or trough-shaped depression formed by intense wind erosion of a dune or other sand deposit.

Blowout dune — A dune formed in association with a blowout. See blowout.

Brackish water — Water with a salinity between that of normal sea water and normal fresh water.

Breaker — A wave that becomes unstable, steepens, and breaks as it moves into shallow water, producing a mass of foam and turbulent water.

Calcareous — Containing calcium carbonate.

Canal — An artificial watercourse of relatively uniform dimensions, cut through an inland area to connect two or more bodies of water and designed for navigation.

Channel — (a) A watercourse; a natural passageway or depression through which water flows, forming a connecting link between two bodies of water. (b) An artificial waterway, such as a canal, generally constructed and maintained by dredging.

Channel-mouth bar — Sandbar deposited at the mouth (gulfward end) of a washover channel by strong ebb currents that are produced when lagoon water, swollen by storm surge, flows back to the Gulf after a storm. Channel-mouth bars also form at the mouths of other channels.

Cold front — The leading edge of an advancing cold air mass. The passage of a cold front is usually accompanied by a rise in pressure, a shift to northerly winds, a decrease in temperature, and precipitation.

Constructive waves — "Flat-crested" waves (waves having a low wave-height to wavelength ratio) that transport and deposit sediments onto the beach.

Coppice dune — A small mound of sand stabilized in and around a clump of vegetation.

Coquina — A sedimentary rock composed mostly of weakly to moderately cemented shells and shell fragments.

Crossbedding — An internal arrangement of layers in sediments or sedimentary rock, characterized by beds or layers that are inclined or sloping with respect to a horizontal plane.

Cyclone — A circular, atmospheric low-pressure system around which wind blows in a counterclockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Deflation scar — A barren flat produced by wind erosion of sand and finer material.

Delta — The low, nearly flat, triangular or fan-shaped body of sediment deposited at the mouth of a river, stream, or tidal inlet.

Desert pavement — A concentration of wind-polished pebbles or shells covering an area from which the sand has been removed by wind erosion (deflation).

Destructive waves — Steep waves (waves having high wave-height to wavelength ratios) that end in plunging breakers which in turn erode the beach and transport sediments gulfward.

Divide — The boundary between two adjacent drainage basins.

Dominant wind — The wind that has the greatest effect in transporting sediment in a given area, determined primarily by wind velocity and duration.

Dune — A mound, ridge, bank, or hill of loose, wind blown material (generally sand). Dunes may be barren and thus capable of movement, or vegetated and stabilized.

Dune ridge — Same as fore-island dune ridge.

Dynamic — Characterized by active processes and conditions that produce continuous activity and change.

Ebb flow —Tidal current associated with an outgoing tide (a decrease in the height of an astronomical or a wind- or storm-generated tide). The direction of flow is generally seaward.

Ebb-tidal delta — Fan- or delta-shaped accumulations of sediment deposited by gulfward-flowing tides (ebb tides) at the gulfward end of a tidal inlet or channel.

Ecosystem — A unit in ecology consisting of the environment and the physical, chemical, and biological factors that exist in and affect it.

Environment — Surroundings characterized by a unique set of physical, chemical, and biological features, conditions, influences, or forces; for example, a vegetated barrier flat, hurricane washover channel, or wind-tidal flat.

Eolian — Of or relating to wind and its effects in eroding, transporting, and depositing sediments.

Ephemeral pond — Temporary pond.

Erosion — The process by which rock, sediments, and soil at the Earth's surface are loosened, dissolved, or worn away by natural agents, such as water and wind.

Evaporation — The process by which a substance passes from the liquid state to the vapor state.

Exotic plant species — Plants that do not occur naturally in an area but instead are brought in from other locations.

Eye (of a storm) — The roughly circular area (4 to 40 miles in diameter) of relatively light wind and calm weather in the center of a tropical cyclone.

Flood-tidal delta — Fan- or delta-shaped sediment accumulations deposited by flood currents (lagoonward-flowing tides) at the lagoonward end of a tidal inlet or channel.

Flotsam — The wreckage of a ship or its cargo found floating on the sea or washed ashore.

Forebeach — The seaward-sloping surface lying between the low-water line and the landward reach of wave uprush.

Foredune — A dune occurring at the landward margin of the beach and generally forming part of a fore-island dune ridge. See fore-island dune ridge.

Foredune ridge — Same as fore-island dune ridge.

Fore-island dune ridge — A ridge of dunes parallel to the shoreline of an ocean or gulf, occurring immediately landward of the beach and at least partially stabilized by vegetation.

Fractional scale — The numerical ratio between the linear distances on a map and the corresponding actual distances on the surface mapped. A fractional, or ratio, scale of 1:48,000 means that one unit on the map represents 48,000 of the same units on the ground.

Geographic north — Same as true north.

Graphic scale — A bar or line on a map, marked off in units such as feet, miles, or kilometers, that indicates the proportion between the linear distances on the map and the corresponding actual distances on the surface being mapped.

Grassflat — Shallow subaqueous environment of high biologic productivity, characterized by a dense growth of marine grasses and typically found in shallow areas of a lagoon.

Grassland — Land supporting a dense growth of grasses, such as the heavily vegetated barrier flat of a barrier island.

Groins — Long, rigid, man-made structures extending seaward from the shoreline and constructed for the purpose of blocking longshore currents and protecting the shoreline from erosion.

Heavy mineral — A rock-forming mineral generally having a specific gravity of 2.8 or greater.

High sea level — Average maximum level of the sea's surface during tidal cycles.

High tide —The highest point in the alternate rise and fall of sea level, which results from the gravitational attraction of the moon and sun and is modified by effects of the wind on the water surface.

Holocene — Same as Recent.

Hurricane — A tropical cyclone with sustained wind velocities greater than 74 miles per hour. See cyclone.

Index map — A map depicting the location of one or more smaller areas within a larger area and also indicating special features in the larger area.

Indigenous plant species — Plants that are native to (occurring naturally in) an area, rather than having been introduced by man.

Infrared photograph — A photograph produced by film that records wavelengths just beyond the red end of the visible spectrum, as well as certain wave lengths of visible light.

Jetsam — Cargo discarded from a ship and washed ashore.

Jetty — A man-made structure extending from the shore into a body of water that prevents natural filling of a navigable channel. Jetties are often built in pairs, with one extending from each side of the channel entrance.

Lagoon — A shallow body of water near the sea, partly or completely separated from it by a low, narrow strip of land, such as a barrier island.

Lagoon-center sand — The generally barren, muddy sand occupying the deeper, central parts of a lagoon.

Lagoon-margin sand — Barren sand, usually submerged, bordering a lagoon shoreline and characterized by sand waves and ripples.

Lagoon system — The group of geologic environments, primarily subaqueous, that compose a lagoon.

Landform — Any physical, recognizable form or feature of the Earth's surface having a characteristic shape and produced by natural causes; for example, a dune or tidal flat.

Latitude — Imaginary lines on the Earth's surface running east and west parallel to the Equator. Each line indicates locations that are an equal number of degrees north or south of the Equator.

Leeward — Downwind; facing away from the wind.

Light mineral — A mineral, such as quartz, that has a specific gravity of about 2.8 or less.

Line camp — A corral, or set of corrals, located along the line of a cattle drive and used to hold cattle temporarily.

Lithosphere — The solid part of the Earth that lies below the hydrosphere (the water on the surface of the Earth) and atmosphere (air surrounding the Earth).

Littoral drift — Same as longshore drift.

Location map — An index map showing the location of a specific feature or features or the location and outline of a smaller area within a larger area.

Longitude — Imaginary lines on the Earth's surface that extend north and south between the geographic poles. Each line indicates locations that are an equal number of degrees east or west from the Prime Meridian, which runs through Greenwich, England.

Longitudinal dune — A long, narrow sand dune, oriented parallel to the direction of the dominant dune-building winds, that is wider on the windward end, tapering to a point on the leeward end.

Longshore current — An ocean current created as waves approach and break at an oblique angle to the shoreline. Longshore currents flow parallel to and close to the shore.

Longshore drift — (a) The moving, or drifting, of material, such as sand and shell, caused by a current flowing near and parallel to a shoreline. (b) The material being moved by such a current.

Low tide — The lowest point in the alternate rise and fall of sea level, which results from the gravitational attraction of the moon and sun and which is modified by effects of the wind on the water surface.

Made land — Land built with artificial fill above water level for the primary purpose of creating new land area.

Magnetic declination — The horizontal angle between true north and magnetic north in any given location.

Magnetic north — The uncorrected direction indicated by the north-seeking end of the needle of a magnetic compass.

Map legend — A brief explanatory list of the symbols, map units, patterns, colors, and other features appearing on a map.

Map scale — An indication of the proportion between linear distances on a map and the corresponding actual distance on the surface mapped.

Map unit — One of the mappable features, such as a geologic environment, distinguished from other physical features in a mapped area on the basis of a unique set of characteristics and indicated on the map by a unique color, pattern, or symbol.

Maritime — Oceanic.

Marsh — Poorly drained area that is frequently flooded with fresh or salt water and that supports hydrophytes (plants that can grow partially submerged in water).

Matchline — The line marking the common boundary between two separate maps or map segments of adjacent areas and along which a map reader may mentally or physically join the two pieces to produce one continuous map.

Mean sea level — The average height of the surface of the sea for all stages of the tide during a 19-year period.

Modified land — Land, the natural environments of which have been changed by man, either purposely or inadvertently.

Motte — Thicket, grove, or clump of trees, such as a live oak motte.

Mud drape — A thin layer of very fine sediments settled from water and deposited on a surface when water turbulence is not great enough to maintain suspension, usually during the waning stages of a flood or in some low-energy aquatic environment.

Norther — A cold front marked generally by strong north winds. See cold front.

Oblique dune — A long, narrow sand dune similar to a longitudinal dune but oriented at an angle (usually slight) to the direction of the dominant dune-building winds.

Orthophotographic map — A map prepared from one or more photographs formed by a perspective projection in which the distortion due to relief and camera angle has been removed.

Packery — A meat-packing house.

Parabolic dune — A sand dune shaped like a parabola (or horseshoe), with its ends pointing upwind.

Pass — A channel, natural or artificial, connecting a body of water with the sea, for example, a narrow opening between two closely adjacent islands.

Patch reef — A small flat-topped organic reef forming a part of a reef complex.

Peninsula — An elongate body of land nearly surrounded by water but connected on one end with a larger land area.

Pirogue — A canoe made from a hollowed-out log.

Pleistocene — Geologic block of time that encompasses the "ice ages" or periods of glaciation during the Quaternary Period. More specifically refers to an epoch of the Quaternary Period, between the Pliocene of the Tertiary and the later Holocene of the Quaternary.

Presidio — A military post; a fort.

Prevailing wind — The wind direction that is recorded most frequently during a year.

Quartz — A mineral, usually colorless and transparent, and composed of silicon and oxygen (SiO2), that constitutes most of the sand along the Gulf shoreline.

Radiocarbon date — The age of a rock or sediment determined by measuring the concentration of carbon-14 remaining in an organic material, such as a shell, contained in the rock.

Recent (Holocene) — An epoch of the Quaternary Period, extending from the end of the Pleistocene, when the last continental glaciers melted, into the present. Some researchers informally use 4,500 years ago as the final time boundary for the Holocene in the Gulf of Mexico and refer to the 4,500 years since as the Modern Epoch.

Reworked subaerial spoil — Dredged material originally deposited in a heap but later scattered and acted upon by wind, waves, or currents.

Ripple marks — A series of small, parallel ridges and troughs that resemble water ripples but are formed on the surface of sand or other sediment by the movement of wind or water.

Salinity — A measure of the concentration of salt or total dissolved solids. The amount of salt or dissolved solids in normal sea water is about 3.5 percent (35 parts per thousand or 35,000 parts per million).

Sand — Rock and mineral fragments ranging from 0.063 to 2 mm (0.0025 to 0.078 inch) in diameter.

Sandflat — A sandy plain devoid of vegetation.

Sand migration — Movement of sand by some natural agent such as the wind.

Sand wave — A relatively large, linear wave (sandbar) resembling a water wave but formed in sand by wind or water currents. Wave orientation is usually perpendicular to current direction.

Sediments — Rock, mineral, or organic particles deposited by agents such as water and wind.

Serpulid reef — A reef, usually a patch reef, built of the contorted calcareous tubes formed by annelid worms of the family Serpulidae.

Serpulid worm — An invertebrate belonging to the family Serpulidae that characteristically builds a contorted calcareous tube on a submerged surface. Serpulids are annelids (worms belonging to the phylum Annelida) and are characterized by a segmented body with a distinct head and appendages.

Shoreface — The relatively steeply sloping zone between the low—tide line of a shore and the more nearly horizontal surface of the offshore zone.

Shoreline — The intersection of the surface of a water body with the shore or beach.

Slipface — The steeply sloping surface on the lee side of a dune, standing at or near the angle of repose of loose sand, and advancing downwind by a succession of slides, or avalanches, wherever that angle is exceeded.

Specific gravity — The ratio of the mass or weight of a substance to the mass or weight of an equal volume of a standard substance — usually water. The specific gravity of quartz is about 2.6, which means that quartz weighs 2.6 times as much as an equal volume of water.

Spit — A low, narrow tongue of sand extending from a mainland shoreline and formed by the longshore drifting of sediments. See longshore drift.

Spit accretion — The building of a spit by longshore drifting of sediments. Spit accretion is often responsible for the building and extending of barrier islands in the direction of longshore currents. See spit, longshore drift, longshore current.

Spoil — Material excavated during dredging of channels in bays and lagoons or through land and commonly deposited nearby in heaps.

Spoil heap — A pile or mound of the material excavated during dredging operations.

Stabilize — To prevent loose sediment from being transported by wind or water, such as by binding and covering naturally with vegetation or by covering artificially with some material too heavy to be blown by the wind or moved by water.

Stabilized blowout dune — A blowout dune whose movement is arrested by the growth of vegetation. See blowout dune.

Storm runway — A linear flat or trough, usually produced by wind deflation and along which water flows during storms.

Storm surge — An abnormal, sudden rise of sea level along a coast during a storm. Onshore wind stresses and barometric pressure reduction result in water piling up against the shore as the storm hits land.

Strait — A relatively narrow waterway connecting two larger bodies of water.

Subaerial — Above water; emergent.

Subaqueous — Below the surface of the water; submergent.

Subaqueous spoil — Dredged material, presently below the water's surface, that has been deposited in a body of water, such as a lagoon. Subaqueous spoil usually has been reworked by waves and currents.

Substrate — (a) Rock or sediment below the soil zone. (b) Surface on which a fixed organism is attached.

Subtropical — Refers to the climate of the subtropics, falling between tropical (see tropical) and temperate climates and characterized by moderate or mild temperatures. A subtropical climate is more like the tropical than the temperate climate.

Surf zone — That part of the shoreface where waves break, located between the most seaward wave breakpoint bar and the lower part, or toe, of the forebeach where the waves begin their uprush.

Swale — A long, narrow, generally shallow trough-like depression between two ridges.

Swash — The uprush and backwash of waves on a beach.

Swash bars — Low, elongate bars of sand and/or shell formed along and parallel to the beach by wave swash; water is commonly trapped in narrow troughs landward of newly formed bars.

Swash zone — The sloping part of the forebeach that is alternately covered and uncovered by the uprush and backwash of waves.

Swell — Smooth, symmetrical, flat-crested waves that have traveled out of the area of the ocean where they were formed.

Terrigenous — Refers to material eroded from the land surface.

Texture (sediments) — The physical nature of sediments relating not only to particle size, but also to sorting, roundness, sphericity, and grain surface features.

Threshold velocity — The minimum wind or water current velocity necessary to initiate sediment movement. Threshold velocity varies as conditions such as sediment size vary

Tidal flat — An extensive, nearly horizontal tract of land in a very low-lying coastal area that is alternately covered and uncovered with water by the rise and fall of the tide.

Tidal inlet — A narrow passage cutting into a shore and through which water flows landward with the rising tide and seaward with the falling tide.

Tornado — An intense, destructive storm of small diameter, characterized by a funnel-shaped cloud in which winds circulate at extremely high velocities.

Transpiration — A process whereby water absorbed by plants is evaporated into the atmosphere from the plant surface.

Transverse dune — A sand dune elongated perpendicularly to the direction of the dune-forming wind and having a gentle windward slope and a steep leeward slope.

Triangulation station — A point on the Earth's surface, the position of which is determined by means of bearings taken from two fixed points a known distance apart. Surveyed stations are usually permanently marked.

Tropical — Type of climate characterized by high temperature, high humidity, and abundant rainfall.

Tropical storm — A tropical cyclone with sustained wind velocities between 40 and 74 miles per hour. See cyclone.

True north — The direction from any point on the Earth's surface toward the geographic North Pole, or the northern point of intersection between the Earth's surface and axis of rotation.

Turbidity — Cloudy appearance of a fluid due to the presence of suspended material.

Vaquero — Spanish word for a cowboy in Spanish America and the Southwest.

Washover channel — Channel scoured through a barrier island or peninsula at a low, weak point during attack by storm waves and tides.

Washover fan — A fanlike body of sediment transported through or eroded from a storm washover channel and deposited at a point where the channel opens onto a broad flat.

Water table — Upper level or surface of ground water in an unconfined aquifer.

Wave breakpoint bars — Offshore submerged sand bars (usually numbering three on the Texas Gulf Coast) that parallel the shoreline and underlie the breaking point of waves.

Wave height — The vertical distance between the crest and trough of a wave.

Wavelength — The distance from one point on a wave (such as the crest or trough) to the corresponding point on the succeeding wave.

Wave steepness — A physical characteristic of a wave defined by the ratio of wave height to wavelength. Waves with high ratios (1/25 to 1/7) are considered steep compared with those with lower ratios (less than 1/100).

Wave tank — A laboratory tank in which waves are artificially generated for experimental purposes.

Wind deflation — A form of wind erosion in which loose, dry sand and finer material is removed from an area.

Wind-deflation flat — A barren, relatively flat area from which sand and other loose fine material has been removed, or eroded, by the wind.

Wind shadow — The area or zone on the downwind side of an obstacle where air motion is blocked or reduced by the obstacle. Sediment may accumulate in this area as a result of lower wind velocity and turbulence.

Wind-shadow dune — A dune formed on the downwind side of some obstacle. See wind shadow.

Wind-tidal flat — Tidal flat flooded primarily by wind tides. See tidal flat and wind tide.

Wind tide — A rise in water level on the downwind side of a body of water, such as a lagoon, caused by the force of wind on the water surface.

Wind tunnel — Man-made tunnel through which artificially generated wind is directed for experimental purposes, such as to observe the effect of wind velocity on sand migration.

Windward — Upwind; facing into the wind.

Wisconsin glacial stage — The fourth and last glacial stage of the Pleistocene Epoch in North America.

Zone of convergence (longshore drift convergence) — Stretch of shoreline along central Padre Island, near 27°N latitude, where southward- and northward-flowing longshore currents generally meet or converge.

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Last Updated: 28-Mar-2007