Area Of Critical Environmental Concern (ACEC). An area within public lands where special management attention is required to protect and prevent irreparable damage to important historic, cultural or scenic values, fish and wildlife resources, or other natural systems or processes, or to protect life and safety from natural hazards. (FLPMA, Section 103(a)).

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alluvial fan. A gently sloping (ten degrees or less) fan shaped deposit of sediments formed where an intermittent stream undergoes an abrupt reduction in slope. Coarsest materials are found near the apex of the fan (nearest the mountain or hill front) whereas the finest materials occur farthest from the mountain front.

alluvium. Material, including sand, clay, gravel, and mud, deposited in river beds, lakes, alluvial fans, valleys, and elsewhere by modern streams.

Animal Unit Month (AUM). A unit of measure of the impact of a grazing animals on range land. An AUM is equivalent to the amount of forage required by a 1,000 pound cow for 1 month (BLM, 1980, p. 68). A calf might give an AUM value, sheep another, etc..

anthropology. the science that deals with the origins, physical and cultural development, racial characteristics and social customs and beliefs of mankind.

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anticline. A configuration of folded, stratified rocks in which the rocks dip or incline in two directions away from the crest, like the two halves of a pitched roof. The reverse of a "syncline".

arid. Zero to ten inches precipitation per year. Potential for evaporation exceeds annual precipitation.

ash. 1. Inorganic residue remaining after ignition of combustible substances, quantitatively determined by definite prescribed methods. 2. Volcanic dust and particles less than 4 mm. in diameter.

ash fall. 1. A rain of airborne volcanic ash falling from an eruption cloud. Characteristic of vulcanian eruptions. 2. A deposit of volcanic ash resulting from such a fall and lying on the ground surface. (AGI)

ash flow. 1. An avalanche of volcanic ash, generally a highly heated mixture of volcanic gases and ash, traveling down the flanks of a volcano or along the surface of the ground and produced by the explosive disintegration of viscous lava in a volcanic crater or by the explosive emission of gas charged ash from a fissure or group of fissures. Ash flows of the type described at Mount Pelee are considered to represent the feeblest type of the nuee ardente. The solid materials contained in a typical ash flow are generally unsorted and ordinarily include volcanic dust, pumice, scoria, and blocks in addition to ash. 2. A deposit of volcanic ash and other debris resulting from such a flow and lying on the ground surface. Syn: ignimbrite. (AGI)

ash flow tuff. A tuff resulting from an ash flow.

ash, volcanic. Uncemented pyroclastic material consisting of fragments mostly under 4 mm. in diameter. Without a qualifying adjective, the term should be applied only to essential ejecta.

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Aso lava. A type of indurated pyroclastic deposit produced during the explosive eruptions that formed the Aso Caldera of Kyushu, Japan. Chiefly a lavalike tuff consisting of lenses or spindles of black and gray obsidian lying in a tuffaceous matrix that displays a streaky, varicolored banding or eutaxitic structure. Possibly erupted as extremely hot, gas rich dust and ash carrying large clots of molten, vesiculated volcanic glass. Similar to deposits variously described as welded tuff, welded pumice, ignimbrite, and tuff-lava. (AGI)

aulacogen. A long lived deeply subsiding trough, at times fault bounded, that extends at high angles from a geosyncline far into an adjacent foreland platform.

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autecology n. Study of the requirements, tolerances, and responses of a single organism (Oosting 1956:12). The ecology of a species or of individual organisms in relation to the environment (Webster).

avalanche, volcanic. Avalanche usually refers to a large mass of ice and snow, sometimes accompanied by other material, moving rapidly down a mountain slope. A volcanic avalanche is a general term relating to movement of large masses of volcanic material, ususally tephra, moving rapidly down a mountain slope without regard for temperature and gas content.

axiolite, axiolitic. No definition in AGI, 1962. Defined in Ross and Smith, 1961.

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bajada. 1. The slope or continuous apron comprised of merged alluvial fans. 2. A series of merged alluvial fans along the base of a mountain range.

bench. A strip of relatively level earth or rock, raised and narrow. A small terrace or comparatively level platform breaking the continuity of a declivity (AGI).

block. An angular fragment over 256 mm. in diameter showing little or no modification in form due to transportation; similar in size to a boulder. (AGI, 1962).

bolson. In arid regions, a basin filled with alluvium and intermittent lakes and having no surface water outlet.

Bryozoa. Phylum of tiny colonial animals equipped with a circle or loop or tentacles surrounding the mouth that build calcareous structures of many kinds, mostly marine. Ordovician to Recent. The phylum of invertebrate animals which are popularly called "moss animals". (Called Polyzoa by some zoologists.)

capture. See stream capture.

carbonate bank. A submarine plateau composed of limestone, much like the present-day Bahamas.

carbonate mineral. Mineral formed by the combination of the complex ion (CO3)2- with a positive ion. Common example: CaCO3, calcium carbonate, the primary constituent of limestone.

California Desert Conservation Area (CDCA). The 25 million acre CDCA contains over 12 million acres of public lands administered by the BLM. The boundaries are formed by the Nevada border and Colorado River on the east. The United States / Mexico boundary forms the southern boundary. Western boundaries are mostly desert areas on the east sides of the Peninsular Ranges, the Transverse Ranges, the Tehachapi Mountains and the Sierra Nevada.

cinder cone. A type of volcano which is built exclusively or in large part of pyroclastic ejecta dominated by cinders (particles 0.01 to 1 inch in diameter). Seldom exceeds 1,500 feet in height. Slopes up to 30 or 40 degrees.

craton. A relatively immobile part of the earth, generally of large size.

crinoid. an echinoderm of the class Crinoidea, having a cup-shaped body to which are attached branched, radiating arms, comprising the sea lilies, feather stars, and various fossil forms.

crystalline. The orderly arrangement of atoms in a crystal. Rock which is made up of crystals. Examples: igneous and metamorphic rocks.

crystallization. The process of forming crystals by cooling a molten substance.

desert pavement. A residual deposit produced by continued removal of clay and dust by wind which leaves a surface covered with close packed pebbles. 2) Such as desert pavement. Desert crust. When loose material containing pebbles or larger stones is exposed to wind action the finer dust and sand are blown away and the pebbles gradually accumulate on the surface, forming a sort of mosaic which protects the finer material underneath from attack. This is the desert pavement. Synonym: pebble mosaic (AGI). See discussion in Chapter XI -- Geomorphology.

diamictite. A sedimentary rock consisting of unsorted rock debris, suggestive of glacier ice-rafted material.

dune. A low hill, or bank, of drifted sand (AGI).

edaphic adj. Of or relating to soil, esp. as it affects living organisms (Webster). Pertaining to, or influenced by, soil conditions (Munz 1965).

endemic adj. 1. Prevalent in or peculiar to a particular locality or people. 2. Ecol. Native or limited to a certain region (Webster).

ephemeral pool. A pool of water which contains water only during the wet seasons. Drys up completely at some time during the seasons.

ethnography. a branch of anthropology dealing with the scientific description of individual cultures.

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eutaxite, eutaxitic. Applied to a structure of certain volcanic rocks with a streaked or blotched appearance due to the alternation of bands or elongated lenses of different color, composition or texture; the bands, etc., having been originally ejected as individual portions of magma which were drawn out together in a viscous state and formed a heterogeneous mass by welding. The term is most appropriately used in describing the structure of a majority of welded tuffs. (AGI)

fan. An accumulation of debris brought down by a stream descending through a steep ravine and debouching in the plain beneath, where the detrital material spreads out in the shape of a fan, forming a section of a vary low cone (AGI).

fault. A surface of rock rupture along which there has been differential movement. A fracture in rock in which the two blocks on either side have moved in respect to each other.

fault block mountains. A mountain bounded by one or more faults. When two faults bound a mountain they may be responsible for the movements which have uplifted the block which was eroded to form a mountain.

fold. A bend, flexure, or wrinkle in rock produced when the rock was in a plastic state.

fossil. any remains, impression, or trace of an animal or plant of a former geological age, as a skeleton, footprint, etc.

granite. A coarse-grained igneous rock dominated bt light-colored minerals with minor amounts of dark-colored minerals. Quartz and feldspar dominate. Makes up about 95% of all intrusive igneous rocks found on the continents.

igneous rock. An aggregate on interlocking silicate formed by the cooling and solidification of a natural silicate melt.

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ignimbrite. 1. A silicic volcanic rock forming thick, massive, compact, lavalike sheets that cover a wide area in the central part of North Island, New Zealand. The rock is chiefly a fine grained rhyolitic tuff formed mainly of glass particles (shards) in which crystals of feldspar, quartz, and occasionally hypersthene or hornblende are embedded. The glass particles are firmly "welded" and bend around the crystals, and evidently were of a viscous nature when they were deposited. The deposits are believed to have been produced by the eruption of dense clouds of incandescent volcanic glass in a semimolten or viscous state from groups of fissures. Syn: welded tuff. 2. The deposit of a fiery cloud or pyroclastic flow, extensive and generally thick with well developed prismatic jointing.

indurated. Rendered hard; confined in geological use to masses hardened by heat, baked, etc., as distinguished from hard or compact in natural structure. In modern usage the term is applied to rocks hardened not only by heat, but also by pressure and cementation.

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inselberg. A lone, resistant, steep sided hill or mountain, formerly part of a larger mountain range, that is surrounded by alluvial fan or playa deposits.

intrusive rock. A rock that solidified from a mass of once molton material that invaded the earth's crust but did not reach the surface.

lacustrine. 1. Produced by or belonging to lakes. 2. Of, or pertaining to, or formed or growing in, or inhabiting, lakes.

lahar. 1. Landslide or mudflow of pyroclastic material on the flank of a volcano. 2. Deposit produced by such a landslide. May be wet or dry, hot or cold.

lapilli. Essential, accessory, and accidental volcanic ejecta ranging mostly from 4 mm. to 32 mm. in diameter.

lithification. The process by which unconsolidated rock-forming materials are converted into a consolidated or coherent state, i.e., turned into a rock.

mesa. A table land; a flat topped mountain or other elevation bounded on at least one side by a steep cliff; a plateau terminating on one or more sides in a steep cliff (AGI).

metamorphic rock. "Changed-form rock", any rock that has been changed in texture or composition by heat, pressure, or chemically active fluids after its original formation. Conditions of formation usually found miles below the earth's surface, thousands of atmospheres of pressure, cloe to the melting point.

mineral. A naturally occurring solid element or compound. It is inorganic, has a definite composition, or ranges within definite limits. It has an orderly internal atomic structure and may express this in the form of visible crystals.

mudflow, volcanic. See "lahar".

nuee ardente. See ash flow.

palynology. The study of pollen and other spores and their dispersal, and applications thereof.

patina. A thin, light colored, outer layer produced by wind action and weathering.

pediment. A sloping rock surface forming a ramp up to the front of a mountain range in an arid region. It may be covered by thin alluvium.

pediplain. A widely extending, dominantly gentle sloping or rolling surface which is rock cut and alluviated.

pelean eruption. Designating or pertaining to a type of volcanic eruption characterized by explosions of extreme violence and the formation of nuees ardentes, or ash flows. The lavas involved in this type of eruption are generally extremely silicic and viscous.

phenology n. Study of periodic biological phenomena, as breeding, flowering, and migration, esp. as related to climate (Webster).

plain. A region of general uniform slope, comparatively level, of considerable extent, and not broken by marked elevations and depressions; it may be an extensive valley floor or a plateau summit. Any extent of level or nearly level land (AGI).

playa. 1. The flat floor of a bolson in an arid region. It is underlain by silts, clays, and complex associations of different salts. 2. The shallow central basin of a desert plain, in which water gathers after a rain and is evaporated (AGI).

playa lake. An intermittent lake which occupies the flat floor of a bolson.

pumice. An excessively cellular glassy lava, generally of the composition of rhyolite.

pyroclastic. A general term applied to detrital volcanic material that have been explosively or aerially ejected from a volcanic vent. Also a general term for the class of rocks made up of these materials.

pyroclastic rock. Fragmental rock blown out by volcanic explosion and deposited from the air. Rocks formed from bombs, blocks, cinders, ash, tuff, and pumice.

refugia. As used in ecology, refers to small areas, or even microclimates, where a species can survive when it could not do so in the surrounding area.

relict n. An organism or species of an earlier era surviving in an environment which has changed considerably (Webster). A localized plant left over from an earlier geological period (Munz 1965).

rock. An aggregate of minerals of different types which may be in varying proportions. Contrast with "mineral".

salina. A saline playa, that is mostly underlain by complex associations of different salts.

scoria. Volcanic slag. Pyroclastic ejecta, usually of basic composition, characterized by marked vesicularity, dark color, heaviness, and a texture that is partly glassy and partly crystalline. Fragments of scoria between 4 mm. and 32 mm. are essentially equivalent to volcanic cinders.

sediment. The loose fragments of minerals and rocks which are transported by the wind, rivers, and glaciers as well as currents in lakes and the ocean.

shale. a laminated sediment in which the constituent particles are predominantly of the clay grade.

stratification. The structure produced by the deposition of sediments in layers or beds. May be found in pyroclastic and sedimentary rocks.

stream capture. The diversion of the upper part of a stream by the headward growth of another stream. Also called beheading, piracy, and stream robbery.

supergene. Applied to ores or ore minerals that have been formed by generally descending water. Ores or minerals formed by downward enrichment.

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syncline. A configuration of folded, stratified rock in which the rocks dip downward from opposite directions to come together in a trough. The reverse of an "anticline".

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synecology n. study of groups of organisms, often in a natural setting (Oosting 1956:12). The study of the environmental interrelationships among communities of organisms (Webster).

tafoni. Cavernous weathering features which have arch-shaped entrances, convex walls, overhanging upper margins, and fairly smooth, gently sloping floors.

talus. A collection of fallen disintegrated material which has formed a slope at the foot of a steeper declivity. Synonym: scree (AGI).

tephra. A collective term for all clastic volcanic materials which during an eruption are ejected from a crater or from some other type of vent and transported through the air; includes volcanic dust, ash, cinders, lapilli, scoria, pumice, bombs, and blocks. Syn: volcanic ejecta. (AGI)

terrace. Benches and terraces are relatively flat, horizontal, or gently inclined surfaces, sometimes long and narrow, which are bounded by a steeper ascending slope on one side and by a steeper descending slope on the opposite side. Both forms, when typically developed, are step like in character (AGI). Benches are smaller than terraces. An example of a terrace can be found in lower Opal Wash, while benches are found on the west side of Wild Horse Canyon, north of the Ceremonial Site.

topographic. Of or pertaining to the forms found at the earth's surface. The configuration of the earth's surface as it forms valleys, ridges, peaks, etc.

tuff. A rock formed of compacted volcanic fragments, generally smaller than 4 mm. in diameter.

turtleback. Large smoothly curved topographic surface underlain by folded metamorphic rocks in the Death Valley region; resembles a structural nose with amplitude up to several thousand feet.

turtleback fault. Low angle fault which has brought Cenozoic sedimentary and volcanic rocks into contact with metamorphics at a turtleback surface; interpreted as a folded overthrust or as a plane along which normal faulting or extensive landsliding has occurred.

Vertical Angle Bench Mark (VABM). A marked point of known elevation, established through survey of vertical angles (????).

volcanic ash. See "ash, volcanic".

volcano. Landform developed by the accumulation of cooled molten rock products near a central vent of opening.

wash. A watercourse, usually dry, with a loose surface of sand, gravel, boulders, etc..

watercourse. A natural channel for water.

weather. The state of the atmosphere at a given time and place, described by specification of variables such as terperature, moisture, wind velocity and barometric pressure.

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welded tuff. A tuff which has been indurated by the combined action of heat retained by the particles and the enveloping hot gases.

yardang. Irregular ridges, commonly alternating with round-bottomed troughs, formed by eolian erosion.

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Date and time this article was prepared: 12/24/2004 11:54:19 AM