Part 5

About midway between Edwards and Wolcott, the Eagle River suddenly changes direction and flows northward for about a mile before resuming its former westward course. This sudden change is caused by a sharp north-south fold in the sedimentary rocks on the northwestern flank of the Sawatch Range. A magnificent series of roadcut and hillside exposures along the highway here illustrates the close relation between rock layers and river course. Within about a mile, the highway cuts through rocks of Pennsylanian, Permian, Triassic, Jurassic, and Cretaceous age, spanning a geologic time interval of more than 200 million years.

The south end of the Sawatch Range, at Monarch Pass, contains steeply dipping Late Paleozoic limestones and coal beds. The coal has been mined on a small scale; the limestone is now quarried for use as a flux in iron smelters at Pueblo.

Elk Mountains and West Elk Mountains

The Elk Mountains and West Elk Mountains appear to be westward continuations of the Sawatch Range. Structurally, however, they are not faulted anticlines like most of the other ranges in Colorado, but are composed of a series of layers of Paleozoic sediments thrust westward over one another. These rocks, often crumpled and highly metamorphosed, are cut by numerous sills, dikes, and other intrusions, many of which have caused mineral enrichment locally.

At Maroon Bells, in the canyon of Maroon Creek, and at Redstone on the Crystal River, these metamorphosed sediments are well exposed. Here, red sandstones and shales have been altered to quartzites and slate. At Marble, metamorphism of a thick limestone bed has produced white marble of great beauty, known as Yule Marble. This decorative stone was quarried extensively until about 1940. It was used in the Lincoln Memorial and several other monumental structures; in the town of Marble it has been used for the doorsteps of log cabins! The largest block quarried, for the Tomb of the Unknown Soldier in Arlington National Cemetery, measured 14 by 7.4 by 6 feet in the rough, and weighed 56 tons.

Crested Butte, at the south end of the Elk Mountains, is a small intrusive igneous mass called a laccolith. Hard and resistant to erosion, it stands over 2,000 feet above the adjacent valley floor.

San Juan Mountains

The San Juan Mountains are the most extensive range in Colorado, and also the most heterogeneous. Covering more than 10,000 square miles of the southwestern part of the state, these mountains are formed mostly of Tertiary volcanic rocks, the result of repeated outpourings of lava and ash from a cluster of volcanoes. Water-laid gravels composed of volcanic sand and pebbles are interlayered with basalts and ash beds; the total thickness of these beds reaches many thousands of feet.

The widespread volcanic activity which formed most of the range began in mid-Tertiary time and continued for several million years. A few Quaternary volcanic flows are known in the region, but there is no active volcanism there at present.

The western side of the main range, including some of the highest peaks, consists primarily of uplifted and faulted Paleozoic sedimentary layers. These layers, highly dissected by erosion, can be seen near Ouray, at Molas Lake, and at Durango. Large patches of Precambrian granite and metamorphic rocks protrude through the sediments, as in the Needle Mountains; they indicate that this part of the range is a faulted anticline like many other Colorado ranges.

Early Cenozoic glacial deposits occur in some parts of the San Juans. These are unusual features, as glaciation of this age is unknown elsewhere in Colorado.

Three small ranges rise just west of the San Juans: the San Miguel, Rico, and La Plata Mountains. Each consists of several small masses of Tertiary igneous rock intruded into Paleozoic conglomerates, shales, and limestones.

Mineralization has been intense in the San Juans; most of it took place during the Late Tertiary volcanic period. Rich veins penetrate Precambrian gneiss and granite, and Paleozoic limestones are often enriched also. Several mines are still active near Ouray, Silverton, Telluride, and Rico.

Uinta Mountains

The eastern end of Utah’s Uinta Mountains extends into Colorado. Unlike other ranges in Colorado, these mountains trend east-west. Structurally, the range is a faulted anticline. It is quite asymmetrical, however, and is tilted and folded upward on the south, and overturned or thrust-faulted on the north. Steeply dipping Mesozoic and Paleozoic sediments on the south side of the range, sparsely vegetated and often thrown into spectacular folds, are a prominent feature of northwest Colorado scenery.

In Colorado the crest of the Uintas reaches an elevation of about 8,500 feet. It consists of Precambrian rocks, but these are not the igneous and metamorphic rocks that characterize the Precambrian core of other Colorado mountains. They are easily recognized as sediments—dark red conglomerates, sandstones, and mudstones—virtually unmetamorphosed though they were deposited nearly a billion years ago. Called the Uinta Mountain Formation, these rocks are found only in this part of Colorado and adjacent areas of Utah. They are probably related to similar Precambrian rocks found in Montana and Canada.

At the east end of the Uintas two isolated uplifts, Cross Mountain and Juniper Mountain, are faulted blocks of Paleozoic rocks standing like islands in a sea of Cenozoic valley fill. They are the last outposts of the Uinta anticlinal pattern as it wanes toward the southeast.

Dinosaur National Monument, a Uinta Mountain tourist attraction, encompasses a vast area of wilderness on both sides of the Yampa River in Colorado. Here many of the features of the east end of the Uinta Mountain structure can be seen. A unique display of the world’s largest fossils can be visited in the Utah portion of the Monument.

THE PLATEAUS

The western quarter of Colorado is a region of flat-lying Paleozoic, Mesozoic, and Cenozoic sedimentary rocks which have not been bent up into mountains except in a few isolated instances. This area lies more than a mile above sea level, however, and because of the gradient such an elevation affords, it is deeply sculptured. The Colorado River and its tributaries have sliced into the plateau surface, separating it into many isolated tablelands or mesas. Some are capped with sedimentary rock, others with Tertiary basalt.

Simple folds and faults have given the mesas different elevations. Thus the average elevation of the White River Plateau is 11,000 feet, that of the Roan Plateau 9,500 feet, and that of Mesa Verde only 7,000 feet. West of Durango the plateaus dip gently southward, as can be seen at Mesa Verde. Igneous intrusions and extrusions have altered plateau topography in some areas. West of Mesa Verde, for instance, an intrusive stock forms a prominent dome in the Southern Ute Indian Reservation.

West of the northern Colorado mountains, and north and west of the White River Plateau, a rolling upland extends from Colorado into Utah and Wyoming. It is interrupted by the Uinta Mountains and a number of smaller related uplifts such as Juniper Mountain and Cross Mountain. South of the Uinta axis the area is known as the Uinta Basin.

The northern part of this area is structurally the south edge of the Green River or Washakie Basin in Wyoming. The Rangely anticline, in the northeastern corner of the Uinta Basin, is one of Colorado’s richest oil fields; it is discussed in Chapter III.

Although surfaced with much younger sediments than the rest of the Plateau Province, this area is structurally similar. On the whole, sedimentary layers are relatively flat-lying, and where they are uplifted they are deeply sculptured by streams and rivers. The sedimentary rocks in this region contain uranium and placer gold in addition to great oil and gas deposits. The southeastern part of the Uinta Basin, usually called the Piceance Basin, is the site of a great deposit of oil shale (see Chapter III). The term “basin” may here seem unusual to the casual observer, for the oil shales occur on the Roan Plateau at places well over 10,000 feet in elevation. However, the entire region was basin-like—lower than the surrounding ranges—for many millions of years, and during Tertiary time thousands of feet of valley and lake deposits were laid down in it.

The White River Plateau, north of Glenwood Springs, is composed of almost horizontal Paleozoic sedimentary rocks that fold downward sharply along its south and west edges. The fold is 135 miles long and is clearly marked by the Grand Hogback, the eroded edge of hard Cretaceous and early Cenozoic rock layers. Shale and coaly layers involved in the same fold have eroded more readily, leaving the resistant sandstone as a prominent ridge.

The Uncompahgre Plateau, southwest of Grand Junction, is structurally very like the White River Plateau. Its features can be well observed in Colorado National Monument. It has been elevated several thousand feet more than the Book Cliffs and Grand Valley areas to the north. Sharp folding and faulting near the Colorado River at the north boundary of the National Monument show that differential movement between the two regions was sharp and localized.

A series of northwest-trending anticlines along the Utah border in southwestern Colorado are of special geologic interest. They represent peculiar structures in which salt and gypsum have played a major part. These minerals were deposited in thick layers late in Paleozoic time; subsequently they were covered by thousands of feet of sand, shale, and limestone. Because of their low density and high plasticity they have since crept upward along weak spots in the overlying sediments, often contorting these rocks as they moved. Breaking through to the surface, the salt and some of the gypsum washed away more rapidly than the surrounding rock, leaving long faulted troughs such as Gypsum Valley and Paradox Valley. In most of these structures the gypsum can still be seen, although the more soluble salt has eroded away. Oil wells in this part of Colorado and in adjacent parts of southeast Utah have penetrated thousands of feet of evaporites, including pure salt, gypsum, and potassium salts.

The peculiar weathering characteristics of flat-lying sedimentary rocks in an arid climate are well demonstrated in Colorado National Monument, Mesa Verde National Park, and elsewhere in the Plateau Province. Those fortunate enough to make a river trip through the Yampa or Green River Canyons in northwestern Colorado or on the rivers of eastern Utah and northern Arizona will have an unusually fine opportunity to observe close at hand the weathering and erosion in this area. Resistant sandstone and limestone layers break into sheer cliffs, often many hundreds of feet high, while the softer layers of mudstone and shale form gentle slopes and terraces. Vast arching caves often develop where resistant layers are undermined—caves sometimes containing ancient Indian dwellings.