Stone

Quarrying of Paleozoic limestones and dolomites along the east flank of the Rampart Range northwest of Colorado Springs has badly defaced a prominent mountain backdrop. Recent seeding efforts by quarry operators are returning the exhausted part of the quarry to its original lightly vegetated condition, and hopefully, as the quarry is depleted, the scar will disappear. (John Chronic photo)

In Colorado, as in most parts of the world, building stone for local use is quarried locally. Two of the state’s stones, however—Yule Marble from the Crystal River Canyon, and Lyons Sandstone of the Front Range—have been more widely used.

The Yule Marble, or Yule Colorado Marble, was produced by metamorphism of Leadville Limestone in an area intruded by the Treasure MountainGranite, thirty-five miles south of Glenwood Springs. This exquisite marble, which has graced many famous monuments and buildings (among them the Lincoln Memorial and the Tomb of the Unknown Soldier), is known for its almost uniform snowy whiteness and regular, fine crystallization. Although its beauty, massive character, and uniformity made it a sought-after ornamental stone, quarrying was economically marginal because of the remoteness of the site. In spite of this, nearly $7,000,000 worth of the marble was produced before the quarry closed in 1940.

Pure white marble was quarried for many years at the Yule Colorado Marble Quarry, about three miles southeast of the village of Marble. (U. S. Geological Survey photo)

The Lyons area, north of Boulder, provides pink, hard, even-grained sandstone which splits readily into slabs or flagstones. These are used in the Denver-Boulder area for sidewalks and patios as well as for facing buildings. Quarries owned by the University of Colorado provide a constant supply of handsome facing material and flagstone for new university buildings, although in recent years the high cost of stone construction has limited its use on the campus.

Lyons Sandstone is quarried near Lyons, Colorado. The salmon-colored sandstone splits along surfaces defined by slight differences in size and arrangement of the sand grains. (John Chronic photo)

Most of the buildings of the University of Colorado are faced with Permian Lyons Sandstone, which is widely used for buildings and flagstones throughout the Boulder-Denver area. The University Museum, shown here, was established in 1902, and contains over a million scientific specimens, including many Coloradofossilsand minerals. Exhibits in the Hall of Earth portray Colorado’s geologic history. (Tichnor Bros. photo)

The Lyons Sandstone was deposited as beach and bar sand along the edge of a sea which lay east of the Front Range in Permian time. After deposition, the sand was deeply buried and compacted. Now tilted up along the Front Range uplift, it comes to the surface along the east side of the range. Only between Fort Collins and Boulder does the stone have the desirable combination of hardness, thin-beddedness, and color which makes it desirable for ornamental use. The pink color of the Lyons Sandstone is derived from iron oxides, mostlyhematite, disseminated between the sand grains. Dendrites (often erroneously calledfossilferns or plants) ornament some slabs; they were formed by crystallization of manganese dioxide from groundwater as it slowly percolated through the rock.

Outcrops of the Cretaceous Greenhorn and Niobrara Limestones provide most of the cement materials in Colorado. A number of plants along the mountain front, including a completely automated and dust-free one near Lyons, provide the major population centers with millions of tons of cement each year.

Colorado is richly endowed with gypsum, useful in cement and plaster manufacture and for ornamental stone and sculpture. Along the eastern front of the mountains, gypsum occurs in the Triassic Lykins Formation; in the Mountain Province, it is abundant in Pennsylvaniansedimentary rocks. Particularly high-quality Pennsylvanian gypsum is quarried at the town of Gypsum, west of Eagle.

The Colorado portion of the Paradox Basin, in thePlateauProvince, contains immense deposits of Pennsylvanian gypsum. Here, rocks near the surface have been pushed up into sharp northwest-trending faultedanticlinesby upward movements of gypsum and salt from depths of several thousands of feet. The soluble salt and gypsum cores of these structures have been washed away more rapidly than the surrounding layers of sandstone and shale, leaving depressions such as Gypsum Valley, Paradox Valley, and Sinbad Valley, on the crests of the anticlines. Red and yellow Triassic sandstones and shales, especially the Chinle Formation and the Wingate Sandstone,dipaway from thesevalleys. Exploratory wells indicate that vast masses of salt and gypsum are present beneath the surface, and may extend to depths greater than 10,000 feet.

More than thirty different gems and ornamental stones are known to occur in Colorado. Amazonstone, amethyst, garnet, tourmaline, aquamarine, topaz, lapis lazuli, quartz crystal, smoky and rose quartz, sapphire, several varieties of agate, zircon, and other attractive stones are gathered within the state, mainly in the Mountain Province. Turquoise is known at several places in the volcanic area of southern Colorado. Alabaster is mined along the northeastern mountain front near Fort Collins and Loveland. Localities of interest to gem hunters are described inColorado Gem Trails and Mineral Guide, by Richard M. Pearl.

Gem Village, in southwestern Colorado on U. S. highway 160 between Durango and Pagosa Springs, is a favorite stopping place for tourists wishing to see or buy colorful and attractive Colorado stones such as petrified wood, agatized dinosaur bones, chalcedony, and jasper.

Although not all aspects of water and water supply are geologic, water is an important geologic agent, determining the shape of the surface, the distribution of minerals, and the location of caves. Water used in Colorado comes entirely from precipitation within the state, as all of Colorado’s rivers flow from Colorado outward toward the surrounding lower-elevation states.

A cross section through the Front Range northwest of Denver shows the redistribution and use of western slope water in eastern Colorado through the Colorado-Big Thompson Project. This project has cost about $160,000,000, but it is repaying the investment many times over by providing electric power and increasing farm production.

Moisture carried by prevailing westerly or northwesterly winds falls primarily on Colorado’s western slope, although at some times of year precipitation may come from the northeast or southeast. West of the continental divide, where population is sparse, there is a surplus of water. East of the divide, where more than 90 per cent of the population lives, water is in desperately short supply. The high and largely unpopulated Mountain Province receives by far the greatest proportion of precipitation, while agricultural areas of the Prairie andPlateauProvinces receivemuch less. Needless to say, the major problem involving water in Colorado is how to move it from areas where it is abundant to areas where it is needed.

In many parts of the state, complex water laws and complicated irrigation canals and water systems were developed soon after the area became settled. Gradually but inevitably, water resources have been transferred from the western slope to the eastern. However, such transfer must be undertaken with due regard for the rights of downstream users, notably California, Arizona, and New Mexico.

One of the largest water movement schemes in the state is the Colorado-Big Thompson Project. Water that otherwise would flow into the Colorado River is piped from Grand Lake through the Alva B. Adams tunnel under the high mountains of Rocky Mountain National Park, and into the Big Thompson drainage near Estes Park. It then travels through a series of reservoirs and tunnels into the South Platte River basin, where it is used for irrigation and household water. The water is pumped up the western gradient of this system by electric power produced as it flows down the eastern slope. Surplus electric power serves the Colorado-Wyoming area.

Another large project is the Denver Water Board’s Dillon Reservoir Project, in which western slope water collected at Dillon is pumped twenty-three miles under the continental divide through the Harold D. Roberts tunnel to the North Fork of the South Platte River for use by the city of Denver. The exit point of this tunnel can be seen a few miles west of Grant along U. S. highway 285. This project is continuously growing as Denver’s water needs mount.

In each of these projects, engineering geologists played a prominent part in locating dams and tunnels that would not leak or fail, and that could collect and transport a maximum amount of water during the high-runoff spring season for distribution through the rest of the year. Fortunately for geologists, the tunnels and bores necessary to the projects allowed them to learn a great deal about the structure of the interior of the high mountains, and helped to improve their interpretation of earth history in this most interesting region.

The necessity for storing irrigation water along the eastern mountain front has led to the creation of hundreds of new lakes in the region. Although water levels vary with the season, many of the lakes provide opportunities for water sports and recreation for the burgeoning inland population.

Two large dams have recently been built in western Colorado for another purpose: to control the flow of water in the Colorado River drainage basin. Electric power for western Colorado also comes from these dams. One of the dams is on the Gunnison River at Curecanti, upstream from the Black Canyon of the Gunnison National Monument, and the other is on the Frying Pan River near Ruedi. The latter was completed over the objections of geologists, who believed that the extensive gypsum deposits underlying the damsite would cause its failure. Cement pumped deep into the rocks in the vicinity has so far prevented serious rupture.

There is strong resistance by conservation groups to the construction of more dams on Colorado River drainage, primarily because the Colorado and its tributaries pass through many irreplaceable canyons, some of them parts of National Parks and Monuments, that are very much a part of our western heritage.

In the San Luis Valley, runoff from the San Juan and Sangre de Cristo Mountains sinks into layers of sand in the Alamosa Formation. Flowing along the sand layers toward the center of the valley, it providesartesian waterfor irrigation of valley farmlands.

Groundwater is extremely important to Colorado, especially in the Prairie Province and the San Luis Valley. Below these two areas lie a number of distinct and productive groundwateraquifers, several of them artesian. In Otero County, for example, there are five major aquifers: three separate Quaternary gravel deposits, the Cretaceous Dakota Sandstone, and the Cheyenne Sandstone Member of the Purgatoire Formation, also Cretaceous. All these aquifers are characterized by their high porosity and permeability,which allow water to flow rapidly through them. Wells in the younger, shallower aquifers produce as much as 2,000 gallons per minute; those in the older, deeper aquifers produce about eighty gallons per minute, some of it with an artesian “head.”

The San Luis Valley supports intensive agriculture, made possible by a greatartesian watersupply. A thick series of soft interlayered clays and sands, the Alamosa Formation, slopes down toward the center of the basin from the surrounding mountains. Water entering the sandstone beds at the mountain edges flows through the sand layers held there by the impermeable clay beds. By the time it reaches the center of the valley, it has developed considerable hydrostatic head, and the water rises in wells without pumping. Unfortunately, both the irrigation water and the soils in the San Luis Valley are highly alkaline. Constant evaporation from the irrigated fields has concentrated the alkali near and on the surface, rendering some of the land less usable than it was originally.

Colorado has many caves, most of them carved by underground water in Paleozoic limestone. The Cave of the Winds at Manitou is the only one in the state which has been developed as a tourist attraction. It is in highly faulted Ordovician and Mississippian limestone near the mountain front, where the faulting, coupled with the high relief, has accelerated solution of the rock by allowing groundwater to percolate downward rapidly. The cavern was probably carved during the Pleistocene Ice Age, when surface water and groundwater were much more abundant than at present. Deposition ofstalactitesandstalagmiteshas occurred within the last few thousand years, as supplies and movement of water have decreased.

Spanish Cave, above timberline on Marble Mountain in the Sangre de Cristo Range, is probably the nation’s highest limestone cave. It is in thick folded and faulted Pennsylvanianreeflimestone, at an elevation of over 12,000 feet. The cave has many intricate passageways branching from its main vertical tubes and channels.

Fulford Cave, south of Eagle, is in the Mississippian Leadville Limestone of the northern part of the Sawatch Range. Many othercaves are situated south of Fulford, near Woods Lake, where the limestone is widely exposed and highly dissected.

Fairy Cave, northeast of Glenwood Springs, is the best known of the many caverns in the Paleozoic limestones that form the southern flanks of the White RiverPlateau.

In Cave of the Winds near Manitou, Paleozoic limestones, cracked and tilted by uplift of the Front Range, have been honeycombed by ground water. Calcitestalactiteshang from the ceiling, whilestalagmitesgrow up from the floor. (Cave of the Winds Company photo)

In thePlateauProvince another type of cave is formed not so much by groundwater as by weathering of the flat-lying alternating beds of massive resistant sandstone and less resistant, thinlybedded mudstone and shale. Where the resistant layers are undermined, great arching caves develop. These are best observed atMesaVerde National Park, where many of them once sheltered Indian communities. They can also be seen in Colorado National Monument and along the Colorado River and several of its major tributaries.

Along the edge atMesaVerde, caves in Cretaceous Mesa Verde sandstone were used for shelter by Indians. Springs near the bases of the caves, which provided the Indian communities with water, probably contributed to the undermining of the sandstone cliffs. (Colorado Department of Highways photo)

The multitudes of mineral and hot springs in Colorado are a fascinating and interesting facet of the Mountain Province. Some are located along majorfaults, where the rocks are so broken and shattered that groundwater can move freely toward the surface. Colorado Springs, Manitou Springs, and Eldorado Springs are on the fault complex that forms the east edge of the Front Range. Glenwood, Juniper, Steamboat, and Poncha Springs are on well defined faults also.

Glenwood Hot Springs flow from Pennsylvanian shales of the Belden Formation, where sedimentary layers are faulted by the sharp upward tilting against the south side of the White RiverPlateau. Behind the hotel and on the right can be seen the Mississippian Leadville Limestone, cut by the Colorado River. (From a painting by William H. Jackson, courtesy of Colorado State Archives and Public Record)

Many other springs do not seem to be controlled so strongly by faulting, but owe their presence to sources of volcanic or magmatic heat which exist near to the surface of the ground. Some springs of this type issue from Precambriangranite, or Cenozoic volcanic rock, while others flow from sedimentary rock layers. Waunita Hot Springs and Pagosa Springs, although near volcanic rocks, reach the surface through porous sandstones and shales ofCretaceous age. Mt. Princeton Hot Springs comes fromalluviumbut its heat source is the intrusive igneous rock which makes up part of the adjacent mountain.

Springs of another general type are also present in Colorado whereaquifers, generally sandstones, are dissected by erosion. These springs, usually not highly mineralized or warm, are most often found in thePlateauProvince. Such springs are frequent at the bases of the great sandstone cliffs ofMesaVerde and Colorado National Monument.

Manitou’s carbonated springs, which attract many tourists, have their origin in the arrangement and nature of the rocks through which the water flows. Water from the Pikes Peak region, slightly acid from its contact with the granitic rock, flows into the Manitou limestone all along Ute Passfault, which extends from Cheyenne Mountain northwest to Woodland Park. Descending through channels along the fault, the water becomes pressurized. Because of its pressure and its acid content, it partly dissolves the calcium carbonate of the limestone, and from then on carries carbon dioxide in solution. As the water comes to the surface at the low point of the fault exposure, near the west edge of Manitou, the pressure is released and the carbon dioxide effervesces, just as a bottle of soda water effervesces when the cap is removed.

The preceding part of this chapter mentions many ways in which man’s destiny in Colorado has been shaped by geologic factors. Early Coloradoans settled near gold and silver placers, later ones near mines that produced ores of other metals, or in the towns that sprang up around the mills and smelters that processed these ores. Our present distribution of population is partly a heritage from these first settlements, partly a result of later discoveries of oil, gas, and radioactive minerals, and partly a response to the state’s extreme topographic variation, which controls and delineates agricultural areas and transportation routes.

In recent years, man has begun to appreciate the fact that he may benefit in other ways from knowledge about geology. A new geology has developed—environmental geology—which may be defined as the total of all geological conditions and influences affecting the life and development of man.

Environmental geology is a broad science, concerned not only with the location of cities and towns, but with the uses people make of the land and its economic products, and with the relationship between the geological character of the land and the present and future location of roads, dams, bridges, factories, homes, recreation facilities, sanitary land fills, and even sewage plants.

Two aspects of environmental geology which are particularly pertinent to Colorado’s residents are discussed below.

Landslidesand slumping rock or earth are a frequent menace to Colorado’s development in the Mountain Province. Often activated by heavy rains or deep manmade cuts, they can cause—andhavecaused—much damage to roads, buildings, and other works of man.

The flanks of North and South Table Mountains, near Golden, are mantled by thick landslide debris; intermittent movement of the individual slides has repeatedly affected the railroad, irrigation ditches, and roads. As many as six different slides have moved within a single year. In one slide area, asphalt road material is estimated to be thirteen feet thick; successive layers of pavement have been laid one on top of another to keep the street up to grade.

Landslides and landslide-prone areas may not be obvious to the untrained eye. Each year buildings and roads are constructed on unsuitable rock and soil foundations, in places where some degree of land slip is almost inevitable. Building in such areasis risky, but sometimes worth the risk; if condition are less than ideal, risks can be reduced by specialized types of construction.

Floodsare a perennial threat to much of the state, because of the high relief of the drainage basins and the torrential nature of the spring and summer rainfall. Their damaging effects were realized early in Colorado’s history, when canyons were used as highways and railroad routes.

Colorado’s most expensive flood was probably the flood in the South Platte River basin south of Denver in 1965, which caused $508,000,000 worth of damage and drowned six people. The losses can be attributed to man’s failure to realize the significance of the South Platte drainage routes and flood plains. Homes, shopping centers, and many other buildings occupied—and still occupy, as of 1971—land that has been intermittently flooded for many years. The following description of this flood, by H. F. Matthai of the U. S. Geological Survey, may help to convey some warning to residents or potential residents of the South Platte valley and other river valleys in Colorado:

“The morning of June 16 was most pleasant, but conditions changed rapidly shortly before noon. A tornado touched ground 15 miles south southeast of Denver about 1 p.m. Within the next hour, another unroofed 30 homes in the little town of Palmer Lake, 40 miles south of Denver. About 2 p.m., a dense mass of clouds descended and concealed the top of Dawson Butte, 7 miles southwest of Castle Rock; and the little light remaining faded until it was dark black and frightening, according to some people. A nearby rancher’s wife described the intense quiet as awesome, but the calm did not last very long.

“The deluge began, not only near Dawson Butte, but also at Raspberry Mountain, 6 miles to the south, near Larkspur. The rain came down harder than any rain the local residents had ever seen, and the temperature dropped rapidly until it was cold. The quiet was shattered by the terrible roar of wind, rain, and rushing water. Then the thudding of huge boulders, the snapping and tearing of trees, and the grinding of cobbles and gravel increased the tumult. The small natural channels on the steep slopes could not carry the runoff; so water took shortcuts, following the line of least resistance. Creeks overflowed, roads became rivers, and fields became lakes—all in a matter of minutes.

“The flow from glutted ravines and from fields and hillsides soon reached East and West Plum Creeks. The combined flow in these creeks have been described as awesome, fantastic, and unbelievable; yet none of these superlatives seem adequate todescribe what actually occurred. Large waves, high velocities, crosscurrents, and eddies swept away trees, houses, bridges, automobiles, heavy construction equipment, and livestock. All sorts of debris and large volumes of sand and gravel were torn from the banks and beds of the streams and were dumped, caught, plastered, or buried along the channel and flood plains downstream. A local resident stated, ‘The banks of the creek disappeared as if the land was made of sugar.’

“The flood reached the South Platte River and the urban areas of Littleton, Englewood, and Denver about 8 p.m. Here the rampaging waters picked up house trailers, large butane storage tanks, lumber, and other flotsam and smashed them against bridges and structures near the river. Many of the partly plugged bridges could not withstand the added pressure and washed out. Other bridges held, but they forced water over approach fills, causing extensive erosion. The flood plains carried and stored much of the flood water, which inundated many homes, businesses, industries, railroad yards, highways, and streets.

“The flood peak passed through Denver during the night, and the immediate crisis was over by morning; but those in the inundated areas were faced with a Herculean task. The light of day revealed the nature of the destruction—mud in every nook and cranny, soggy merchandise, warped bowling alleys, drowned animals, the loss of irreplaceable possessions, to name a few types. The colossal cleanup job, which would take months, began.”

Hydrogeological studies by the U. S. Geological Survey and Corps of Engineers give knowledgeable estimates of flood danger for different populated areas of the state, and recommend that homes, roads, and other structures be placed above likely flood levels.

Alluvial fan. A cone-shaped mass of sediment built by rivers or streams as they issue from mountains onto more level ground.

Alluvium. Stream deposits formed in recent geologic time, composed of sand, gravel, and stones.

Ammonite. One of a large group of extinctmollusksrelated to the living chamberedNautilus.Ammoniteshells, usually cone-shaped or coiled, are divided into many chambers by crenellated septa.

Angular unconformity. A surface separating tilted or folded layers of rock from overlying less disturbed layers.

Anticline. An upwardfoldor elongated arch in rock layers.

Aquifer. A rock layer that is water-bearing.

Artesian water. Groundwater that is under sufficient pressure to rise above the level at which it is encountered in a well. It does not necessarily rise completely to the surface.

Basalt. An extrusive igneous rock, fine-grained and dark colored, composed mainly of calcium-richfeldsparand the black mineral pyroxene.

Basement. A name commonly applied to metamorphic origneous rocksunderlying the sedimentary rock layers.

Batholith. A large body of intrusive igneous rock, 40 square miles or more in outcrop area, which extends downward to an unknown depth.

Bedrock. The solid rock which underlies soil, sand, clay, or other loose surface material.

Belemnite. The cigar-shaped internal shell of an extinct marinemollusksimilar to a squid.

Brachiopod. One of a large group of marine shelled animals having two unequal, bilaterally symmetrical shells.

Bryozoa. A large group of tiny colonial marine animals that secrete calcareous or horny coverings in a great variety of shapes.

Caldera. A large basin-shaped depression caused by explosion or collapse around a volcanic center.

Cassiterite. A heavy, brown to brownish black mineral composed of tin and oxygen (SnO₂) that is an ore of tin.

Cephalopod. A marinemolluskwith a head surrounded by tentacles. Squids and octupuses belong to this group, as dofossilforms having straight or coiled shells divided into numerous interior chambers.

Chalcopyrite. A reddish-gold colored ore of copper (CuFeS₂).

Cirque. A deep, steep-walled recess in a mountain, caused by glacial erosion at the head of a valley.

Concretion. A nodular or irregular concentration of minerals such as calcite or limonite, formed by precipitation of the mineral from groundwater around a nucleus.

Conglomerate. A rock containing coarse fragments of an older rock, usually as rounded water-worn stones or pebbles.

Conodont. One of a group of tiny dark brown tooth-likefossilsthought to be dermal or dental parts of some extinct group of fish.

Diatreme. A volcanic vent or pipe drilled through rocks by the explosive energy of gas-charged molten rock, now containing igneous rock and often altered or unaltered fragments of the surrounding rock.

Dike. A vertical or nearly vertical sheet of igneous rock which cuts across the structure of adjacent rocks.

Diorite. An intrusive igneous rock composed of sodium-richfeldsparand dark minerals, with only small amounts of quartz.

Dip. The angle at which a layer of rock is inclined below the horizontal.

Dome. A roughly circular upfold in which the rock layersdipoutward in all directions from the center.

Dowsing. Searching for underground water or ore with a divining rod, usually a forked stick supposed to locate spots where the desired substance may be found under the surface.

Echinoderm. One of a large group of marine invertebrate animals, most of which have pentagonal symmetry and a skeleton of many calcite plates. Many forms are spiny. The group includes starfish and sea urchins.

Evaporite. Chemical sediments precipitated when water (usually sea water) evaporates.

Extrusive rocks.Igneous rocksformed when molten rock material is ejected onto the surface. Synonymous with volcanic rocks.

Fault. A break in the rocks in which there has been displacement of the two sides relative to each other.

Fault block range. A mountain range bounded on two or more sides byfaults.

Feldspar. A group of light-colored aluminum silicate minerals that are major constituents ofigneous rocks. They contain potassium, sodium, and calcium in differing proportions.

Fold. A bend in rock layers.

Foraminiferida. One-celled marine animals with microscopic, perforated, many-chambered calcium carbonate shells, often called forams.

Fossil. The remains or traces of an animal or plant which has been preserved in the rock.

Fusulinid. One-celled marine animals (forams) with shells which look like a grain of wheat in shape and size, frequently abundant in Colorado Pennsylvanian rocks.

Galena. A heavy gray metallic mineral (PbS), often cubic in form, that is the most important ore of lead.

Gangue. Nonvaluable minerals occurring inveinswith ore minerals.

Glaciation. Alteration of the earth’s surface by erosion and deposition byglacierice.

Glacier. A body of ice originating on land by recrystallization of snow, and showing evidence of movement by flowing.

Gneiss. A coarse-grainedmetamorphic rockusually banded with streaks of darker, finer-grained rock.

Granite. An intrusive igneous rock consisting essentially of sodium or potassiumfeldsparand quartz, often speckled with dark-colored minerals.

Graptolite. Extinct marine organisms without known close living relatives, with small black sawblade-like chitinous hard parts preserved asfossils.

Hematite. A steel gray or metallic grayish black or reddish gray mineral (Fe₂O₃) that is an important ore of iron.

Hogback. A sharp-crested ridge formed by a resistant layer of steeply dipping rock.

Huebnerite. A heavy reddish brown mineral (MnWO₄) that is a major ore of tungsten.

Igneous rocks. Rocks formed by solidification from a molten state, either at the surface (extrusive) or below the surface (intrusive).

Intrusive rocks.Igneous rocksformed when molten rock material solidifies without reaching the surface.

Joint. A fracture in the rock, along which no discernible movement has taken place.

Kerogen. Solid bituminous material in oil shales.

Laccolith. A lens-shaped mass of igneous rock intruded into layered rocks.

Lava. Fluid or molten rock such as that which issues from a volcano.

Lode. A rock mass, often avein, containing valuable minerals.

Massif. A mountainous mass that has relatively uniform geologic characteristics and which may embrace a number of peaks.

Mesa. A flat-topped mountain bounded on at least one side by a steep cliff.

Metamorphic rock. Rock formed by alteration of pre-existing rock, especially by great temperatures and pressures.

Mollusk. Any one of the large group of invertebrate animals which includes the snails, clams, octopuses, squids, and their extinct relatives.

Molybdenite. A soft bluish gray, metallic mineral (MoS₂) that is a major ore of molybdenum.

Monocline. A steplikefoldin otherwise horizontal or gently dipping rock layers.

Moraine. An accumulation of unsorted rock material built up by the action ofglacierice.

Native gold. Gold occurring in nature uncombined with other elements.

Peneplain. A land surface worn down by erosion to a nearly flat or broadly undulating plain.

Petzite. A heavy black or steel gray metallic telluride ore of gold and silver (Ag₃AuTe₂).

Placer. A sand or gravel deposit containing particles or nuggets of gold or other heavy valuable minerals.

Plateau. An elevated, comparatively flat surface of land, usually larger than amesa, sometimes composed of many mesas, and often dissected by deep stream valleys.

Porphyry. An igneous rock, usually intrusive, which contains conspicuous large crystals in a fine-grained matrix.

Pyrite. A brass-yellow metallic mineral (FeS₂) that is an important source of sulfur. It is commonly known as fool’s gold.

Reef. A moundlike limestone structure built in the sea by sedentary organisms such as corals.

Rhyolite. A light-colored volcanic rock with quartz andfeldsparas the principal constituents.

Schist. Ametamorphic rockcharacterized by parallel orientation of flat-grained minerals like mica.

Sedimentary rocks. Rocks formed of fragments of other rock transported by wind or water, or formed by precipitation from solution.

Sphalerite. An amber-yellow to black mineral (ZnS) that is an important ore of zinc.

Stalactite. A cylindrical or conical deposit of calcite hanging from the roof of a cavern, formed by evaporation of water droplets containing calcium carbonate.

Stalagmite. Columns or ridges of calcite rising from the floor of a cavern, formed by water containing calcium carbonate dripping from astalactite.

Stock. A mass of igneous intrusive rock that covers less than 40 square miles, has steep sides, and extends to an unknown depth.

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