Texas Bureau of Economic Geology The Big Bend of the Rio Grande: A Guide to the Rocks, Geologic History, and Settlers of the Area of Big Bend National Park

Aborigines—the inhabitants who lived in the Big Bend before the time of the first European explorers.

Adobe brick—the name applied to a sun-dried brick. The bricks are made from adobe, a clayey deposit found in the desert basins of southwestern United States and Mexico.

Agatized wood—the replacement of wood by agate (silica) in such a manner that the original grain, form, and structure of the wood are preserved. The agate is a variegated waxy quartz (silica) in which the colors are in hands or in distinctive color patterns.

Alamo—the Spanish name for a cottonwood tree. The term is commonly applied to streams, springs, and sometimes towns in the Southwest where there is a growth of cottonwood trees.

Algal reef—a reef composed largely of algal remains in which the algae were the principal lime-secreting organism.

Ammonite—one of the large extinct groups of mollusks related to the living chambered nautilus. The divisions between chambers (sutures) are complex and angular as compared with the simple sutures in the nautilus. See Nautilus.

Angle of repose—the maximum slope or angle at which a material such as soil or loose rock deposit remains stable. When the angle of repose is exceeded, mass movement by slipping down the slope may be expected.

Anticline—a Structure characterized by arched strata and formed by folding of layered rocks. The two sides of the fold are inclined (dip) away from each other along a common ridge or axis, similar to the two opposite slopes in the roof of a quonset hut. Opposite of syncline, which is a downfold in the bedded rocks.

Anticlinal ridge—a ridge formed by the arched beds in an anticline. Erosion may modify the folded ridge but commonly the crest of the anticline corresponds closely to the highest part of the ridge.

Arroyo—the channel of an intermittent stream, normally with nearly vertical banks of unconsolidated rock material 2 or more feet high, and a flat-floored channel often covered with rock debris. The term arroyo is sometimes applied to all intermittent drainage channels found in the Southwest.

Ash (volcanic)—See Volcanic ash.

Asymmetric fold—a fold in which the beds in one limb dip more steeply than in the other. If one limb becomes overturned, the term "overturned fold" is also used. See Overturned fold.

Badlands—a region nearly devoid of vegetation where erosion has cut the land surface into an intricate maze of narrow gullies (arroyos), sharp-crested ridges, and pinnacles. Traveling across such a region is difficult or almost impossible, hence the name badlands.

Badland topography—a region in which badland features form the dominant land surface.

Baile—the Spanish word for a dance.

Basalt—fine-grained, dark-colored igneous rock. It may be either intrusive or extrusive. Basalt is a common kind of lava. See Intrusion and Extrusion.

Basin (topographic)—generally any depression on the earth's surface into which all the adjacent land drains, whether occupied by water or not, is termed a basin; an area having no outlet is a closed basin.

Bed—the smallest division of a stratified rock sequence, marked both above and below by a more or less distinct plane that separates one individual bed from the adjacent rock layers (beds).

Bedded formation—a formation containing successive beds, layers, or strata.

Bedrock—solid rock, whether stratified or not, which underlies unconsolidated surface soil, sand, gravel, or clay.

Bedrock mortar—a depression in bedrock made by the Indians by grinding and used by them for hammering, crushing, and grinding grain. The procedure was similar to that used by early druggists who used a mortar and pestle for grinding drugs.

Bonito—the Spanish word for pretty.

Box canyon—a canyon having steep rock sides and a steep end, and normally a zigzag course.

Carranza—the President of Mexico during 1916.

Chicle—the Spanish word for chewing gum.

Cinco de Mayo—May 5, the date (in 1862) that Mexico defeated Maximilian's army. One of the two important Mexican holidays; it is celebrated as is July 4 in the United States.

Cinnabar—the principal ore of mercury (HgS). Often called quicksilver.

Comanche moon—See Indian summer.

Conglomerate—rounded water-worn rock fragments, cemented together by another mineral substance, similar in appearance to concrete. Sometimes called pudding stone.

Consequent stream—a stream whose course was controlled by the original slope of the land.

Contact—the place or surface where two different kinds of rock come together.

Continental deposit—sedimentary deposits laid down upon the land surface as contrasted with those laid down in the sea. They may be deposited in lakes, along Streams, or by the wind on dry land surfaces where there are neither lakes nor streams.

Continental shelf—the name applied to the shallow and gradual sloping ocean floor from the shore line out to a depth of about the 100-fathom (600 feet) line, beyond which the descent to the abyssal depths is abrupt. Most of the marine sediments (sandstone, shale, and limestone) are deposited on the continental shelf.

Continental slope—that part of the ocean floor extending from a depth of about 100 fathoms (600 feet) to about the 2,000-fathom depth (12,000 feet).

Correlation—the process by which stratigraphers attempt to determine the mutual time and rock relations of local sections.

Creep—the slow movement of finely broken rock materials or soils from higher to lower levels.

Cross-bedding—inclined beds or laminations between the main bedding planes; the inclined beds are oblique to the main planes of stratification. False bedding.

Cross section—a profile portraying a vertical section of the earth's crust.

Cuesta—a Spanish word that generally applies to an unsymmetrical ridge with one slope long and gentle, generally agreeing with the top surface of the resistant bed that forms it, and the other slope steep or even precipitous, formed on the cut edges of the bed or beds that form the gentle slope.

Cut-and-fill features—a feature resulting from the removal of one or more beds before the deposition of the overlying bed. Such features are often developed along stream meanders when there is cutting (erosion) on one side and fill (deposition) on the other.

Devils River Limestone—a formational term applied during the early twentieth century to most of the massive Lower Cretaceous limestone units in the Big Bend. In this report, the Devils River Limestone has been subdivided into the Glen Rose, Telephone Canyon, Del Carmen, Sue Peaks, and Santa Elena Formations (see pp. 15, 118 and table 1).

Dike—a tabular body of intrusive igneous rock that cuts the bedding of the host rocks.

Dike swarm—a set of generally parallel or radiating dikes.

Dinosaur—a class of extinct reptiles that lived during the Mesozoic Era. The term is rooted from the Greek words "dino" meaning terrible and "saur" meaning lizard; Terrible Lizard.

Dipping bed, dip—the angle at which any bed, stratum, or planar feature is inclined from the horizontal.

Displacement—see Fault displacement. Displacement may also be caused by igneous intrusions, folding, and slump (landslides).

Dome—a roughly symmetrical upfold in the rock layers; the beds dip more or less equally in all directions from a central point. A homely example would be to place a round vegetable bowl upside down on the table and spread the table cloth evenly over the bowl; the upward bulge in the table cloth would represent the dome. Opposite of a basin.

Dome crest—the top of the upfold (dome); highest point on the dome.

Dome mountain—a mountain resulting from domical upfold in the bedded rocks. Its form and the lack of plications in the exposed rocks suggest that the originating force pushed upward from beneath rather than laterally. This type of mountain is common in the western United States.

Downdropped or downthrown side—see Fault, downthrown side.

Ensilage—chopped forage (fodder) stored in a silo and used for feeding livestock.

Epoch—a subdivision of the periods in the geologic time scale. Example, Eocene. See geologic time scale, figure 5.

Era—a large division of geologic time including several periods. See geologic time scale, figure 5.

Erosion—the wearing away and removal of the rocks or soils of the earth's crust.

Eruption—in geology, the emergence of molten rock from a vent at the earth's surface.

Escarpment or scarp—the steep face of a ridge or mountain range. The escarpment normally abruptly separates the mountain range from adjacent level or gently sloping areas.

Escopeta—the Spanish word for shotgun.

Estufa (canyon)—the Spanish word for stove (generally cookstove); Estufa (stove) Canyon.

Extrusion—the act or process of thrusting or pushing out. In geology the term is normally applied to the emission of lava or other volcanic material at the earth's surface.

Extrusive rocks—a term applied to those igneous rocks derived from magma (molten rock beneath the surface) or magnetic materials that poured out or were ejected at the earth's surface; as distinct from the intrusive igneous rocks that cooled and solidified from magmas that invaded older rocks at depths.

Family—a taxonomic division used in the classification of animals and plants.

Fault—a fracture or fracture zone in the rocks along which there has been relative displacement of the two sides parallel to the fracture. The displacement may be a few inches, hundreds of feet, or many miles. See Fault displacement.

Fault block—a rock mass bounded on at least two sides by faults.

Fault block mountain—a fault block raised to form a mountain. Such mountains are normally carved by later erosion and are hounded by an escarpment on at least one side.

Fault displacement—displacement or dislocation is the relative movement of rocks on opposite sides of a fault. The movement may be vertical, horizontal, or both, and the displacement is measured as the distance between the broken ends of the same bed or formation as measured along the fault.

Fault, downthrown or downdropped side—the block of rock that has moved downward along the fault relative to the other side See Relative movement.

Fault escarpment (scarp)—an escarpment that coincides more or less closely with a fault. The escarpment normally occurs on the high side of the dislocation in the Big Bend National Park. See Fault-line scarp.

Fault-line scarp (escarpment)—a scarp (escarpment) which has been produced by differential erosion along a fault line. Erosion normally causes the escarpment to recede so that the fault-line scarp may be hundreds of feet or even miles from the true fault line.

Fault, normal—see Normal fault.

Fault scarp—the cliff formed by a fault. Most fault scarps have been modified by erosion since the faulting.

Fault, thrust—see Thrust fault.

Fault trace—the intersection of a fault and the earth's surface.

Fault trough—a relatively depressed fault block lying between Iwo faults with roughly parallel trace. See Graben and Rift valley.

Fault, sapthrown side—the block or mass of rock on the side of a fault that has been displaced relatively upward. The term refers to the relative movement and not an absolute displacement. Opposite of downthrown side.

Fauna—the animals (collectively) of any given formation, age, or region.

Fetlock—part of horse's leg where a tuft of hair grows behind the pastern joint.

Fiesta—the Spanish word for a holiday or celebration.

Fissure—a crack, break, or fracture in the rocks. Fissures normally have well-defined boundaries that constitute the sides; some of them have been filled with minerals, but many of them are open passages.

Flaggy—strata from 10 to 100 mm (about 0.4 to 4.0 inches) thick which have a platy or tabular character.

Flagstone—a rock that splits readily into slabs suitable for flagging.

Flood plain—all great rivers annually flood portions of the level land along their banks, especially near their mouths, and cover them with sedimentary deposits. The area thus flooded is called the flood plain. Generally called bottom land in western United States.

Flora—the plants (collectively) of a given formation, age, or region.

Fold—a bend in the strata (bedded rocks). The upfolds (arches) are called anticlines the downfolds synclines.

Fold axis—the line following the apex of an anticline or the lowest part of a syncline.

Foot wall—the mass of rocks beneath the fault plane.

Formation—the fundamental unit in rock stratigraphic classification. A formation is a body of rock characterized by lithologic homogeneity; it is prevailingly but not necessarily tabular and is mappable at the earth's surface or traceable in the subsurface.

Fossil—the remains or traces of animals or plants which have been preserved by natural causes in the rocks of the earth's crust.

Foxfire—the phosphorescent light emitted by rotting wood.

Fracture—a break in the rocks.

Fresh-water deposit—sediments deposited in a lake or other water body that does not contain the mineral salts normally found in sea water.

Fresno—the Spanish word for ash tree.

Frijoles—the Spanish word for pinto or speckled brown beans.

Frost action—the action of freeze (expansion) and thaw which is a powerful weathering agent and eventually causes disintegration of rocks.

Glacial climate—the cold climate normally associated with periods of glaciation.

Glacier—a mass of ice having definite lateral limits and motion in a definite direction that has originated from the compaction of snow in the snowfield that feeds the glacier.

Glass (volcanic glass)—natural glass produced by the rapid cooling of molten lava. The rate of cooling is too rapid to permit crystallization, which results in the formation of obsidian and pitchstone.

Glochids—a cluster of tiny spines on the pads of the prickly (blind) pear.

Graben—a block of the earth's crust, generally long as compared to its width, that has sunk between two or more faults. The surface of the depressed block is normally low as related to the surface on the opposite sides of the bounding faults.

Grains—in geology, the particles or discrete crystals which comprise an igneous rock; the individual particles that comprise a sedimentary rock, as the sand grains in a sandstone.

Grain size—a term relating to the size of mineral particles that make up a rock whether igneous or sedimentary. Such terms are normally expressed as fine-, medium-, or coarse-grained.

Granite—a coarse-grained, light-colored, igneous rock composed of pink or gray feldspar, quartz, and commonly mica or hornblende.

Granular—a textural term applied to igneous rocks in which the mineral grains are more or less granular, and to sedimentary rocks made up of granular grains.

Gravel—an unconsolidated accumulation of pebbles mixed with sand grains and granules. The word gravel is generally applied when the size of most of the largest pebbles does not exceed that of an ordinary hen's egg. If the gravel is cemented with mineral matter, it is normally called a conglomerate.

Grinding—the wearing away of rock materials through the effect of continual contact, pressure, and abrasion by other rock particles.

Gully erosion—the removal of soil, weak rock materials, sand, etc., by running water during heavy rainfall. On farms, the term is often applied to any erosion channel that cannot be eliminated by plowing.

Habitat—the environment in which the life needs of a plant or animal are supplied.

Hanging wall—the mass of rock above the fault plane.

Herbiverous—animals that feed on herbs or other vegetable matter. Opposite of carniverous (flesh eaters).

Heterogeneous—as applied to rocks, commonly conglomerates, the pebbles show great variation as to size, kind, composition, color, hardness. Antonym, homogeneous.

Hill—hill and mountain are relative terms with respect to each other—the highest elevations are designated mountains and the lower elevations, hills. Some would class all elevations of 1,000 feet or more as mountains and the gradational features between the higher and lower features as foothills. Normally, the term hill is used to designate a mass that rises conspicuously above the level of the surrounding country, regardless of the elevation, and culminates in a well-marked crest or summit.

Hogback—a term commonly applied to a sharp-crested ridge formed by a hard rock ledge that dips steeply beneath the earth's surface. Due to resistance to erosion the hard rock ledge rises boldly above the weaker rocks that surround it. See Cuesta.

Hornfels—a fine-grained, tough, hard, non-laminated metamorphic rock resulting from contact metamorphism (baking). See Metamorphism. Most hornfels in the Big Bend are black: they are conspicuously exposed on the west flank of the McKinney Hills.

Horsetrap—a fenced enclosure with sufficient forage to sustain a few horses, The mounts used regularly on a ranch are kept in the horsetrap, where they can forage for themselves but are readily available.

Huacamole—a green salad consisting chiefly of mashed avocado, with chopped onion, tomatoes, and seasoning spices.

Huaraches—the Spanish word for sandal. The early Mexican huaraches were made from untanned leather, not decorated, and were fastened to the feet by rawhide strings. In recent years, most of the locally made Mexican huaraches along the Rio Grande have been made from pieces of automobile tires.

Ice age—the glacial period.

Igneous—rocks formed by solidification from a magma, or partially molten materials. One of the two great classes into which most rocks are divided. Contrasted with sedimentary rocks.

Igneous intrusion—A body of igneous rocks that invaded older rocks and solidified beneath the earth's surface.

Impervious—term applied to strata such as clay and shale which do not permit the penetration of water.

Indian summer—a period of warm, quiet, hazy weather that may occur in late September, October, or early November; in some years there may be only a few days or none at all, while in other years. there may be one or more extended periods. The Indian summer period includes the time of the Mexican or Comanche moon.

Indurated rock—hard rock. In normal usage. the term applies to rocks hardened by cementation, by pressure, or by heat, or any combination of these agencies.

In situ, or in place—in its natural position or place; in geology, said specifically of a rock, soil, or fossil found in the situation or place where it originally formed or was deposited.

Intermittent stream—a stream which flows but part of the time, as after a rainstorm or during but part of the year.

Intermontane basin—a basin lying between mountain ranges, common in the Great Basin region.

Intrusive or intrusive rocks—in geology, a mobile mass that penetrates into or between other rocks and which has solidified beneath the earth's surface.

Invertebrate fossil—fossil of an animal having no backbone or spinal column. See Vertebrate.

Jacal—the Spanish word for a small adobe, rock, or rock and adobe hut.

Joint—in geology, a fracture or parting which interrupts abruptly the physical continuity of a rock mass.

Jointed rocks—rocks that are fractured in a consistent pattern. See Joint.

Laccolith—a concordant, intrusive body that has spread laterally between rock layers, doming the overlying rocks but not deforming the underlying rock; thus a laccolith normally has a horizontal or nearly flat base and is mushroom- or tackhead-shaped.

Lagoon—a pool or lake of shallow water, normally salty, near the seashore and often connected to the ocean by a narrow passage. The quiet water on the land side of a barrier island; example, Laguna Madre on the Texas Gulf Coast.

Land form—the term land form is applied by physiographers to each one of the multitudinous features that taken together make up the surface of the earth.

Land sculpture—the carving of physical features by natural causes, chiefly by running water but also by waves, wind, and glaciers.

Landslide—the downward sliding or falling of a relatively large mass of earth, rock, or mixture of both from a mountain or any steep slope to a lower level. Landslide action is commonly triggered by excessive rainfall.

Lava—fluid rock such as that which issues from a volcano or a fissure to the earth's surface; also the same material solidified by cooling on the earth's surface.

Lava flow—(1) a river or sheet of fluid, viscous, or partially solidified lava that issues from a volcanic vent or from a fissure in the earth's crust. (2) The solidified, stationary mass of rock formed when the lava congeals on the earth's surface.

Ledge—a term commonly applied to one or several beds of hard rock occurring in a hill-side; a narrow shelf-like projection from the face of a steep declivity.

Lignite—a brownish-black low-rank coal in which the alteration of the vegetable material has proceeded further than in peat but not so far as in subbituminous coal.

Limestone—a bedded sedimentary deposit consisting chiefly of calcium carbonate (CaCo₃). Limestones contain 50 percent or more of the carbonates of calcium or magnesium and are widely distributed on the earth's surface.

Lithology—a term commonly used to refer to the composition of a rock.

Load—the sediments moved by a stream, whether in suspension (floating) or being rolled and pushed along the stream's bottom.

Magma—Naturally occurring molten rock.

Mammal—a warm-blooded vertebrate animal that brings forth its young alive and suckles the young.

Marl—A calcareous clay, or intimate mixture of clay and calcium carbonate, usually with fragments of shells. The term marl is also often applied to soft, impure limestone.

Massive—in this report, the term massive is used to describe sedimentary strata that are 3 or more feet thick, normally hard, free of minor joints and laminations, and having homogeneous physical characteristics.

Meandering (stream)—the characteristic habit of a stream that winds freely on a broad flood plain.

Member (of a formation)—a division of a formation, generally of distinctive character, containing a specific fauna or of local extent.

Mesa—a tableland; an elevated, flat-topped land form bounded on at least one side by a steep cliff. Mesa is a Southwestern term and is more often applied to small areas (several square miles) rather than to large features which are more properly called plateaus.

Metamorphic rock—a rock that has been metamorphosed. See Metamorphism.

Metamorphism—a change induced in a rock by high temperatures and pressures. For example, when a limestone is deeply buried in the earth's crust, the higher temperatures cause the calcite to recrystallize and a marble is formed. The limestone has been metamorphosed.

Metate—the Spanish word for a dish-shaped or hollowed-out stone used for grinding grain.

Mexican moon—see Indian summer.

Mineral—a naturally occurring, homogeneous inorganic substance, having crystalline structure and normally a definite chemical composition. Most minerals contain impurities.

Mineral deposit—an accumulation of minerals.

Mineral-spring—a spring whose waters contain mineral salts.

Morrale—the Spanish word for a knapsack, nosebag for feeding horses, shopping bag, hunter's bag, etc.

Mortar—in this report, a rock vessel in which the Indians ground or pounded grain with a rock pestle to make flour. See Metate.

Mud cracks (sun cracks, shrinkage cracks)—a regular system of cracks formed when mud sediments lose water. The cracks form a polygonal pattern.

Muy malo—the Spanish words for "very bad"; often applied to degree of sickness or to a person's general physical condition.

Nautilus—One of the cephalopods having an externally chambered shell, coiled planispirally, with enlarging whorls in direction of living chamber. The divisions (sutures) between the chambers are straight or simply curved as contrasted with highly irregular sutures of its extinct relative, the ammonite.

Neck—see Volcanic neck.

Normal fault—a fault in which the hanging wall has dropped relative to the footwall.

Nugget—a water-worn piece of native gold.

Obsidian—natural volcanic glass. Most obsidian is black, although color may be red, green, or brown.

Order—a category in the classification of animals and plants, ranking between a family and a class; an order is composed of several families. See Family and Class.

Ore—a metalliferous mineral or aggregate of metalliferous minerals that can be mined and treated at a profit.

Organic—pertaining to living things (animals and plants) or to those plants and animals that once possessed life.

Outcrop—the exposure of rock at the earth's surface.

Overthrust fault—fracture along which one mass of rock has overridden another wherein the hanging wall (relatively) has moved over the footwall.

Overturned fold—a fold in which the beds on one limb have been tilted past the vertical position so that they are inverted in the outcrop.

Paleontology—the science that deals with the study of both fossil plants and animals from past geologic ages.

Parting—a small fracture in the rocks.

Pass—a gap, or other relatively low break, in a mountain range, or valley between ranges, through which a stream, road, or trail passes.

Peat—A dark brown or black soft substance produced by the partial decomposition, disintegration. and carbonization of vegetation in bogs or marshes. The formation of peat is an early stage in the development of coal.

Period—the fundamental unit of the standard geologic time scale. Example, Devonian Period. See geologic time scale, figure 5.

Pervious bed (rock)—a bed or stratum with pores through which fluids can move.

Petrify—to become stone. Organic substances, such as shells, bones, and wood, embedded in sediments, become converted into stone by the gradual replacement of their tissue, particle by particle, by mineral matter. Commonly, not only the outward form but even the minutest details of the organic tissue are preserved.

Petrified wood (silicified wood)—wood replaced by silica.

Petroleum—a naturally occurring substance composed predominantly of carbon and hydrogen compounds. Petroleum may occur as a liquid, gas (natural gas), and/or in a solid (asphalt) state depending on the nature of these compounds and the existing conditions of temperature and pressure.

Physiography—physiography has to do with description of land forms.

Plug or plug-like mass—an intrusive mass that cut through and deformed the older rocks.

Porosity—the ratio of the aggregate volume of voids in a rock or soil to its total volume.

Porous—containing voids, pores, interstices, or other openings which may or may not be connected.

Presidio—the Spanish name for fortress or prison and also a place where troops were garrisoned. In addition to the fortress, the early presidios also included a religions chapel.

Primitive fossil—fossilized remains of the earliest simple life.

Producer's gas—a combustible gas manufactured from coal.

Prospect—the name given to any mine workings, the value of which has not been determined.

Prospecting—searching for new mineral deposits; also, preliminary exploration to test the value of a mineral deposit already known to exist.

Relative movement—in studying structural features, for example, faults, it is often impossible to determine which block actually moved, Thus, most displacements can be discussed only in terms of relative movement, that is, block A moved relative to block B. The downthrown side in a normal fault is the one that shows the relative downward movement—both sides may have moved down, but the downthrown side moved down farther in relation to the upthrown side.

Remuda—the Spanish word for a group of horses. Also a term commonly applied to a herd of spare horses or spare pack animals.

Relief—(1) The elevations or the inequalities (collectively) of a land surface, (2) The difference in elevation between the high and low points of a land surface.

Retorsely barbed—barbs bent or turned backward.

Rhizomes—an underground bulbous stem that produces roots below and leaves and bloom stalk above. Example: iris.

Rhyolite—the extrusive equivalent of a granite. Rhyolites are normally fine grained or have visible crystals scattered through a fine groundmass; some rhyolites are glassy.

Rift valley—an elongate valley formed by a depressed block of the earth's crust that lies between two or more faults with approximately parallel trends. See Graben.

Rimrock—rocks or rock ledge forming a natural and commonly precipitous boundary of an elevated land form.

Ripple-mark—an undulating surface produced in sand or non-coherent materials by the wind, by currents, or by wave action.

Roadrunner—a long-tailed, slender, ground cuckoo that inhabits the open region of the Southwest. It spends most of the time on the ground and can run with great swiftness. It is commonly also called the chapparal [bird] and by the Latin-Americans, paisano (fellow countryman ).

Rock—strictly, any naturally formed aggregate or mass of mineral matter, whether or not coherent, which constitutes part of the earth's crust. Commonly, however, the term rock is applied only to firmly consolidated mineral masses, which would exclude loose unconsolidated sand or gravel and soft clay.

Roll—this term is variously used to describe minor deformations or dislocations. In this report it applies to small folds in thin-bedded weak rock that was folded when supporting strata were removed by solution and the overlying mass collapsed or slumped.

Runoff—the water which flows on the earth's surface.

Saddle—a low point in the crestline of a ridge, often a divide between the heads of streams flowing in opposite directions.

Sand—an unconsolidated detrital material consisting predominantly of quartz grains.

Sandstone—a cemented or otherwise compact detrital sediment composed predominantly of quartz grains, the sizes of the grains being those of sand.

Sapping—the process of weathering and erosion that causes the disintegration and removal of weak rock layers underlying a more resistant bed; due to the lack of support, the upper resistant bed collapses and fragments tumble down the mountain slope.

Saturated—a rock or soil is saturated with respect to water if all its interstices (voids) are filled with water.

Scarp—an escarpment, cliff, or steep slope.

Schist—metamorphic rock with a foliated structure.

Sedimentary rocks—rocks formed by the accumulation of sediments following deposition from a transporting medium such as water (aqueous deposits), air (eolian deposits), or both. The sediments consist of rock fragments of various sizes (conglomerate, sandstone, shale); the remains or products of animals or plants (certain limestones and coal); the products of chemical action or of evaporation (limestone, salt, and gypsum); or mixture of the above materials. Some sedimentary deposits (tuffs) are composed of volcanic ash and are deposited either on land or in water. A characteristic feature of sedimentary rocks is their layered structure, known as bedding or stratification. Each layer is a bed or stratum; the layers are deposited in a flat, or nearly flat, position. When sedimentary rocks are observed that are not in a nearly flat position, they have been deformed by forces that deformed the earth's crust. See Igneous rock for contrast of origin.

Seep—a place where water oozes from the earth, often forming a spring.

Semiarid—descriptive term for an area of low rainfall; a subdivision of climate in which the associated plants are spiny shrubs, stunted trees, and short grasses, whereas a subhumid climate is characterized by tall trees and grasses.

Shale—a laminated sedimentary rock in which the constituent particles are predominantly clay (consolidated mud).

Sheet erosion—erosion accomplished by sheets of unchanneled running water.

Sheetflood—under certain conditions, essentially unchanneled sediment-laden water flows over an erodable surface in a sheet. This is termed sheetflood.

Shut-in or Shut-up—a narrow gorge with steep sides in a broader valley.

Sierra—a Spanish word for mountains. More strictly applied to mountain ranges that have a jagged crest.

Sill—a tabular intrusive body of igneous rock which conforms to the bedding or structure of the host rocks.

Slump—when the soil, rocks, or any earthy material on a slope becomes unstable and moves down the slope under the influence of gravity, it is said to have slumped.

Sorting—as applied to sediments, the dynamic process by which material having the same peculiar characteristics, such as size, shape, or specific gravity, is separated from a larger heterogeneous mass.

Species—a group of individuals (animals or plants) having substantially the same structure, habits, geographic and geologic range, which normally interbreed, producing like kind.

Sponge spicule—a small, slender, spindle-shaped, sharp-pointed body that forms part of the skeleton or supporting framework of the sponge.

Stratification—the planar structure in beds or layers of strata. A common characteristic of sedimentary rocks.

Structure—strictly, and properly, structure is the sum total of all the structural features (rock deformations) in an area. However, commonly, a particular feature, such as an anticline is referred to as a structure.

Symmetrical fold—a fold with an essentially vertical axial plane so that the two limbs are symmetrical.

Syncline—a fold in rocks in which the strata dip inward from both sides; a downfold. The opposite of anticline.

Tank—a small artificial depression in which water collects. It is used chiefly for watering livestock. The term is commonly used in the arid Southwest and is synonymous with the term pond in many areas.

Tequila—an alcoholic beverage made from the century plant (maguey).

Terrace—a relatively flat, horizontal or gently inclined surface, sometimes long and narrow, which is bounded by a steeper ascending slope on one side and a steeper descending slope on the opposite. Generally formed along stream courses.

Throw—the amount of vertical displacement along a fault.

Thrust fault—a fault that is characterized by inclination of the fault plane where the hanging wall (upthrown side) has moved over the footwall (downthrown side), thus causing shortening in the earth's crust.

Tinaja—the Spanish word for a natural bowl, bowl-shaped cavity, or hole in the bedrock in which water accumulates.

Tornillo—the Spanish word for screw and applied to a species of scrubby tree in the Southwest that has a seed pod with coiled revolutions similar to those of a common wood screw.

Trans-Pecos Texas—that part of Texas that lies west of the Pecos River; the area bounded on the northeast and east by the Pecos River and on the southwest and south by the Rio Grande. Some persons consider southeastern New Mexico as a part of the Trans-Pecos region.

Truncate—geologically applied to rock units and structure cut by an erosion surface.

Tuff—consolidated volcanic ash.

Tuna—the Spanish word for the fruit of a prickly pear.

Unconformity—a surface of erosion or non-deposition, especially the former, that separates younger rock layers (strata) from older rocks. Commonly, the stratification in the two sequences of rocks intersects at an angle and this relationship is normally designated as an angular unconformity.

Upthrow—the block or mass of rock on that side of a fault that has been displaced upward.

Vent (volcanic)—an outlet, through which volcanic ejecta erupted on the earth's surface.

Vertebrate—an animal having a backbone or spinal column, such as in mammals, birds, and fishes.

Villa—one of the Mexican revolutionists during 1910-18.

Viva Carranza—hurrah for, or long live Carranza.

Viva Villa—hurrah for, or long live Villa.

Volcanic ash—uncemented material consisting of fine rock fragments or volcanic glass, produced during the explosive action of volcanoes. If the ash is later cemented into a compact (indurated) rock mass, it is called tuff.

Volcanic cone—a cone-shaped eminence formed by volcanic discharge.

Volcanic crater—a steep-walled depression on top of a volcanic cone, directly above the vent that feeds the volcano.

Volcanic neck—the solidified material filling the vent of a dead volcano.

Volcanic rocks—the igneous rocks that have been poured out at the earth's surface by volcanic activity. The term is synonymous with extrusive rocks.

Volcano—a vent in the earth's crust from which molten lava, ash, cinders, and gases issue; the mound of volcanic ejecta.

Weathering—action by chemical and physical processes which attack rock at the earth's surface.

Windcharger—a small wind-powered electric generator. Those used in the Big Bend during the mid-1930's were commonly mounted on a tower up to 20 feet high and when the velocity of the wind was 15 to 20 miles per hour they would light two to three 50-watt bulbs. Naturally, there was some variation in the brightness of the lights unless the voltage was regulated by storage batteries. The windchargers were stocked by some of the larger trading posts and they could be obtained from some mail order houses for about $17.50 each.

Wind gap—a low depression or notch in a mountain range where a stream formerly flowed. Wind gaps are places where mountain ranges are most easily crossed and as such were exploited by transportation routes, first by stages and wagons, then by highways and railroad.

Back Cover (1987 edition): View from the South Rim of the Basin overlooking Elephant Tusk, Backbone Ridge, and Dominguez Mountain (three peaks in background from left to right) and Fresno Creek. (photo by Raquelle Smalley Keegan)