Prairie, Peak, and Plateau: A Guide to the Geology of Colorado

III Geology and Man in Colorado

Colorado’s first permanent settlers arrived in 1858, when gold was discovered in river sands near what is now the city of Denver. The ensuing gold rush, coming ten years after the rush to California, rivalled it in fury and brought sudden wealth to lucky miners and the adventurous merchants who grubstaked them. Several hundred mining towns or “camps” sprang into existence almost overnight, their sites determined by the geology of the mountain areas. The cities of Denver, Boulder, and Golden were established as milling and shipping centers for the products of the mines. In 1876 the now-wealthy area, previously part of Kansas Territory, became the State of Colorado.

For more than a hundred years Colorado’s minerals—products of her long and diverse geologic history—have influenced her development in many ways. The state’s early wealth, stemming from bonanzas in gold and silver, is evidenced by palatial homes, hotels, and public buildings constructed during the first few decades of mining activity. Some of these are still standing—the opera houses at Central City and Aspen, Central City’s famous Teller House, and the Grand Imperial Hotel at Silverton are examples.

Many of the stories and legends of Colorado’s gold camps are recounted in Stampede to Timberline, by Muriel Sibell Wolle, delightfully illustrated with sketches of old mining towns as they appear today. Mining in Colorado, published by the U. S. Geological Survey, also makes fascinating reading, as it contains many historical anecdotes and eyewitness accounts of gold-rush days.

Development of the metal-mining areas in Colorado followed a definite sequence. Placer gold was usually discovered first. Recovery of placer gold was followed by mining of gold from veins or “lodes.” Although at first only native gold was mined, gold-bearing compounds such as telluride were soon recognized as an additional source, especially at Gold Hill, Cripple Creek, and of course the camp that came to be known as Telluride. As gold sources were depleted, silver, first produced as a byproduct, became of prime interest. Lead and zinc were in turn byproducts of silver mining. Other metals, notably copper, vanadium, tungsten, and iron, were produced later. Molybdenum is the Johnny-come-lately of the state’s mining industry, but is now the chief metal produced. A uranium boom in the 1950s brought a short rush to western Colorado and new vigor to the economy.

Oil was discovered near Canon City in 1862. The nearby Florence field and a small, shallow field near Boulder preceded much greater discoveries in the Denver Basin, the Uinta Basin, and southwest Colorado. Oil reservoirs, confined to areas of sedimentary rock, are found primarily in the Prairie and Plateau Provinces of the state, and recovery of the oil has done much to distribute population to these areas.

Coal is also restricted to sedimentary rock areas. Coal production in Colorado has waxed and waned with the years, but has provided fuel for export, for the railroads, for the manufacture of electric power, and for many of the state’s industries.

A good picture of present mineral production in Colorado can be obtained from the following summary for 1971, prepared by the Colorado Bureau of Mines:

Product	Value
Molybdenum	$105,389,456
Petroleum	90,494,459
Sand and gravel	32,842,503
Coal	30,251,443
Natural gas	18,695,225
Uranium	18,048,692
Vanadium	15,863,554
Cement	13,377,520
Zinc	13,310,787
Lead	6,582,025
Tungsten	6,360,020
Limestone and dolomite	5,397,570
Silver	4,198,054
Fluorspar	3,887,210
Copper	3,875,976
Stone	1,961,279
Gold	1,832,791
Clay	962,986
Iron	880,047
Pumice	309,370
Tin	278,862
Gypsum	253,856
Pyrites	142,640
All others	1,091,927
Total	$376,288,252

Colorado is now the nation’s leading producer of molybdenum, tin, and vanadium, and second in output of tungsten. In oil production it ranked twelfth among the states in 1968, but ninth in reserves, with 420,000,000 barrels of proven reserves on 1 January 1969. An as yet untapped source of oil lies in the oil shales of western Colorado.

As part of the natural environment, water plays a major role in man’s activities. Water problems in Colorado revolve mainly around the best use of runoff in a state whose major catchment basins are across the continental divide from her largest population centers and most fertile farm land. Groundwater, closely related to surface water distribution and movement, is a geological problem, and in Colorado as in other states many government and private geologists serve farm and industrial communities in the search for usable supplies.

CAUTION: Old mines are dangerous! They may contain water or deadly gases, or be on the verge of collapse. Keep away from abandoned prospect pits and mine shafts. WARN AND WATCH YOUR CHILDREN.

GOLD, SILVER, AND OTHER METALS

Colorado’s placer and lode sources of gold, which gave first impetus to the series of mining booms in the state, were fantastically rich. Summit and Lake Counties, for instance, each produced more than $5,000,000 in placer gold between 1859 and 1867. During the same nine-year period, more than $9,000,000 in lode gold was produced from Gregory Gulch, a tiny canyon between Central City and Black Hawk. Other districts rivalled or surpassed these figures.

Early in the game it was recognized that almost all the deposits occurred along what came to be known as the “mineral belt,” a fifty-mile-wide zone extending southwest from the Boulder region. Most of the metals mined in the state come from this belt, but there are three notable exceptions: Cripple Creek, Silver Cliff, and western Colorado vanadium and uranium districts. In the first few years of the Colorado rush, gold ores and placer gold were discovered only in the northeastern part of the mineral belt. Gradually the belt was found to extend further and further southwest: Tincup was discovered in 1861, Silverton in 1870, Lake City in 1871, and Telluride in 1875. Aspen, on the western edge of the belt, was not discovered until 1879, perhaps because the area was difficult of access and lacked the easily recognizable native gold.

In the northeast part of the mineral belt, gold and other minerals occur in veins in Precambrian granite and gneiss. In the Leadville and Aspen areas, ores are associated with altered Paleozoic limestones. At the southwest end of the mineral belt, in the San Juan Mountains, ore veins are found near or in Tertiary volcanic rocks. Native gold, gold-bearing compounds, and other metallic ores in these veins originated where mineral-rich solutions from deep within the earth penetrated fissures and joints in the surrounding rock. Regardless of the age of the host rock, almost all the ores of Colorado were deposited in the early or middle Tertiary Period, about 35 to 70 million years ago.

Gold and silver are no longer mined extensively in Colorado, although any summer Sunday will see weekend operators panning near mountain streams or trundling rock from one-man mines. The recent rise in the price of silver has encouraged many miners to reopen old shafts. The most active mines in the state today are those producing molybdenum, lead, zinc, and vanadium. (Vanadium, although a metal, usually occurs in Colorado with radioactive minerals, and so is discussed with them rather than with the metals.)

The Colorado mineral belt extends from Boulder County on the northeast to San Juan County on the southwest. Almost all of the prominent mining districts in Colorado lie along this belt. Cripple Creek and Silver Cliff, however, lie far to the east of the general trend.

Telluride; Denver; Colorado Springs; Alamosa
BOULDER: Ward; Gold Hill; Boulder; Nederland
GILPIN: Central City; Black Hawk
JEFFERSON: Golden
CLEAR CREEK: Empire; Georgetown; Silver Plume; Idaho Springs
SUMMIT: Breckenridge
EAGLE
PITKIN: Aspen
GUNNISON: Tincup
CHAFFEE
PARK: Climax; Alma; Como; Fairplay
TELLER: Cripple Creek
FREMONT
OURAY: Ouray; Camp Bird; Ironton
SAN JUAN: Silverton
HINSDALE: Lake City
LA PLATA: Durango
MINERAL: Creede
CUSTER: Silver Cliff

All told, some 430 metal mining districts have been established as legal entities in the state of Colorado. Each of these districts had the right to draw up its own regulations concerning prospecting, claims, and mining rights, within a framework established by the Federal government. Only a few of the districts ever became really significant producers. The geology and history of several of the leading areas are presented in the pages that follow.

Boulder County

Gold Run, near Gold Hill, was the scene of one of the earliest strikes in Colorado. Gold was found here in December 1858, and was sluiced from stream sands and mined from veins early in 1859. Active placer mining lasted only about a year, however, and lode mining dropped off rapidly as near-surface oxidized ores were worked out. When a smelter was erected at Black Hawk in 1868, and sulfide ores could be treated, there was a revival of activity. In 1869 the Caribou and Poorman mines near Nederland were discovered; they quickly became the most active mines in the county. The Ward district opened soon after.

In 1872, a gold-silver telluride called petzite was found in veins at Gold Hill. Renewed prospecting in this area resulted in location of mines near Sunshine, Salina, and Magnolia. During the years that followed, new mines appeared almost as fast as old ones were depleted. In 1892, the peak year, more than $1,000,000 in gold and silver was produced; total production has been about $25,000,000.

In 1900, a black mineral common in the Nederland area was recognized as ferberite, an ore of tungsten, and a new rush to the area started. During the next eighteen years Boulder County was the main tungsten producer in the United States; about 24,000 tons of tungsten trioxide, worth $23,000,000, were produced here. The ore was found in nearly vertical veins six inches to three feet thick, in a lenticular area about nine miles long extending from Nederland northeast to Arkansas Mountain, four miles west of Boulder.

Boulder County is characterized by an abundance of small mines. Old shafts, pits, and mine buildings can be found throughout the central part of the county. Little mining is done here today; many of the towns that once peppered these hills have fallen into decay or disappeared entirely.

Central City and Idaho Springs

The Central City-Idaho Springs area was the principal metal mining region in the state until the late 1880s. In 1858, rich placer deposits were discovered in gravels and river terraces along both forks of Clear Creek. Exploration upstream led to discoveries of rich oxidized quartz veins at Central City, Black Hawk, and Idaho Springs. These veins, which generally trend northeast-southwest, extend through the mountains in a zone about six miles long and three miles wide between the two forks of Clear Creek.

The ores filled a multitude of cracks and fissures in the Precambrian bedrock. The veins are usually less than five feet thick, and are almost vertical and often clustered in zones up to thirty feet wide. The position of one of the vein systems may be seen clearly between Black Hawk and Central City—the ore-bearing rock has been mined out, but a series of collapsed tunnels marks the line where the veins crossed the valley. A monument here commemorates the discovery of Gregory Gulch, one of the richest localities in the state.

Several rich veins were mined in both directions—southwest from Central City and northeast from Idaho Springs—until the mines met. The Argo tunnel, marked by dilapidated buildings and extensive dumps on the north side of Idaho Springs, connected the two districts; it was completed in 1904.

The “Patch,” a deep crater-like hole on Quartz Hill, about one mile southwest of Central City, is an intriguing feature in this area. It was produced by glory-holing, a mining technique in which a deep tunnel is deliberately caved by blasting, so that ores above the tunnel can be removed. This glory hole was dynamited below an irregular mass of highly broken rock where many ore-rich veins converged. After the caving, ores were taken out through the remaining part of the tunnel.

The principal ore minerals of Central City and Idaho Springs are native gold, pyrite, sphalerite, galena, chalcopyrite, and tennantite. Prospecting for uranium was carried out during the 1950s but no uranium was ever mined here.

The area has produced almost $200,000,000 worth of gold, silver, lead, zinc, and copper. A few mines still operate seasonally or on a small scale, but tourists, many of them riding Jeeps across the mountainous terrain to visit mines and ghost towns, are often more visibly active than the mines.

Georgetown, Empire, and Silver Plume

A few miles southwest of Idaho Springs, another mining area had a similar, though less productive, history. In 1859, placer and lode gold were discovered near what is now Georgetown. Placer mining dominated here between 1859 and 1863. Gravel and crushed rock from decomposed quartz and sulfide veins were washed through sluiceboxes in the same way as placer gravel, gold being caught in riffles or gunny sacking on the bottoms of the troughs. The veins were found to be decomposed to depths of about 40 feet; below this the gold occurred closely associated with sulfides such as pyrite, sphalerite, galena, and chalcopyrite, from which it could not easily be separated. However, smelters were developed in 1866 for treatment of these sulfides, and gold, silver, lead, and copper were recovered. Gradually, as the gold was worked out, silver and lead became the important products of the mines.

Sluicebox mining was a common sight near the early gold camps, where primary recovery was from placer deposits or decomposed quartz and sulfide veins. (State Historical Society of Colorado photo)

Leadville

Placer gold was discovered in 1859 in California Gulch, about seven miles north of the present town of Leadville. The rush that followed was short but sweet; the camp was called Oro—gold! About $5,000,000 was produced from the placer mines within two years, though by 1861 the area was all but deserted, for the easily won placer gold was gone.

Early-day Leadville sprawled among its mine dumps at an elevation of 10,200 feet. The Sawatch Range, in the background, contained many smaller mining communities, now deserted. Mt. Massive, the state’s second highest peak, forms the crest of the continental divide here. (State Historical Society of Colorado photo)

In 1875 a smelter was erected a few miles downstream from Oro to process cerussite—silver-rich lead carbonate—that occurred in the placer sands. For years this mineral had been considered a nuisance because, being much heavier than sand, it tended to separate out with the gold. The new town of Leadville sprang up near the smelter and shortly afterward more lode deposits were discovered south of the placer workings. From $63,000 in 1875, production climbed to $2,500,000 in 1878 and more than $15,000,000 in the peak year of 1882.

Geologically, the ores of this district occur as Tertiary replacements and veins in Ordovician, Devonian, and Mississippian limestones. The “Blue” or Leadville Limestone, of Mississippian age, contains the richest ore. Ore deposits were formed after the limestones had been faulted and cracked extensively by mountain-building movements; the ores themselves probably crystallized from molten or gaseous materials involved in related igneous intrusions. River gravels and glacial debris mask the true nature of the lode deposits, but studies in the mines show that the fault systems along which ores are deposited trend north or north-northeast.

The Leadville district is now experiencing its third mining boom as a newly recognized lead-zinc orebody is being developed. Production is expected to reach 700 tons of ore per day by 1971. Total production of gold, silver, lead, zinc, and copper in the district has reached $500,000,000.

Breckenridge

Breckenridge was also discovered in 1859, with placer gold the first attraction. The placers gave out in 1862 after about $3,000,000 in gold had been recovered. Earliest attempts to mine the rich silver and lead veins of the district were in 1869.

As at Leadville, the sedimentary rocks of the area were intruded by granitic masses in Tertiary time, but here the sedimentary rocks are mostly Pennsylvanian sandstones and shales. These rocks were badly faulted and broken during the intrusion, and the ores were deposited as the granitic material cooled. The lode deposits occur mostly in small veins well hidden by surface sands and gravels. Some of the veins yielded exceptionally beautiful crystallized wire and flake gold, specimens of which are on display at the Colorado School of Mines library in Golden and in the Denver Museum of Natural History.

Dredging for alluvial gold was attempted in 1898 in the Breckenridge district, but this method of extracting gold was not successful until 1905. A number of dredges operated between 1910 and 1925. These floating behemoths shovel up gold-bearing gravels from the bottom and one side of the pond on which they float, sort out the gold in giant sluiceboxes, and spew out the leftover gravels in great arc-shaped heaps that can be seen near Breckenridge and Fairplay and in a number of other valleys in Colorado. They depend for their operation on a plentiful supply of water and a shallow water table, but they can sift through quantities of gravel at relatively low cost. All told, about $7,000,000 in gold has been dredged from this district.

Fairplay

This gold dredge, still floating in its pond just south of Fairplay, operated from 1941 to 1952. With chains of buckets like those in the foreground, it dug gravel 70 feet below water level, carving a 35-foot bank above water level; in effect it mined to a depth of 105 feet. This dredge extracted nearly 115,000 ounces of gold from about 33 million cubic yards of gravel (John Chronic photo)

Another gold field discovered in 1859 was in the northwest corner of South Park, along the headwaters of the South Platte River. Several mining camps were established here. After early production of rich placer deposits, claims were consolidated and large flumes constructed so that gold could be recovered by hydraulic mining. In this type of mining, streams of water from high-pressure hoses are directed at gravel surfaces. The gravels are washed into long sluiceboxes, where gold is caught in riffles. Hydraulic mining continued upstream from Fairplay until about 1900.

In 1922 a dredge was constructed near Fairplay to process gravel along the South Platte and in the valley floor. An even larger dredge, constructed in 1941, operated until 1952, when rising labor costs overrode the narrow margin on which it operated. At the time operations ceased, the dredge was recovering about six cents in gold for each cubic yard of gravel processed.

Placer gold has always been the principal mineral product of the Fairplay area, but native gold also occurs in the surrounding mountains in quartz veins, and many small mines were developed to extract it. Sulfide ores were also mined; they contained silver, lead, and zinc as well as gold. In the Mosquito Pass and Horseshoe Amphitheater areas, there is renewed activity now because of the recent rise in the price of silver.

Silverton

Gold was discovered in the San Juan Mountains of southwest Colorado in 1870. The earliest mine, near what is now Silverton, was located by a group of prospectors sent out by Governor Pile of New Mexico Territory. Since the site was on Ute Indian land, real mining did not begin until a treaty allowing it was ratified in 1874.

Production in the Silverton district has been from veins in Tertiary volcanic rocks within an elliptical area known as the Silverton cauldron. Here the volcanic rocks, part of the several thousand feet of lava flows and ash falls of the San Juan volcanic field, were cracked and faulted by a second period of igneous activity. Ores formed in the cracks and fissures.

In the 1870s the Silverton district was very remote, and difficulties with transportation retarded activity there. In 1882, however, a narrow-gauge railroad was built connecting Silverton with Durango, and the problem of transporting ore out of the isolated mountain valley was simplified. The railway still exists; a train makes daily passenger runs during the summer—the only remaining operating narrow-gauge line in the United States. The track follows the Animas River canyon, whose cliffs and crags are dotted with long-abandoned mines, prospect holes, and mine buildings, monuments to the tenacity and determination of the men who mined here.

Production in this district was more than $22,000,000 in gold and $20,000,000 in silver between 1874 and 1923. New activity is evident here, as in other silver-rich areas of Colorado, because of recent demand for silver, lead, and zinc.

Silverton lies in a remote mountain valley in the San Juan Mountains. Silver, gold, lead, and zinc have been mined here since 1874. Storm Peak, composed of Tertiary volcanic rocks, forms the backdrop; the narrow-gauge railroad track is visible in the foreground. (Jack Rathbone photo)

Ouray

Ouray was settled in 1875, when gold and silver deposits were found near Mount Sneffels. Since 1877, mines in Ouray County have produced over $35,000,000 in gold and $32,000,000 in silver. The district is still quite active: in 1965, mines in this area produced more than $9,000,000 in gold, silver, copper, lead, and zinc, about a third of total Colorado production of these metals for that year.

A few miles south of Ouray, along Uncompahgre Gorge, an old mine clings to the slope below the Million Dollar Highway (U. S. 550). Abrams Mountain rises in the background. The Precambrian Uncompahgre Quartzite outcrops up to about the road level; Miocene Sunshine Peak Rhyolite caps the peak. (Jack Rathbone photo)

A mile north of Ouray a prominent intrusive stock marks the center of mining activity closest to Ouray. The richest deposits of the Ouray area, however, lie about five miles southwest, near Mount Sneffels and Red Mountain Creek. There, several large mines, including the famous Camp Bird mine, have operated for many years, extracting ore from hundreds of veins that underly the surface. Some of these veins are two to four miles long. They are in Tertiary volcanic rocks of the San Juan Formation. Quartz and calcite are the common gangue (non-economic) minerals, and pyrite, sphalerite, galena, and chalcopyrite are the most abundant ores. Most of the silver is in the galena; gold occurs in streaks and nodules associated with quartz.

About ten miles south of Ouray, along the “Million Dollar Highway” (U. S. 550), the Red Mountain district lies on the northwest edge of the Silverton volcanic cauldron. It contains a number of small pipelike bodies very rich in silver-copper and silver-lead ores. Following the mid-Tertiary volcanism and ore intrusion, surface rocks in this area were intensely oxidized: resulting iron oxides now form the gaudy reds and yellows of Red Mountain and the slopes near Ironton. This alteration, as well as the fact that much of the area is covered with fallen rock, stream gravels, or glacial deposits, compounds difficulties of locating the small though high-grade ore deposits.

The Idarado Mine, on the east side of U. S. highway 550 near Red Mountain, used to produce ores from nearby volcanic pipes; now it produces from veins some distance to the northwest. The area is honeycombed with tunnels and shafts.

Aspen

Silver was found at Castle Creek and on Aspen Mountain in 1879. A group of prospectors from Leadville, apparently after examining maps of the Geological and Geographical Atlas of Colorado published in 1877, explored along the line of Paleozoic limestones encircling the Sawatch Range. As they had hoped, they found ores similar to those at Leadville in rocks of the same age.

Mining began at Aspen in 1880. Here, as at Leadville, intrusion of granite porphyry into or near the Leadville Limestone had broken and deformed the layers, and ores were deposited in fissures and as replacements during cooling of the intrusions. The intricacy of faulting which controls the ore pockets in the limestone is well shown on the map of Aspen Mountain in Chapter II.

Glaciation occurred in this area, and glacial deposits cover most of the ore bodies and outcrops so that little bedrock is exposed. Mapping was accomplished by extrapolating to the surface the bedrock patterns shown in mine pits, shafts, and tunnels.

Aspen produced some of the richest silver ores in the world, and thrived as a boom town for most of two decades. In 1888 the value of ores produced reached over $7,000,000; the next year it topped $10,000,000. After the silver crash of 1893 production declined rapidly; the last mines were closed in the 1920s. Total production of silver, lead, zinc, and copper reached about $100,000,000. There was virtually no gold in the ores at Aspen.

Creede

Creede and its mines are located in an area of Tertiary rhyolite and dacite, light-colored volcanic rocks.

Happy Thought Mine
Amethyst Mine
West Willow Creek
AMETHYST FAULT
Last Chance Mine
Del Monte Mine
Commodore Mine
Jackpot Mine
Coppervein Mine
Bachelor Mine
BULLDOG MOUNTAIN FAULT
Kansas City Star Mine
Commodore Tunnel
Mustang Tunnel
Nelson Tunnel
Exchequer Mine
SOLOMON FAULT
CAMPBELL MOUNTAIN
Holy Moses #2
Holy Moses Mine
Ridge Mine
Solomon Mine
Monte Carlo Mine
Mollie S. Mine
East Willow Creek
Ramey Tunnel
Dora Belle Mine
Mammoth Tunnel
Homestake Mine
Mammoth Mine
MAMMOTH MOUNTAIN
Nancy Hanks Mine
Pipe Dream Mine
THE NARROWS
Windy Gulch
CREEDE
Willow Creek

The Creede district ranks as one of the most productive silver areas in the United States. It came into being largely as a result of a discovery by N.H. Creede in 1889. When exploring in this area, he was reported to have exclaimed “Holy Moses!” on examining a rich piece of ore, thus giving the name to the mine which initiated the rapid development of the district. By the end of 1892 the Holy Moses and nearby mines had produced ore valued at more than $4,000,000. The area was so rich that it managed to survive 1893’s great decline in the price of silver; by 1920 almost $42,000,000 in gold, silver, lead, and zinc had been mined there.

The ores, silver-bearing galena, sphalerite, native gold, pyrite, and chalcopyrite, are in quartz or amethyst veins in faulted and shattered Tertiary volcanic rocks. Nearly all the ore deposits lie along a complex system of vertical faults, the Amethyst fault zone, which runs more or less northwest-southeast through this region. Both the faulting and the enrichment of the fault fissures are believed to have taken place in mid-Tertiary time, shortly after deposition of the volcanic host rocks.

Cripple Creek

Cripple Creek, on the flanks of the Pikes Peak massif, has produced more than $400,000,000 worth of gold. The Sangre de Cristo Mountains are visible in the distance beyond the Arkansas River valley. (Jack Rathbone photo)

In 1890, two sheepherders stumbled on some richly mineralized rocks near Cripple Creek. A boom developed immediately, for the rocks contained both gold and silver. Since then, the area has produced more than 2,000,000 ounces of silver and nearly 19,000,000 ounces of gold.

Cripple Creek has produced almost half of all the state’s gold and silver. The ores are located in or at the edge of a large mass of middle Tertiary volcanic rocks which form an elliptical basin or caldera several miles across. The caldera, surrounded by Precambrian gneiss and granite of the Pikes Peak massif, was probably formed by collapse of a volcanic center that had erupted through the older rock. The collapse shattered the rocks around the basin margin, and subsequent volcanic activity introduced mineral-rich solutions into the many faults and fissures produced by the collapse. Tellurides of gold, silver, and copper, as well as pyrite, sphalerite, galena, tetrahedrite, and other minerals, are characteristic.

Climax

At Climax, the ore occurs scattered through the intrusive Climax Granite Porphyry and the intruded Idaho Springs Formation. Visitors can tour the surface workings during the summer months.

Tertiary dikes

Shell of Climax stock

Core of Climax stock

Ore zone

Precambrian granite

Fault

Dykes

Molybdenum now ranks as the number one metal mined in Colorado. Over $105,000,000 of “moly” was mined here during 1969, almost all of it from the Climax Mine, the world’s largest single source of this metal. The Climax deposit is located high on the west slope of Ten Mile Range in central Colorado, about 100 miles southwest of Denver. It is in the central part of the Colorado mineral belt, near the Mosquito Fault, a prominent structural feature which extends about sixty miles along the north-south trend of the mountains. Rocks on both sides of this fault are intruded by Tertiary granite dikes, sills, and stocks. The Climax Mine is in a stock just east of the fault, near the axis of a broad anticline in Precambrian metamorphic rocks.

Ore minerals at Climax are molybdenite, huebnerite, and cassiterite; pyrite is recovered also for the manufacture of sulfuric acid. The ore is very low in metal content, containing only one-third of a percent of molybdenum, 0.005% tungsten trioxide, and 0.0001% tin. The great size of the ore body and efficient recovery by modern methods make Climax a profitable mine, however. Production has risen each year since the mine began operation.

Urad Mine near Berthoud Pass is a newly developed near-surface molybdenum mine similar to Climax. Nearby at the Henderson Mine the ore body is more than half a mile below the surface of the ground.

RADIUM, URANIUM, AND VANADIUM

Over a large area of the Plateau Province in western Colorado, Mesozoic sedimentary rocks are locally stained bright yellow, orange, or green. Such staining suggests mineralization, and radioactive compounds were recognized here before 1900. At that time, however, there was little or no market for them or for the vanadium frequently associated with them. When Marie Curie required radium for experiments with her newly discovered element, the raw materials were sent from western Colorado; by and large, though, production of radium from these ores was prohibitively expensive.

In 1905, vanadium was found to be effective in toughening steel. The Vanadium Corporation of America was formed to mine the Colorado ore. This company mines a rich zone in the Jurassic Entrada Sandstone, where vanadinite occurs with carnotite and other uranium ores. In the early days of vanadium mining, the uranium ores were discarded with other gangue materials; now, of course, uranium is produced from them.

Since 1945, uranium production has been an important Colorado industry; in 1969 about $17,500,000 worth was produced. Uranium occurs in the state in two very different situations. In the Plateau Province, where it was first discovered, it occurs in sedimentary rocks as patches of pitchblende, carnotite, and a greenish yellow mineral called schroekingerite. It is most abundant in the Triassic Chinle Formation and the Jurassic Entrada and Morrison Formations, where it was probably deposited by downward movement of rainwater from overlying uranium-rich Tertiary volcanic rocks. Concentrations of uranium often occur in or near organic matter such as coal, fossil bone, or petrified wood, so mines tend to be located along rock layers carrying abundant organic material.

Another type of uranium ore is found in the Mountain Province. Veins in Precambrian rocks of the Front Range and several other ranges contain pitchblende which seems to have been deposited by hot groundwater rising through broken and fissured Precambrian rocks. Often exceedingly rich, such ore is mined in the manner of most of Colorado’s metals. The Schwartzwalder Mine, a few miles northwest of Golden, has produced more ore of this type than any other mine in Colorado.

OIL, NATURAL GAS, AND OIL SHALE

Petroleum and natural gas have been found in large quantities in the Prairie and Plateau Provinces in Colorado, as well as in smaller quantities in North Park in the Mountain Province. They generally occur in porous sandstone and limestone layers, where they have been trapped by overlying finer-grained, less permeable layers in or near folds and faults.

Several oil and gas seeps were found along the mountain front shortly after the arrival of the earliest settlers. Near Canon City, on Oil Creek, a plaque commemorates the first production:

Oil Creek—site of the first oil well in the west—second place in the United States to produce petroleum from wells. In 1862 ... A. M. Cassedy drilled an oil well 50 feet deep. By February, 1863, production was one barrel a day. Later, several thousand gallons of petroleum were produced by primitive methods, and kerosene and lubricating oil were shipped by ox team as far as Denver and Santa Fe.

About twenty miles to the southeast, near Florence, the Cretaceous Pierre shales were drilled in 1876. Oil was found in a system of intersecting fractures and joints. Some of the early wells in the Florence field are still producing, making this Colorado’s oldest and longest producing field. It has yielded more than 10,000,000 barrels of oil.

Small quantities of oil have been produced near Boulder since about 1900, also from Pierre sandstones and shales. In this area, wells were located by “dowsing” or “witching,” as was fashionable at the time. Several old rigs can be seen near Boulder Reservoir. As at Florence, oil has been trapped in fractures of otherwise dense and impervious shale. Some gas is produced and is used by local farms.

More recently, oil was found far beneath the surface in the northern part of the Prairie Province. Here, in the Denver Basin, oil is produced from several levels in the Dakota Sandstone. The oil has accumulated in lenses of beach sand deposited along the shoreline of the Cretaceous sea. The general trend of the shoreline, and of the oil fields, is northeast-southwest. The shore appears to have been similar to Georgia’s present coastline: a swampy tidal zone separated from open sea by lagoons, sandy bars, and clean sand beaches.

Individual oil pools in the Denver Basin are small, but there are many of them; they lie nearly a mile below the surface, under much of Morgan and Logan Counties and adjacent parts of Nebraska. Exploratory and development drilling keeps total oil production at about 50,000 barrels a day. Oil and gas produced here is piped to Denver and other Colorado cities.

In southeastern Colorado, oil and gas occur in late Paleozoic limestones and sandstones similar to those which outcrop at the edge of the Wet Mountains. Prospecting by geophysical methods and by drilling has revealed several small, rich accumulations, one of which is thought to contain about 30,000,000 barrels of oil.

The Rangely field, in northwestern Colorado, is the most productive field in the state. Located in the northeastern part of the Uinta Basin, it is an outstanding example of an anticlinal field, where oil is trapped in a large, gentle dome. The shape of the dome shows up well on the surface; rock layers can be seen dipping outward in all directions from the town of Rangely. Oil was found by drilling on the crest of the dome. At first, oil was produced from fractures in the Cretaceous Mancos Shale at less than 1,000 feet depth. Later, deeper drilling showed that oil had also accumulated in the Permian Weber Sandstone, at 5,000 to 7,000 feet. At present this field is producing about 28,000 barrels of oil a day, but the figure is dropping each year as the field is depleted.

Oil and gas are produced in southwestern Colorado from the eastern edge of the Paradox Basin and the northern edge of the San Juan Basin. In the Paradox Basin, oil comes from Pennsylvanian limestone mounds or reefs. Production in the Colorado part of the basin has been at most a few thousand barrels per day; more is produced in adjacent Utah. In the San Juan Basin, gas and oil are trapped in thin porous layers of Cretaceous and Pennsylvanian sandstone, between impervious layers of shale. Most of the production is in New Mexico, although some oil comes from the Colorado part of the basin.

The greatest known potential oil resource in the world lies in the oil shales of western Colorado. The richest of these shales cover an area of 1,600 square miles north of the Colorado River, south of the White River, and just east of the Colorado-Utah line. The oil shales are part of the Tertiary Green River Formation, which extends over much of northwest Colorado, northeast Utah, and southern Wyoming. Oily material called kerogen is locked in these rocks, too solid to flow out of the fine pore spaces of the shale. To free it the shale must be mined, finely crushed, and heated until the kerogen converts to liquid oil. This is an expensive process, and as yet production of petroleum from the oil shale has not been possible at a cost which will compete with production of oil and gas from wells. The United States Bureau of Mines, as well as a number of oil companies, have sought for more than fifty years to discover a less expensive method for extracting oil from the shale. No doubt at some time in the future a competitive technique will be developed, or a growing shortage of other oil will bring world prices to a level with which present production techniques can compete.

Oil and gas production in Colorado is decreasing at present, even though great efforts are being made to find new oil pools. Petroleum prospecting and wildcat drilling are carried out in as yet unproductive basins in the Plateau Province, in intermontane basins such as the San Luis Valley, and on the Plains. Known reserves will continue to provide the state with significant income for many years to come, and if oil shale recovery becomes profitable. Colorado’s hydrocarbons will become the most prominent of her commodities.

COAL

Coal resources of Colorado amount to about 60 billion tons. Only one per cent of this has been mined. Thousands of tons are now being produced daily from large mines in central, southern, and northwestern parts of the state.

Colorado’s coal deposits were formed during late Cretaceous and early Tertiary time, when seas were receding from this region and the land was rising. They represent accumulations of leaves and other plant material in swamps and flood plains similar to those now found in the delta of the Mississippi River and in the swamps of southeastern United States. Almost all Colorado coal is bituminous or soft coal.

Coal was recognized early in Colorado history by settlers along the mountain front, and was mined west and north of Denver in the 1860s. Several large underground mines still operate in this district, supplying local power plants, but production does not compare with that of the Walsenburg-Trinidad area in southern Colorado or the Hayden area in northwest Colorado.

The Walsenburg-Trinidad region, part of the Raton coal field, has produced coal since the building of the Santa Fe Railroad in the early 1870s. For many years coal from these mines moved the Santa Fe trains and many of the numerous smaller railroads that served Colorado’s cities and mining camps. The location of the mines helped to determine the location of the Colorado Fuel and Iron Company smelter in Pueblo. Now, most southern Colorado coal is used to produce electric power. Many small mines, miles away from the power plant west of Trinidad, are deserted.

A large coal-burning power plant has recently been built between Hayden and Steamboat Springs, just west of the Yampa River. Here, some of the extensive coal deposits can be seen in road cuts along U. S. highway 40. Until conversion to diesel fuel became almost universal in North American railroads, mines of this district produced coal for locomotives.

In the heyday of the gold and silver mines, coal was also mined near Coalmont, in North Park, and Como, in South Park. Coal from these areas was used for fuel in nearby mining towns and ranches, and for the narrow-gauge railroads that penetrated the mountains here.

At Anthracite, near Crested Butte, high-grade anthracite coal was mined for a time. Identical in origin with other Colorado coal, the anthracite of this region was hardened by heat and pressures from Tertiary igneous intrusions forcing their way into local sedimentary rocks during post-Cretaceous mountain building.

A multitude of other coal camps are scattered about Colorado: Cokedale, Delcarbon, Coaldale, Roncarbo, Carbondale, and Cardiff stand out because of their suggestive names. These early small camps are, like their metal-mine cousins, largely deserted today.

CONSTRUCTION MATERIALS

Sand, Gravel, and Clay

Sand, gravel, and crushed rock rate high among geologic products in Colorado; more than $27,000,000 worth of these materials were produced in the state in 1969. Highway and construction activities have brought recent expansion in the number and size of quarries and gravel pits. Increasingly, Coloradoans are insisting that quarries and pits be excavated only where they will not mar the natural beauty of the landscape, and many old pits are now being filled in. Unfortunately, the scars left by some quarries—such as that on the Rampart Range near Colorado Springs—are difficult to erase.

Clay of good quality occurs in Cretaceous deposits in many parts of Colorado, most frequently in the Dakota or Laramie Formations. In the area around Golden, the Coors Porcelain Company for many years mined clay for use in pottery and low temperature ceramic ware. Scars from this mining can be seen along the mountain front north and south of Golden, and deep clefts within the town, just west of Colorado School of Mines, testify to the amounts of clay that have been removed. Colorado clay is not pure enough to be used in high temperature ceramics, and the present use for it is in the manufacture of common tiles and bricks.

A recent development in Colorado is the use of Cretaceous Pierre shales in manufacturing lightweight aggregate for building. The shale is mined between Golden and Boulder, near Colorado highway 93. In the nearby plant, it is pulverized and then heated in a large rotating cylinder until the surface of each particle fuses. Then the particles are quickly cooled. The resulting product is much like cinder, light in weight and yet strong. It can be mixed with cement for use in construction work requiring a great strength-to-weight ratio, or made into concrete blocks.

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)

Stone

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 Mountain Granite, 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 Colorado fossils and 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, mostly hematite, disseminated between the sand grains. Dendrites (often erroneously called fossil ferns or plants) ornament some slabs; they were formed by crystallization of manganese dioxide from groundwater as it slowly percolated through the rock.

Lime and Gypsum

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 Pennsylvanian sedimentary rocks. Particularly high-quality Pennsylvanian gypsum is quarried at the town of Gypsum, west of Eagle.

The Colorado portion of the Paradox Basin, in the Plateau Province, contains immense deposits of Pennsylvanian gypsum. Here, rocks near the surface have been pushed up into sharp northwest-trending faulted anticlines by 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, dip away from these valleys. 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.

GEMS AND ORNAMENTAL STONES

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 in Colorado 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.

WATER

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.

Surface Water

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 and Plateau Provinces receive much 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.

Groundwater

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 provides artesian water for irrigation of valley farmlands.

SAN JUAN MOUNTAINS
LIMIT OF FLOWING WELLS
HUBBARD’S WELL
OTTOWAY’S WELL
ALAMOSA WELL
GEORGE NEWSOM’S WELL
CALKIN’S WELL
LIMIT OF FLOWING WELLS
Moraine
Alluvial Slope
SANGRE DE CRISTO MOUNTAINS
Sands, lava beds, gravels, conglomerates, etc.
Alamosa formation
Granites
WEST
SANTE FE FORMATION
SANTE FE FORMATION
EAST

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 groundwater aquifers, 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 great artesian water supply. 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.

Caves

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 of stalactites and stalagmites has 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 Pennsylvanian reef limestone, 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 other caves 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 River Plateau.

In Cave of the Winds near Manitou, Paleozoic limestones, cracked and tilted by uplift of the Front Range, have been honeycombed by ground water. Calcite stalactites hang from the ceiling, while stalagmites grow up from the floor. (Cave of the Winds Company photo)

In the Plateau Province 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, thinly bedded mudstone and shale. Where the resistant layers are undermined, great arching caves develop. These are best observed at Mesa Verde 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 at Mesa Verde, 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)

Springs

The multitudes of mineral and hot springs in Colorado are a fascinating and interesting facet of the Mountain Province. Some are located along major faults, 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 River Plateau. 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 Precambrian granite, 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 of Cretaceous age. Mt. Princeton Hot Springs comes from alluvium but 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 where aquifers, generally sandstones, are dissected by erosion. These springs, usually not highly mineralized or warm, are most often found in the Plateau Province. Such springs are frequent at the bases of the great sandstone cliffs of Mesa Verde 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 Pass fault, 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.

ENVIRONMENTAL GEOLOGY

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.

Landslides and 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—and have caused—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 areas is risky, but sometimes worth the risk; if condition are less than ideal, risks can be reduced by specialized types of construction.

Floods are 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 to describe 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.

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