Geological Survey Bulletin 611 Guidebook of the Western United States: Part A

SHEET No. 14. (click on image for an enlargement in a new window)

FIGURE 11.—Eagle sandstone north of Park City, Mont.

The traveler has now passed the axis of the great Pryor Mountain anticline, which brings to light the Colorado shale south of Billings, and the rocks dip gently and regularly toward the west. Under the influence of this westward dip the shale (Colorado) visible in the bluffs on the south side of the river soon passes below water level, and the cliffs of sandstone (Eagle) on the north begin to approach the railway. Near milepost 20 the cliff is about 2 miles distant and consists of three beds of sandstone with intervening shale or soft sandstone, as shown in figure 11. As the dip is low, only about 10°, the Eagle sandstone approaches the river slowly, but at milepost 25 it can be seen in the hills on the south side of the river. The top of the sandstone passes below water level at a siding called Youngs Point, beyond Park City (see sheet 14, p. 86), and here about 300 feet of shale and another sandstone immediately overlying the Eagle are visible across the river. These beds make up the lower part of the Claggett formation, which dips gently westward and gradually disappears beneath water level. At milepost 32 all the white sandstone has passed from view and the hill slopes are comprised of the overlying Judith River formation. This formation has no decided characteristics by which it may be recognized and identified, but it contains fewer beds of sandstone, and consequently makes smoother hill slopes than the underlying Claggett formation. The slopes composed of the Judith River formation have a whitish-gray tint and are rather monotonous in color and appearance. The rocks composing the upper part of this formation are well exposed in Countrymans Bluff, between mileposts 37 and 38. Here the rocks are undoubtedly of fresh-water origin, as they contain numerous fragments of fossil plants such as could have been deposited only either on land or in bodies of fresh water.

The continued westward dip of the rocks brings the next higher formation (Bearpaw shale) to water level in the vicinity of Columbus. It can not be seen near the railway, but is well exposed a mile north of the town. This shale crops out in the valley of Keyser Creek north of the railway and along the foot of the ridge that begins just across the river from Columbus and extends southeastward as far as the eye can see. This ridge is composed of beds of light-colored sandstone (Lance formation) which are almost identical with those seen in the vicinity of Hysham. In one of these beds a large quarry has been opened a mile north of Columbus, which furnishes a building stone of great excellence. This stone has been used in the construction of buildings in the neighboring towns and in the State capitol at Helena.

FIGURE 12.—Cross section to illustrate the change in the formations between Terry and Livingston, Mont.

These sandstones cap Bensons Bluff, 2-1/2 miles west of Columbus, and come down to water level near milepost 43. The dips here are 6° to 8° to the west, but they flatten within a short distance and the rocks are practically horizontal.

The traveler may remember that at Terry he saw a wedge of somber-colored shale and sandstone immediately overlying the Lance and that the dark color of this wedge is due to the presence of volcanic material, which was washed far to the east from its place of origin somewhere near the Yellowstone Park. The relation of this wedge of volcanic material to the adjacent formations is shown in figure 12. The train is now approaching the place of origin of this material. The gray sandstone of the Lance forms most of the slopes at milepost 50, just beyond Merrill, and about 300 feet above the river the hills have a brownish appearance which indicates that some other formation makes their upper slopes. After crossing the river the same relations may be observed, except that as the train moves westward the brown Lebo shale can be seen at lower and lower levels, owing to the slight westward dip of the rocks.

At Reed Point the white beds of the Lance extend up the slopes only 100 to 150 feet, and above that all the rocks are brown. The Lance probably goes under river level near milepost 60, and beyond that point the hillsides are much smoother and the general tone of the rocks is brown, indicating that the Lebo shale forms the hills at least for a height of 800 or 1,000 feet.

About three-fourths of a mile beyond milepost 63 a large dike, visible on the right (north), cuts directly through the bedded rocks in a direction nearly parallel with the railway. The dike is composed of a dark igneous rock which was injected in a melted condition into an extensive crack in the bedded rocks. It stands up like a wall, and where it cuts across a bed of light-colored sandstone it is easily recognized.

As the train rounds the curve at milepost 66, the traveler looking forward and to the right can get his first good view of the Crazy Mountains (originally called Crazy Woman Mountains). This mountain group stands by itself in the plains and contains the first high peaks which the traveler can see at close range. Its mode of origin is described in the footnote beginning on page 86.

East of this place the volcanic material of the Lebo shale has been so thoroughly washed and sorted by water that it is evenly bedded like ordinary shale and friable sandstone, but near the mountains and the source of supply this material is coarser and some of it has the appearance of being only a little modified by water after it was blown out of some old volcanic vent in the vicinity. Such material, known as volcanic agglomerate, is composed of fragments of lava ranging from minute pieces to blocks 4 feet in diameter. The agglomerate beds have in general a warm gray tint, and a mass of such material gave the name of Greycliff to a siding that was formerly located under the cliff but now has been moved 3 miles to the west. The cliff is fully 100 feet high, but the base of the agglomerate is not exposed and hence its full thickness may greatly exceed that amount. As it is reported to be 2,000 feet thick a few miles to the southwest, it seems reasonably certain that the old volcano which furnished the material was located in that direction, but no trace of it has been discovered.

At Greycliff an upland stretching far to the north is visible across the river on the right. This is underlain by light-colored sandstones of the Fort Union formation, which show here and there, giving to the surface a light-gray appearance. As these rocks dip slightly westward, they should appear near railway level east of Big Timber, but no such rocks occur near the track. This is due to the fact that on approaching Yellowstone Park more and more of the volcanic material is present in the sandstones, giving to them a dark color that makes them indistinguishable from the underlying Lebo.¹

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¹The change in character of the materials composing the Fort Union may not be apparent from the train, but north of the Crazy Mountains, on Musselshell River, all the formations from the Colorado shale (the shale that is exposed across the river from Billings) to the Fort Union change toward the west to an andesitic mass in which formations are not distinguishable. This mass is generally known as the Livingston formation, and when it was named it was supposed to be younger than any formation so far described and to rest unconformably on all the older formations up to and including the Fort Union. This idea was based on its supposed relation to the other formations about Livingston and to the fact that the fossil plants which it carries are different from those in the Fort Union and also in the underlying formations. The anomalous character of the fossil flora has not yet been explained, but the apparent merging of the formations into one great mass of andesitic material toward the west is so apparent that it is now generally regarded as established that the Livingston is not a separate formation, but a peculiar near-shore phase of the other formations, produced by a great supply of volcanic material from an up land on the south.

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After passing Big Timber the traveler obtains on the right (north) his best view of the Crazy Mountains,² an isolated group of sawtoothed peaks which rise sharply to a height of 6,000 feet above the generally even surface of the plain and 7,000 feet above the level of the railway.

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²The Crazy Mountains can not in any sense be considered as a range, for in form they are merely a group of peaks and in structure they are unlike any other range in this part of the country. The highest point, Crazy Peak, has an altitude of 11,178 feet, or about 6,000 feet above the general level of the plateau or bench land at its foot. The Crazy Mountains are therefore higher than many of the more noted mountains of Montana, and they are certainly more conspicuous on account of their compactness and isolation.

Structurally they have no relation to the ranges of the Rocky Mountains, for those ranges in general are formed of upturned or faulted strata, whereas the Crazy Mountains are merely the remains of a great irregular mass, called a stock, of igneous rock that was forced in molten condition into the soft shale and sandstone of the upper Mesozoic and lower Tertiary formation. At the time this great mass of igneous material pushed into the soft sedimentary beds, the surface must have been above the present top of the mountains. J. P. Iddings, who has given the most study to this mountain mass, says that it is not at all certain that the molten material ever poured over or even reached the surface. It is exposed to view now because the beds that once covered it have gradually been washed away by rain and streams.

The stock is 4 miles wide and 6 miles long. It consists of a very coarse grained diorite which disintegrates rapidly when exposed to the weather. In this condition it is easily eroded, and the slopes are very steep, as can be seen from the train. The present mountains are made up not only of the igneous rock, but also of the shale and sandstone into which it was forced. These rocks were heated so intensely that the shale has been baked into a porcelain-like mass that is very hard and resists weathering much more successfully than the diorite. These baked rocks form a zone nearly a mile wide around the core. Through this zone and beyond it the rocks have been cut and hardened by a countless number of dikes that radiate from the central mass in all directions. Here and there the molten matter has found an outlet between the beds of sandstone, resulting in great sheets or sills of the hardened lava. These are very dense and serve as protecting caps to the softer strata beneath. The forcing of so much material between the layers of the sedimentary rocks has raised them up around the stock until they dip from it in all directions.

Nearly the last stage in the evolution of this group of mountains is the sculpturing they have received from local glaciers during the Great Ice Age. These were so small that they did not even coalesce and form an ice cap, but each little glacier scoured out the valley in which it lay and built a moraine at its outer end, where it came down nearly to the level of the bench land.

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