Geological Survey Bulletin 707 Guidebook of the Western United States: Part E. The Denver & Rio Grande Western Route

SHEET No. 3 (click on image for an enlargement in a new window)

Cotopaxi is a small settlement, hemmed in on all sides by high granite walls, but fairly good roads lead from it southwestward to a rich agricultural region at the foot of the Sangre de Cristo Range. Small quantities of the precious metals as well as some copper have been found near the town, but none of the mines are now in operation. Limestone was once quarried here in large quantities for use as flux in iron furnaces, but most of the limestone now so used at Pueblo is quarried near Howard, farther up the valley. The quarries near Cotopaxi were about 3 miles north of the railroad, at the southern end of the belt of Carboniferous rocks. (See sheet 3, p. 100.) The limestone has been preserved here by being downfolded into the granite, and on the east side of the downfold the rock has been broken by a fault and replaced by the granite.

For some distance west of Cotopaxi the sides of the canyon are composed of massive granite, which in places stands up in nearly vertical walls (see Pl. XL, A), but the valley bottom is generally wide enough to afford ample accommodation for the railroad and for the Rainbow Highway. The canyon maintains this width for some distance, but beyond milepost 194 the river passes through the narrowest and most rugged part of the canyon west of Parkdale.

About three-quarters of a mile beyond milepost 194 the railroad emerges suddenly from the granite canyon into a broad valley at the foot of the Sangre de Cristo Range. The course of the railroad, which for a long distance has been nearly southwest, here veers to the northwest along this valley. The change from seemingly endless vistas of rocky canyon walls to a broad valley in which there are farms and green trees is striking and exceedingly restful and is one of the surprises that are constantly awaiting the traveler in this mountainous region.

The change in scenery and in the general character of the country is due to a difference in the underlying rocks, but for some distance this fact is not apparent, as the rocks are not visible from the train, the land near the river being composed of sand and gravel washed down from the high mountains at the back of the valley. The railroad follows the east side of the river, passing by the village of Pleasanton and hugging the granite cliffs that border the valley on the northeast (right). The contact of the soft rocks of the valley with the granite or gneiss is not a normal contact but is due to a fault, the granite having been elevated or the other rocks depressed an unknown distance.

In order to understand the meaning of the surface features along the railroad from Pleasanton to Salida it is necessary to know the geologic structure and the succession of hard and soft rocks.

Mountains are usually formed either because they contain rocks that are somewhat harder than the rocks in adjacent areas or because recent disturbances in the earth's crust have raised one part of the crust with relation to another; or they may be formed by volcanic action. In the Rocky Mountains the principal ranges and peaks have been formed by one or both of the two causes first stated.

The great Sangre de Cristo Range, which towers on the left a mile above the railroad, is no exception, but this range, unlike many others of this general region, is very narrow, being at no point more than 12 or 15 miles wide. At many places its crest is composed of granite and gneiss, which, being harder than the surrounding rock, have remained at their present height, while the softer rocks on either side have been washed away to lower levels. In general, the structure of the mountain at the north end is that of a great anticlinal fold (arch), mainly in Carboniferous rocks, though it affects the lower rocks down to and including the granite. At a point farther south the fold crosses the range at a low angle, and from that point southward the structure is entirely different. The section shown in figure 17 represents in a general way the structure of the rocks at the north end of the mountain—the anticline in the mountain and the syncline (trough) on its east side. From a point above Howard to Pleasanton Arkansas River flows in the valley eroded in this syncline, and the granite on the right of the railroad lies on the east side of the fault, as shown in the section.

FIGURE 17—Cross section of the Sangre de Cristo Range and the valley on its east side at Pleasanton showing the anticline of the mountain and the syncline on the east.

At Pleasanton the railroad is built on the Weber shale and sandstone near the fault, but in passing northward it diverges more and more from the granite wall until it is on the Maroon sandstone nearly in the middle of the trough. This sandstone makes its appearance a short distance above the siding of Vallie. It is very conspicuous on the left, in the hill across the river, and dips about 70° W., or into the great syncline which lies on that side of the railroad. This hill shows to good advantage not only the red Maroon sandstone but a cap of lava, which gives some clue to the recent geologic history of the valley. As seen from the train the lava cap appears to be horizontal, but after passing it the traveler, upon looking back, may see that the lava cap is underlain by a bed of white volcanic tuff²⁸ about 40 feet thick and that both the lava and the tuff slope to the west, or away from the railroad, as shown in figure 18. This westward slope shows that at the time the tuff was deposited and the lava was poured out upon its upper surface, the deepest part of the valley lay considerably west of the channel in which the river flows to-day.

FIGURE 18.—Lava-capped hill south of Howard. The hill, which is opposite milepost 200, is composed of red sandstone dipping steeply to the northwest and is capped by a nearly horizontal sheet of tuff and lava.

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²⁸Volcanic tuff is a name applied to material blown out of a volcano by an explosion of gas or steam. It is generally composed of fine particles of glass but may include fragments of rock of different sizes. The bed of tuff here may have been formed of dust and ashes that settled down on the ground from the atmosphere or were washed into a basin or valley.

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The red sandstone crops out by the side of the railroad as far as milepost 200. Here it is covered by a large mass of tuff and lava which descends below river level and which shows on the northeast side of the valley in places to points beyond Howard. Most of the high hills near Howard are capped with white volcanic tuff and with a sheet of lava, which invariably slopes to the west. These rocks have been traced eastward to a point near the fault at the edge of the granite. As the lava rises steadily toward the east the volcanic vent from which it came was probably near the fault and on high ground, thence it flowed westward down the slope to the river, which was then farther west and somewhat lower than it is to-day.

The volcanic matter doubtless partly filled the old valley of the Arkansas, and then came a great wash of gravel and boulders from the mountains, which must have filled the valley to a depth of several hundred feet. No one yet knows what caused this great deposit of gravel, but it has been assumed to have some connection with the formation of great glaciers in the neighboring mountains. This influx of foreign material dammed the river and forced it over to the east side of its valley, entirely out of its former position. At present the river is cutting away the gravel and lava, but it has not yet cut down to its former level. Remnants of the gravel filling may be seen in the extensive terraces opposite Howard, as shown in Plate XLIII.

At Howard a branch railroad turns to the left, crosses the river, and disappears in the hills in the distance. This line runs to a stone quarry at the station of Calcite, where limestone is being quarried on a large scale by the Colorado Fuel & Iron Co. for use as flux in its large blast furnaces at Pueblo. Howard is a small village, but the well-cultivated farms across the river indicate a prosperous community. The land on both sides of the river is irrigated and yields abundant crops of alfalfa and the more hardy grains. Near the station there are kilns in which charcoal was formerly made. (See Pl. XIV, B, p. 30.) These kilns are the only traces that remain of what was once a large industry in these mountains. The native timber was used in making charcoal, which was in great demand by smelters in almost every mining town. The concentration of the smelting industry into the hands of large corporations and the consequent abandonment of most of the small plants, together with the increased production of coke in the coal fields near by, killed the charcoal industry. Although the decay of this industry temporarily deprived many persons of the means of making a livelihood, it was a blessing to the region as a whole, for the manufacture of charcoal is a wasteful process and one that has consumed much valuable timber that might have been reserved for a more useful purpose.

A little beyond Howard the railroad turns more toward the west and crosses the bedded rocks, which show to good advantage. In this section (see fig. 19) the syncline has been so squeezed by pressure from the east that its sides have been pressed close together or overturned, and consequently all the rocks dip toward the east. The lava is prominent in this part of the valley, but it is limited to the hills on the opposite side of the river. These hills at first appear to be composed entirely of lava, but close scrutiny will show that the red sandstone crops out here and there near the river level. This low place in the sandstone evidently marked the middle of the valley at the time the lava was poured out and filled the valley to a depth of 300 or 400 feet. West of milepost 205 the railroad crosses Badger Creek, which drains a large territory between the Arkansas Valley and South Park. The red sandstones are well exposed in the bend of the river a little farther on and in the approaches to the tunnel beyond milepost 206. They are fairly conspicuous in the river bluffs near milepost 207, but west of this point the red color disappears from both sides of the valley. The last lava-capped hill is nearly opposite milepost 207, and this hill marks the western limit of the old valley, which is now so deeply filled with the volcanic material that it constitutes hills rather than a valley.

FIGURE 19.—Section of the Sangre de Cristo Range and the valley on its east side, through Hunts Peak and Howard.

At Swissvale the railroad is built upon a broad gravel-covered flat. The absence of exposures of hard rock is due largely to this fact and to the fact that the flat, or rather terrace, is composed of the Weber shale and sandstone, which underlie the red sandstone that is so conspicuous farther east. This relation is due largely to the effect of a cross anticline, which trends in the direction followed by the railroad. This anticline brings the Leadville limestone near the surface, but it can not be seen from the railroad until the train passes Wellsville Springs. Its position is marked on the river bank, however, by numerous springs, which carry so much lime in solution that as soon as they emerge from the bank they deposit the lime in the form of calcareous tufa, building up domes of this material around the springs. A rather large spring of this kind is being utilized at Wellsville as a bathing pool, making it a general pleasure resort for the surrounding towns.

Long ago, when the river was flowing at a much higher level than it is now, large springs issued along its banks much as the springs issue along its banks to-day, and they built up immense masses of tufa, which now stand several hundred feet above the railroad. This tufa consists of nearly pure carbonate of lime, and it is now being quarried in a large way for use in refining beet sugar and as flux in iron furnaces.

West of Wellsville Springs the sides of the valley become steeper and the railroad is crowded to the bank of the river under a high cliff of Leadville limestone, which is the lowest formation of the Carboniferous system. The beds of rock in this cliff have been greatly distorted by folding and in places stand nearly vertical, but the bedding has been largely obliterated by the solution and redeposition of the lime, so that the structure can not be determined from the train. After passing the great bend of the river to milepost 210, the synclinal structure may be plainly seen in the bluff on the far side of the river.

The limestone is conspicuous on both sides of the valley almost to milepost 211, where it rises and disappears in the tops of the hills. It is underlain by thin-bedded quartzite, the age of which is not definitely known, though it is considerably older than the other sedimentary rocks which the traveler has recently seen. The quartzite is so much changed by movement and pressure in the crust of the earth that at first sight it may not be recognized as a sedimentary rock. It is cut off in a short distance by a great mass of intrusive rock, which occupies a large area on the northeast side of the river valley and extends up the river as far as the stockyards 2 miles below Salida. Beyond this place the intrusive rocks are restricted to the northeast side of the river, or if they occur on the other side they have been dropped so low by faulting that they are effectually concealed by the gravel in the bottom of the valley. The Arkansas Valley above Salida has doubtless in many places been affected by faulting, so that large tracts have been dropped hundreds and possibly thousands of feet and the depressions so produced filled with sand, gravel, and boulders brought down from the great Sawatch Range on the west. About Salida in particular the evidence of such a dropped block seems to be conclusive, for the river a few miles below the town is flowing on bedrock and it would still be running on or near bedrock at Salida had the bedrock not been depressed below its original level.

The largest town in the mountains west of Canon City is Salida (from the Spanish word outlet; locally pronounced sah-lie'da), which was so named because it stands at the outlet of the upper Arkansas basin. It was settled in 1880 at the time the railroad was being built up the Arkansas Valley, and it is at the junction of the narrow-gage road over Marshall Pass to Grand Junction and that over Poncha Pass to San Luis Park with the main line of the Denver & Rio Grande Western Railroad. Here are the repair shops of the railroad and some other manufacturing plants, and a mile northwest of the town there is a large smelter. It is a town of homes, but in addition there are several hotels for the accommodation of travelers who change from one route to another in order to see the beautiful scenery for which this region is noted. The town lies in a basin that is nearly surrounded by mountains. (See Pl. XLIV.) The Sangre de Cristo Range, which begins near Santa Fe, N. Mex., terminates just south of the town in a prominent point known as Hunts Peak (12,446 feet). The Sawatch Range begins in Mount Ouray (13,955 feet), a little west of the north end of the Sangre de Cristo Range, and stretches northward, including Mount Chipeta, Mount Shavano (14,179 feet), and other high peaks, shown in Plate XLIV. To the north and northeast there is a jumble of lesser ranges without special names.

PLATE XLIV. SALIDA AND THE VALLEY OF THE ARKANSAS. From the hill across the river the town of Salida and the neighboring parts of the valley are spread out as on a map. Hunts Peak, marking the north end of the Sangre de Cristo Mountains is on the left, and Mount Ouray, the south end of the Collegiate Range, is near the middle of the picture. Mount Chipeta shows to the right of Ouray, and Mount Shavano at the extreme right of the picture. The narrow-gage line over Marshall Pass crosses the bridge in the foreground and ascends the canyon in front of Mount Ouray. Photograph by Henry R. Hay, Salida. (click on image for an enlargement in a new window)

As the branch railroad lines that enter Salida are narrow-gage all the freight originating on them and bound for the East must be reloaded into standard-gage cars. This reloading entails considerable expense and loss of time and is a great handicap to the shippers on the narrow-gage lines. Narrow-gage cars can run, however, between Salida and Leadville, because here a third rail has been maintained for the benefit of the mining interests in shipping ores to the smelter.

A description of the route over Marshall Pass and through the Black Canyon begins on page 158.