CENOZOIC ERA

Between the Front Range and the Plains the Cretaceous Dakota Formation forms ahogbackridge which can be traced for 200 miles or more. The well-cemented sandstone resists erosion, and so remains as a ridge when softer layers are stripped away. (Jack Rathbone photo)

The sandstones derived from beach sands sometimes include coarse pebbles of chert which can be traced to sources in Permian rocks of Utah and Nevada. Occasionally the beach and near-shore deposits include marine shells like oysters, indicating that there were brackish and salt water lagoons and marshes along the shore. The Dakota Formation represents the beach of the transgressive or advancing sea. This formation contains oil in eastern Colorado, Nebraska, and Wyoming; the oil itself may have beenderived from decay of organic materials in swamps behind the beaches and bars.

As the sea deepened in eastern Colorado, finer sediments were deposited. These included the black muds of the Benton Shale, and the Niobrara Limestone, a shallow-water deposit containing abundant shells of clams (InoceramusandOstrea) andammonitesand tiny one-celled animals calledForaminiferida. Above the Benton and Niobrara Formations lie the fine gray muds of the Pierre Shale. Several thousand feet thick, the Pierre contains occasional beautifully preserved ammonite shells as well as bones fromfossilfish and swimming reptiles.

Cretaceous rocks in Colorado are rich infossilpelecypods. Each of the fossils illustrated above may grow to a much larger size than shown.

Shales of the Laramie Formation contain many recognizable plantfossils.

The rocks deposited in western Colorado at this time are markedly different from those deposited in eastern Colorado. In the east, deposits are fine and very limy, containing abundant shells and little in the way of coarse debris. In the west, sandstones of theMesaVerde Formation dominate, and coal beds suggest marshy or swampy conditions inshore from the ancient ocean. This is just the pattern we would expect from a low-lying region bordering a shallow sea, a region similar perhaps to the southeastern Atlantic and Gulf coasts of the United States today.

Toward the end of the Cretaceous Period, the sea receded from Colorado. Beaches and bars of the retreating sea left a sandstone layer which now outcrops prominently east of the Front Range as the Fox Hills Sandstone. Above lie interbedded sands and coals, the Laramie Formation. The presence of coal above beach sands shows that the coal swamps moved eastward as the sea retreated.

The exact age of the shoreline deposits and coal beds varies from place to place in such a way as to indicate that the sea withdrew slowly and irregularly. In general the shore moved eastward, but there are localities such as North Park where deposition lasted much longer than elsewhere. In some places no real beach was formed at the ancient strand line.

In western Colorado, the end of Cretaceous time is marked by coarser beds, indicating an increased rate of uplift in Utah.Conglomerateswere deposited in the beds of the McDermott Formation, now visible along the Animas River south of Durango.

It is characteristic of earth history that the younger the rocks are, the more we know about them. This is because younger rocks lie near the surface, have not been disturbed as much by mountain building processes as have older rocks, and have not been affected as strongly by repeated erosion. Many of the events of the Cenozoic Era are documented in detail in the geology of Colorado, and these events have intimately influenced the scenery as we see it today.

The Cenozoic is the Age of Mammals. How it happened that mammals triumphed over reptiles is one of the mysteries of geology. Some scientists think that climatic changesâ€”dropping temperatures and increases in rainfallâ€”swung the balance in favor of the warm-blooded mammals. Others believe that cosmic ray bombardment during some unusual astronomical event may have destroyed many surface-living dinosaurs, while small burrowing mammals (as well as many small reptiles) were able to survive. Still others maintain that the superior intelligence and regulated body temperatures of mammals enabled them to win out in the battle for survival without the aid of climatic or cosmic change.

The names Tertiary and Quaternary, used for the two Cenozoic Periods, are holdovers from early studies in geology in which rocks were divided into Primary (very hard, crystalline rocks such as igneous and metamorphic rocks), Secondary (well consolidated layered rocks), Tertiary (layered rocks which are not fully cemented but which are nevertheless fairly well consolidated), and Quaternary (sediments in which the grains have not become cemented together).

During the first part of the Tertiary Period, uplift began in earnest in Colorado and adjacent states. This uplift was part of the great Laramide Orogeny that built the Rocky Mountain chain from Alaska to New Mexico. The entire area rose above the level of the sea, and mountains were thrust up in a great series of north-south ranges that extended unbroken almost the length of the continent. Between the ranges, thick layers of gravel and sand,derived from the surrounding highlands, were deposited in intermontane basins. Occasional freshwater limestones and shales indicate the presence of lakes.

In Colorado, many details of the formation of the Rockies stand out in bold relief. The Front Range moved upward sharply, mostly as a linear block broken or faulted along both edges. Paleozoic and Mesozoic sediments along the margins of the block were steeply tipped and in some places even overturned, while in some localities Precambrian rocks were thrust out over the younger sediments.

Just east of the Front Range, especially in the area around Denver, the land remained lower and was the site of thick deposits of gravel and sand eroded from the range. The Denver Formation, the ArapahoeConglomerate, and the Dawson Arkose are more than 2,000 feet thick in this area. These are delta and river sediments, all varying a great deal from place to place. Individual layers of sand or gravel are not continuous over extensive areas, but some, such as the Castle Rock Conglomerate, are very prominent locally.

On Wolford Mountain, just north of Kremmling, Precambriangranitelies on top of Cretaceous shale. The older rocks were thrust up and over younger rocks during the Laramide Orogeny. The position of thefaultshows clearly because trees prefer the granite soil above the fault to the shale below. (Jack Rathbone photo)

Along the eastern margin of the Front Range west of Castle Rock and Sedalia, rocks deposited at this time are now folded steeply, indicating that the mountains continued to rise even as basin sediments were being deposited.

In southern Colorado, the Sangre de Cristo and Wet Mountains were also formed as upthrust blocks. Between them, the Huerfano Basin and adjoining Raton Basin received particularly rapid alluvial deposition. In the Raton Basin, quantities of vegetation were deposited in swamps and marshes, forming the thick coal beds which can now be seen in road cuts west of Trinidad and along the Raton Pass highway. Huerfano Basin deposits contain some of the earliest known horse remains, skeletons of a tiny four-toed horse calledHyracotherium(formerly known asEohippus).

Bones ofHyracotherium, the â€œdawn horse,â€ have been found northwest of Walsenburg in Early Tertiary sediments of the Huerfano Basin. (C. R. Knight painting, courtesy American Museum of Natural History)

Other rising ranges provided material for alluvial deposition in North Park, Middle Park, South Park, and the San Luis Valley.Layers ofbasaltand volcanic peaks show that as the mountains rose, the crust cracked and allowedlavato rise to the surface in great quantities. Tertiary basalts are very much part of the Colorado landscape: some can be seen west of Granby, others in Table Mountains east of Golden. Near Boulder, ValmontDikewas intruded, though lava may not have reached the surface in that area. Spanish Peaks in southern Colorado,Mesade Maya, the Rabbit Ears Range, Grand and Battlement Mesas, and many other volcanic features were formed at this time.

The town of Golden nestles between the Front Range and South Table Mountain. Tertiarybasaltcapping South Table Mountain covers beds of the Denver Formation. It thins to the right, or south, indicating that its source was probably to the north or northwest. Buildings in the right foreground are the Colorado School of Mines. (Jack Rathbone photo)

A series of almost verticaldikesradiate from West Spanish Peak. Surrounding sediments are Tertiary. Weathering and erosion along sets ofjointsin the largest dike have shaped it into the â€œDevilâ€™s Staircase.â€ (Jack Rathbone photo)

Most of the rich mineral deposits of Colorado are thought also to have been formed during the early part of the Tertiary Period. Solutions rich in gold, silver, zinc, lead, copper, and sulfides of iron seeped intojointsandfaultsin the crust as the mountains were pushed upward. Ore minerals crystallized out, sometimes inveinsin the ancient Precambrian igneous and metamorphic rocks, sometimes in Paleozoic sediments. These are further discussed inChapter III.

The Eocene Green River Formation includes great thicknesses of oil shale, an untapped petroleum reserve containing perhaps three trillion barrels of oil. The richest part of the oil shale is a dark brown layer called Mahogany Ledge, visible here on cliffs just west of Rifle. If placed in a campfire, fragments of this shale release enough oil to burn with a yellow, smoky flame. (Jack Rathbone photo)

Further to the north and west, the Uinta Mountains rose. They are a fault-block range, but they lie at right angles to the general north-south trend of the Rocky Mountains. South of them the Uinta Basin, one of the largest of the intermontane basins, received shaly deposits in a great lake which existed here for probably several million years. The lake extended over some 100,000 square miles, and during its existence great quantities of tiny organisms lived in its waters. Oily material from theseorganisms was deposited in the mud of the lake sediments, particularly in the eastern end of the basin, there to remain trapped in a great oil-shale deposit.Fossilfish, crayfish, algae, and many forms of insect and plant life have been found as fossils in these lake shales.

West of Pikes Peak, another lake formed, dammed by alavaflow from a nearby volcanic field. Fine volcanic ash falling into this lake preserved the trunks and leaves of many plants as well as abundant insects, fish, and occasional mammal bones. These are now protected and exhibited in FlorissantFossilBeds National Monument. The fossil plants, among them redwoods, poplar, hackberry, and pine, suggest a climate warmer than the present one, and have been taken to indicate that regional uplift to the present altitude had not yet occurred.

Another rich deposit offossilinsects and plants occurs near Creede. Other lake deposits in South Park contain ash layers with fossil algae and snails.

Large petrified trunks of redwoods and other trees can be seen at FlorissantFossilBeds National Monument, west of Colorado Springs. (John Chronic photo)

In southwestern Colorado, extensive Tertiarylavaflows, ash falls, and river deposits form the eastern part of the San Juan Mountains, the largest volcanic area in the state. Mineral collectorsare attracted to this region by the many excellent localities for agate and other siliceous stones.

Still another center of Tertiary volcanism was located in what is now Rocky Mountain National Park. Specimen Mountain, northwest of Trail Ridge, was an active volcano about 30 million years ago, shedding ash andlavaover much of northern Colorado. Therhyolitewhich now caps the hill west of Iceberg Lake, on Trail Ridge Road, was derived from this volcano, but is now separated from it by the deep glaciated valley of the Cache la Poudre River and Milner Pass.

Volcanic ash at times drifted far eastward and blanketed the surface of the plains, burying specimens of many animals and plants. The White River Formation, extending from northeast Colorado northward into South Dakota, is formed of such drifting ash. Many now-extinct mammals have been excavated from this formation.

Sometime after the mid-Tertiary episode of violent volcanic activity, Colorado was uplifted to its present altitude. This was a general uplift, raising the plains andplateauareas as well as the mountains. The uplift was not an abrupt process, but continued for perhaps ten million years. It raised the entire state 3,000 to 5,000 feet above its previous level.

Pawnee Buttes, about 40 miles north of Fort Morgan, rise like castles from the eastern Prairie Province. Remnants of Oligocene and Miocene sedimentary rock that once covered much of northeastern Colorado and adjacent states, they contain jaws, teeth, and other bones of primitive mammals. (Department of Highways photo)

During the remainder of the Tertiary Period, Colorado wasthe site of erosion rather than deposition. However, some stream material was deposited in the mountain valleys, and on the prairies wind-blown and stream-borne sands were spread thinly, interlayered with impure limestones deposited in ponds and lakes. In the San Luis Valley, deposition was probably more continuous than elsewhere, as the exit from the valley was blocked by volcanic flows. The deposits in this valley, sands and clays of the Santa Fe and Alamosa Formations, form a great artesian basin. The rich agricultural development of the valley is made possible by water wells tapping these formations.

Remains of many now-extinct mammals have been found in Tertiarysedimentary rocksof northeastern Colorado, in the general area of Pawnee Buttes. Those illustrated areOreodonfrom Oligocene strata and a â€œgiraffe-camelâ€ (Oxydactylus) from Miocene rocks.

The most significant feature of the Quaternary Period in Colorado, as elsewhere in the northern hemisphere, is the evidence ofglaciation. During the first part of the Quaternary Period, known as the Pleistocene Epoch, great continentalglacierscovered most of Canada and much of northern United States. The ice sheets did not extend southward as far as Colorado, but large valley glaciers developed in many of the mountain ranges of the state and left their traces in many mountain valleys.

Mills and Jewel Lakes, in Rocky Mountain National Park, occupy small glacier-gouged basins inGlacierGorge. The flat-topped peak at the upper left is Longs Peak, elevation 14,256 feet; Pagoda Mountain is in the center of the skyline.Bedrockin this area is Precambriangranite,gneiss, andschistat the Front Range â€œcore.â€ (Jack Rathbone photo)

The conditions leading to Pleistoceneglaciationare not fully understood. Climatic changes may have been initiated by a decrease in solar radiation, changing patterns of ocean currents,reduction of solar heating by volcanic dust, or an increase in general elevation of the land. As the climate became cooler and moister, snowfall increased in the north and at high altitudes. In areas where winter snowfall exceeded summer melting,glaciersdeveloped.

In Colorado,glaciersformed along the crests of the Front Range, the Sawatch Range, the Elk Mountains and West Elk Mountains, the Sangre de Cristo and Mosquito Ranges, the San Juan Mountains, and the Park and Gore Ranges.Glaciationin Colorado was selective: in many places elevation was sufficient for glaciation, but snowfall apparently was not great enough. Where they did occur, the glaciers extended down to elevations of about 8,000 feet. There, temperatures became mild enough to melt the ice.

The mountainglaciershave left many tell-tale signs of their presence. Valleys above 8,000 feet are U-shaped, their upper ends bounded by horseshoe-shaped, steep-walledcirques. In the lower portions of the valleys, at elevations just above 8,000 feet, lie long lines of glacial debris known asmoraines: terminal moraines forming crescents across the valleys to show where melting glaciers dropped their rocky loads; lateral moraines along the sides of valleys; medial moraines where glaciers from two valleys met. Terminal moraines, often forming effective barriers across the present streams, may act as dams, creating lakes such as Grand Lake in Rocky Mountain National Park.

There were at least three distinct glacial episodes in Colorado. This is known because careful studies of glacial debris inmorainesreveal three different degrees of rock weathering. All three stages can be seen in or near Rocky Mountain National Park. The oldest is represented by a moraine about three miles west of Estes Park, where the Big Thompson River traverses a wide U-shaped valley before entering its narrow, unglaciated canyon. The next oldest is represented in terminal moraines further up the valley, at Aspenglen campground. The youngest is shown in a prominent terminal moraine about one mile west of the park entrance in Horseshoe Park.

A large lateralmoraineseparates Hidden Valley from the south side of Horseshoe Park, and an almost equally large lateral moraine is present on the north side of this valley. At Moraine Park, both sides of the valley are edged with lateral moraines also.

Studies in Rocky Mountain National Park have revealed many other details ofglaciationin this area. These are described in Park Service brochures and guidebooks, in the museum at Park headquarters, and in informative roadside signs.

A line of hikers approaches ArapahoGlacier, west of Boulder. Movement of the glacier is evidenced by the crevasses apparent just below the snowfield in the dirty gray glacial ice. (H. H. Heuston photo)

Several smallglaciersare still present in the Colorado mountains, all in shelteredcirquesabove 11,000 feet. These may be remnants of the former larger glaciers, or new glaciers formed after a long warming episode. A hike to one of these glaciers is a rewarding experience for anyone interested in geology. Some of the more accessible are St. Maryâ€™s Glacier west of Denver, Arapaho Glacier west of Boulder (the Boulder Chamber of Commerce sponsors a festive hike to Arapaho Glacier every August), and Tyndall Glacier in Rocky Mountain National Park.

The Ice Age brought drastic changes also to the landscape below 8,000 feet elevation. Heavily loaded with glacial debris, mountain streams disgorged coarse sands and gravels along the mountain front and in the intermontane basins. As theglaciersmelted after each period of expansion, the swollen streams cut deeply into their former deposits and into much older rocks as well. Royal Gorge, the Black Canyon of the Gunnison, and many of the deep, colorful canyons of thePlateauProvince were cut or at least deepened by these waters. The canyons along the east face of the mountainsâ€”Big Thompson, Boulder, Clear Creek, and othersâ€”were also deepened and sharpened by the rushing ice-fed torrents.

On the prairies, rivers dumping their loads of sand covered the older rocks. Sand dunes developed along the river channels. Bones and huge tusks of hairy mammoths were sometimes buried in these soft deposits; now they are occasionally revealed as the dune and river sands are washed or blown away by continuing erosion.

About 20,000 years ago, man arrived in Colorado. Soon after this, the water supply of the valleys diminished greatly, and erosion slowed down correspondingly. The climate gradually became semiarid to arid. Many features of the natural scene were much as they must have been a century ago, without the highways, dams, and television aerials of today. Buffalo and many smaller types of game roamed the plains and foothills; deer, elk, and bighorn sheep were plentiful in the mountains. Nomadic tribes camped and hunted in both mountain and prairie. In the western part of the state, homes could be built in the shelter of great caves, as atMesaVerde, and game could be supplemented with corn and squash planted onplateausurfaces.

Several features of Colorado scenery changed with increasing aridity. Theglaciersof course were gone or nearly gone. Streams were no longer the violent torrents they had been. Many mountain lakes, filled with sediment and vegetation, became instead mountain meadows. And the once fertile intermontane valleys became deserts.

During the last Ice Age, elephant-like mastodons roamed Colorado. As present-day erosion removes sediments, bones, teeth, and tusks are frequently exposed, especially in the Prairie Province. (C. R. Knight painting, courtesy American Museum of Natural History)

Mastodon

On the eastern side of the San Luis Valley, the Great Sand Dunes developed at this time. These dunes nestle against the Sangre de Cristo Range, where strong southwesterly winds blowing across the wide valley tend to funnel toward Mosca and Music Passes. These winds lift loads of sand from the lightly vegetated valley floor, and drop it as they rise over the mountains. Where the sand is dropped, the dunes have formed. They rise to about 700 feet above the valley floor, and cover about forty square miles. The low rainfall of the area, seven to eight inches per year, keeps vegetation from creeping over the dunes and makes them a most distinctive feature of Colorado, a lesson in geology in the making.

* * * * * * * *

Geologic processes in Colorado now seem to be much reduced from what they were a few thousand years ago. Reduction in rainfall has led to reduced erosion. Mountain-building, having reached a climax in Tertiary time, has declined markedly. However, we find evidence that volcanism has occurred within the last few thousand years and faulting within the last few hundred, and Colorado streams rise after sudden mountain storms to approximate the violent torrents of glacial times. Coloradoâ€™s scenery, fashioned during some three billion years of earth history, is ever changing.

The Great Sand Dunes of Colorado were formed during Pleistocene and Recent time by deposition of quartz sand lifted from unconsolidated alluvial deposits in the San Luis Valley. The highest of the dunes rises 700 feet above the adjacent valley floor. (John Chronic photo)

Coloradoâ€™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 â€œcampsâ€ 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 Coloradoâ€™s mineralsâ€”products of her long and diverse geologic historyâ€”have influenced her development in many ways. The stateâ€™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 standingâ€”the opera houses at Central City and Aspen, Central Cityâ€™s famous Teller House, and the Grand Imperial Hotel at Silverton are examples.

Many of the stories and legends of Coloradoâ€™s gold camps are recounted inStampede 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.Placergold was usually discovered first. Recovery of placer gold was followed by mining of gold fromveinsor â€œlodes.â€ Although at first onlynative goldwas 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 stateâ€™s mining industry, but isnow 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 andPlateauProvinces 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 stateâ€™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:

Colorado is now the nationâ€™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 manâ€™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.

Coloradoâ€™splacerandlodesources 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 â€œmineral belt,â€ 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 andplacergold 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 recognizablenative gold.

In the northeast part of the mineral belt, gold and other minerals occur inveinsin Precambriangraniteandgneiss. 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 andjointsin 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 withradioactive 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.

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.

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 fromveinsearly in 1859. Activeplacermining lasted only about a year, however, andlodemining 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 calledpetzitewas found inveinsat 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 verticalveinssix 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.

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