BULLETIN OF THE GEOLOGICAL SOCIETY ÔF AMERICA V ol. 3, pp. 445-452 A u g u s t 8, 1892

THE GEOLOGY OP THE CRAZY MOUNTAINS, MONTANA.

BY J. E. WOLFF.

(Read before the Society December 29, 1891.')

CONTENTS. Page. Topography...... 445 General Structure...... 446 Eruptive Rocks of the northern Area...... 449 Structural Aspects...... 449 Lithologie Characters...... 450 Features of the Southern Area...... 450

T o po g r a ph y .

The Crazy mountains are situated in central Montana, centering about latitude 46°, longitude 110° 15'. They form a high isolated range of the Rocky mountains, lying about 30 miles east of the easterly border of the main mass of the mountains, and rise abruptly from the eastern table­ lands, attaining an extreme elevation of about 11,000 feet above sea-level. The Yellowstone river flows around their southern end a few miles after its exit from the mountains at the lower canyon, and the range is there­ fore in plain view from the Northern Pacific railroad for many miles eastward from the town of Livingston. The mountains trend a little west of north and are about 40 miles long and 15 or 20 wide. A large branch of the Yellowstone, called Shields river,, which flows southward along the western base, has cut a deep, flat transverse valley at its head nearly through to the eastward drainage, and there divides the range into northern and southern halves. Of the two portions thus defined the southern reaches the greater elevation. It has numerous sharp peaks, often of a jagged aiguille type, and the arrange­ ment of the drainage is distinctly radial, since the streams flow westward, southward and eastward from the central mass of high peaks. In moving up one of these streams toward the head we find the valley at first com­ paratively broad, bounded by high bluffs of nearly horizontal sandstones,

LXI— B ult,. G e o i:. Soc. Am., V o l . 3, 1891. (445)

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which become cliffs as the spurs rise toward the peaks; but on approach­ ing the central peaks the valley suddenly narrows for a mile or more and the stream falls from a higher level 400 or 500 feet by cascades and falls, and beyond this the valley again widens somewhat with a more gentle slope to the head. This “ fall-line ” is found on all the radial streams, and is plainly due to the local hardening of the Cretaceous rocks pro­ duced by the central stock of diorite, as described later. The larger valleys have been occupied by glaciers, as shown by rock scoring; and the markings are found 500 or even 1,000 feet above the present stream levels. No lateral moraines were observed, but at the heads of the streams there is considerable morainic material, and also below the exits of the streams from the range, which here contains large bowlders of the characteristic eruptive rocks occurring higher up. The glaciation seems to have been entirely local. The broad benches, stretching out for miles eastward, westward and southward, are covered with water-worn pebbles from the range and may lie high above the present stream beds, which have cut deep through them into the under­ lying sandstones. The change in elevation from these benches to the spurs from the peaks is sudden, the difference of level between the base and the summit of the range averaging perhaps 4,000 feet. That part of the mountains north of the head of Shields river is lower and the summits have the form of ridges or flat-topped dome-shaped masses. Both here and in the southern half, outlying peaked summits or buttes form a topographical feature.

G en e r a l S tr u c t u r e . The general geology is comparatively simple.* The range lies in a region of nearly horizontal Cretaceous rocks, extending indefinitely east­ ward in the great plains and westward to the edge of the frontal range, where sharp uplifts expose the older rocks. These Cretaceous rocks are found throughout the range and either horizontal or, if disturbed, with generally low dips. They consist of yellow or brown sandstones and occasional conglomerates, interstratified with yellow, drab, red, or black shales and impure calcareous beds. The conglomerates in one place contain large pebbles of an older (Carboniferous ?) limestone ; the shales, plant remains and small beds of impure coal. No attempt is here made to assign them to a definite horizon of the Cretaceous, but the base at least seems to belong with the Laramie, which a few miles westward has over 8,000 feet of strata.f

*J. E. Wolff: “ Notes on the Petrography of the Crazy Mountains” ; Neue* Jahrb. Min., etc, 1885, i, p. 09, and 1890, i, p. 192. t W. H. Weed: Bull. Geol. Soc. Am., vol. 2,1890, p. 3C0.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/3/1/445/3414100/BUL3-0445.pdf by guest on 01 October 2021 DOMED CRETACEOUS STRATA. 4 4 7 In the southern half the strata have a general inward dip at the outer edge of the range, both in the spurs and adjacent benches; so that gentle easterly dips are found on the western side, northerly dips on the south, and westerly dips on the east. In the interior this basin structure is in­ terrupted by dome-shaped ;uplifts, of which the most marked is that con­ nected with the central dioritic stock, from which the stratified rocks dip away with gently decreasing dips. This dome structure is sometimes repeated on a smaller scale in the outlying buttes. An example exists on the northeastern border between Little Elk and Big Elk creeks; * the shales and sandstones dip about 30° in three directions from the center of the dome, which has been eroded 300 feet lower than the sides, thus forming a Roughly circular basin a mile or two wide surrounded by lines of cliffs. One small intrusive sheet can be seen in the upper strata, which rapidly thins out. Still farther outward from the center of the dome the strata have steep dips and contain numerous intrusive sheets or bedded dikes. The eroded center seems to be due to the lack of protecting erup­ tive sheets at.that point, making it easy for the erosive agents to cut deep into the soft shales and sandstones. In the northern half of the mountains the dome structure is developed with less regularity and a tendency to longitudinal uplifts with steeper dips and sharp crumples, producing long-crested ridges. An interesting case is found on the, northern side of the deep transverse valley at the head of Shields river, consisting in the southern end of a long anticlinal dome, the strata dipping southward, eastward and westward within the space of a mile. Th.ey are here interleaved with numerous sheets of in­ trusive rocks, which curve around the sides of the dome with them and even preserve this parallelism in sharp minor crinkles a few hundred feet wide, by which the lines of outcrop make elbows. The present crest of the dome is formed by a master sheet or laccolite sixty feet thick, which dips off from three sides; but erosion has cut . through it on the axis of the dome to the underlying soft shales, exposing to view a trans­ verse dike of the same rock, apparently a feeder of the laccolite. The close conformity in greater and lesser crumplings between the intrusive and sedimentary rocks makes it necessary to suppose that the elevation took place after the intrusion of the former, for it does not seem possible that an intrusive rock could force its way into all the details of a sharply crumpled surface. This being the case, the eruptive rocks were exam­ ined with considerable interest at one. of the 'sharp twists for signs of crushing, and with the expectation of some trace of the dynamic meta­ morphism so common in folded intrusive sheets of the Archean and

* The topographic map. is not reproduced here.

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Paleozoic ; but neither in the field nor in the laboratory was any structure detected different from that of the ,rock under normal conditions. The monoclinal buttes developed along either side of the range are very striking, especially on the western side. They owe their present elevation to the sheets of intrusive rock, which dip inward with the strata toward the range at varying dip angles in thé different localities. The most im­ portant of these is the group of three high outlying buttes north of the head of Shields river (“ Three Peaks ” on the map), which are arranged en echelon on a north-south line, the crest lines of the two northerly ones lying progressively east of the third or southerly one. The strata dip about 30° eastward, and the three buttes have high cliffs facing westward and gentle dip slopes eastward. The crests are formed by heavy sheets or laccolites of intrusive rock from 250 to 100 feet thick, with minor sheets at intervals below, interstratified with the shales and sandstones. These master sheets bulge in the crest of the ridge, maintaining their thickness for about a mile in the case of the northern and southern buttes, and then rapidly thin out to a comparatively narrow bedded dike. Ac­ companying this diminution in size the crest of the ridge drops, and the next ridge, formed by another bulging sheet at a different horizon, begins, culminates, and dies out in the same way. This peculiar topography seems therefore due to the intrusion of bulging sheets (laccolites) at different levels in the horizontal strata, the major sheets not having their centers in the same vertical ïine; the whole complex was then tilted and eroded, the soft shales and thinner sheets bein£ quickly taken off, leaving the master sheets standing. In the Henry mountains Mr. Gilbert has described the first conditions of intrusion without subse­ quent tilting. Another peculiar type is found ip the outlying butte near Martinsdale, on the northwestern edge of the range. This has an oval form, is about two miles long, and has an elevation of 600 or 700 feet above the plain at its base. It is fringed by a line of high cliffs facing outward, through which the interior drainage has cut an outlet. The top forms a basin with gently sloping sides. The Cretaceous strata are found around the base dipping gently inward, while the slopes and crest are formed by a great capping eruptive sheet and at least one lower sheet, with thin inter­ vening beds of shale. The thickness of the great sheet was estimated at 365 feet and of the lower 150.. In the center of the basin, on the summit, erosion has cut nearly through the main sheet, leaving tall pinnacles of the eruptive rock standing in groups (sometimes 50 feet high), which are due to the perfect vertical prismatic structure of the sheet. Loose pieces of metamorphosed shale found on the surface at the highest point seem to be remnants of the original covering of the laccolite. The erup­

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tive sheets and basement shales have the form of a gently folded synclinal basin which erosion has spared.

E r u p t iv e R ocks of t h e n o r t h e r n A r e a . Structural Aspects.—The eruptive rocks are of great interest and novelty. They may be classified for purposes of description as dikes, sheets and laccolites, without any essential genetic difference. The writer has found no evidence of surface flows ; all rocks appear intrusive and younger than the enclosing strata. The dikes are innumerable and occur in every part of the range, vary­ ing in width and position. In the canyons cliffs of horizontal strata may be seen a thousand feet and upward in height, which are intersected by mazes of vertical and oblique dikes. Toward the interior of the range these dikes increase in number. As an example, a dike was counted every fifty feet horizontal on a long spur. The sheets are closely connected with the dikes, which sometimes spread out between the strata as sheets, or a sheet may cut obliquely across the strata as a dike to another level. The sheets may be a hun­ dred feet thick and a mile in extent. It is noticeable that sheets occur on the eastern and western edges of the range where dikes are rarer, and it seems to have been easier for the intrusion to force its way laterally. The facts indicating that many of the sheets have been folded with the strata after intrusion have been alluded to., The laccolites differ from the sheets only in their greater thickness and bulging character. The greatest observed thickness of any one laccolite, free from shale bands, was about 350 feet, which would be increased to 500 if a thin shale parting were omitted. They have a well developed prismatic structure at right angles to the cooling surfaces, and hence the upright columns lean to correspond with the amount of dip. The tilted laccolites are, of course, best exposed, presenting cliffs on one side. The intrusion generally follows the bedding for some distance, but is liable to cut obliquely across, and 'without reference to joint planes. In one natural section a long splinter of shale ,200 or 300 feet long and 30 feet thick is seen to have been bent off by thé splitting of the eruptive mass, but is still continuous with the shales at one end. It is rare to see feeding dikes below the laccolites. They are some­ times cut, in common with the shales, by later vertical- dikes of the same or different rock which follow joints in the shales. A limited contact metamorphism is produced by the laccolites and thicker sheets at both contacts, by which the shales are indurated and often changed to a green color by caustic action. The changes in texture and even mineral com­ position produced by different conditions of cooling in the center and at

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the contacts of the laccolites are striking. The rock has a coarse, often granitic, texture in the middle, hut becomes dense and porphyritic within a few feet of the contact. The thinner sheets and dike3 have throughout their mass the character of the contact varieties of the corresponding larger masses. Lithologic Characters.—Brief mention should be made here of the varie­ ties of eruptive rock thus occurring. The most prominent is a dark basic rock found in all three forms (the laccolites reaching over 350 feet in a single sheet), and having a coarse granitic texture in all but the dikes and thinner sheets. This rock, originally found here by the writer in 1883,* was found to be composed of feldspar (in part triclinic), augite and nepheline, with biotite, sodalite, magnetite, olivine, iegirine, etc, acces­ sory ; as an abyssal intrusive rock with the mineral combination nephe­ line, soda-lime feldspar, it filled a gap in the classification of Professor Rosenbusch, and was called by him “ theralite,” as the first undoubted representative of this family. Associated with the theralite in parallel sheets or dikes are lighter colored alkaline rocks with a much higher content of silica, which in the thinner sheets correspond exactly in mineral composition and structure to the effusive rock called acmite-trachyte (often phonolitic) and in the heavy sheets resemble some eleolite-syenites (e. #., those from Arkansas). Other sheets and dikes composed essentially of triclinic feldspar, augite, hornblende, or biotite appear to belong to various groups (diorite-porphy- rite, camptonite, etc). The completed petrographical study of all these varieties is expected to bring out interesting relations between composi­ tion, structure and geological occurrence.

F ea tu r es o f t h e S o u th er n A r e a .

The geology of the central mass of peaks in the southern half remains to be described. The radial drainage and “ fall-line ” features of the topography are due to the presence of a central mass or stock of coarsely crystalline diOrite and granite, which has hardened and metamorphosed the Cretaceous strata for the distance of nearly a mile outward. The streams which head within the area of crystalline rock have to cut through this contact zone or ring of hard rock, beyond which they have cut deeper into the normal soft strata and widened their valleys. The diorite stock is irregularly oval in outline and is about 6 miles wide at the greatest diameter. The rock is composed of triclinic feldspar and hornblende, biotite, augite, hypersthene, often quartz and orthoclase, with the usual accessory minerals, but the composition varies somewhat.

* Neues Jahrb., op. eit.

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The rock has a hypidiomorphous granular structure, is sometimes as coarse as a coarse gabbro, but near the periphery becomes fine grained and porphyritic and often has a marked parallel structure due to motion in the magma. A very basic coarse variety found near the center is noticeable. The diorite is cut toward the center by masses of a light- colored finer-grained granitite, which envelops fragments of the diorite. It is surprising to see the similarity between this Tertiary diorite and granite and the Paleozoic masses of similar rock found exposed on the old eroded surfaces of the Atlantic states, as, for instance, on the northern shore of Boston. In both cases the same black patches are seen in the granite, referable here to enclosed dioritic fragments, and the same alter­ nations of basic and acid rock in streaks or “ Schlieren ” with parallel flow structure. The diorite is found in place in the streams as low as the 8,000 feet contour and can be traced 2,000 feet higher, remaining quite coarse. The Cretaceous shales dip gently off from the dioritic mass as a dome, but at the actual contact were found sometimes turned up on edge and interlaminated with the diorite. They have been profoundly altered by the intrusive rock, preserving in general the stratification of the thicker bands but losing all shaly structure. The result is a dense flinty banded rock, creamy white, green, or black in color, resembling the contact rock called “ adinole,” or coarser, filled with biotite, and more like “ hornfels,” a product of Paleozoic granite contacts. None of the zones of mineralogi- cal change so characteristic of the latter were observed. This effect extends out about 5,000 feet on all sides, gradually dying out, as certain layers only are affected. The diorite stock as well as the adjacent Cretaceous rocks are cut by later vertical dikes of diorite-porphyrite and allied rocks; these dikes swarm in the contact zone, accompanied by horizontal and oblique sheets of similar rock. Mr. J. P. Iddings, who visited this place in 1891, finds that the vertical dikes, both in the stock and in all this part of the range, have a general radial arrangement, with the diorite mass as an approxi­ mate center, repeating a fact observed by him in a smaller diorite stock in the Yellowstone mountains. These long radial dikes extend outward even into the benches at the southern base of the range. This imperfect description can give but a faint idea of the beauty of this great massiv and its contact ring. Its intrusion into nearly horizontal late Cretaceous strata and the enormous subsequent erosion which re­ moved the overlying rocks enable us to see it now in nearly its original condition with deep sections into its center, whereas in the older masses of granitic rock which we usually study the long-continued erosion and

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movements of the crust have removed or altered many of the original features.* The existence of this high range as an outlier is due to the facts that it was the center of violent eruptive activity in post-Cretaceous time, and that the presence of great masses of crystalline rock, combined with the honeycombing of the soft strata by dikes, enabled the whole mass to resist the erosion which levelled the adjoining country. The moderate uplifting of some of the larger sheets with their enclosing rocks also con­ tributed to this result. Warren Upham calls it a striking example of the “ eroded mountain range.” f It is hoped this sketch may present the claim of these mountains as a grand geological model and one, for'the Rocky mountains, easily acces­ sible. From Livingston or adjoining stations on the Northern Pacific railroad it is an easy day’s drive to the foot of the range; the canyons of the larger streams on the east side are easily accessible by horseback a'nd at the entrance even by wagon, and it is possible to ride to the falls in the contact zone. The outlying theralite buttes can all be visited by wagon. *A smaller but apparently similar stock was observed in the northern half of the range,.but not studied in detail. t A Classification of Mountain Ranges, etc. Appalachia, vol. vi, no. 3,1891, p. 204.

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