High Wood Mountains of Montana*

BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA V o l . 6, PP. 389-422, PL8. 24-26 APRIL 17, 1895 HIGH WOOD MOUNTAINS OF MONTANA* BY WALTER H. WEED AND LOUIS V. PIKSSON {Presented, before the Society December 27, 1894) CONTENTS Page Part I.—Geology of the High wood mountains....................................................... 390 Situation.................................................................................................................. 390 Geography and topographic characteristics.................................................... 391 General geologic structure................................................................................... 392 The sedimentary platform................................................................................... 393 Igneous rocks......................................................................................................... 395 General characters ......................................................................................... 395 South and Highwood cores......................................................................... 396 East core......................................................................................................... 396 Shonkin core.................................................................................................. 397 Arnoux core.................................................................................................... 398 Palisade butte................................................................................................ 398 The Shonkin Sag laccolite............. ............................................................. 399 Geologic history.................................................................................................... 499 Part II.—Square butte and its remarkable differentiation zone...........................400 Introductory........................................................................................................... 400 Historical............................................................................. «................................400 General description of Square butte..................................................................400 Its laccolitic origin................................................................................................ 401 Lower zone of dark hoodoos............................................................................... 402 Upper zone of white rock................................................................................... 404 Origin of the platy parting................................................................................. 405 The white band...................................................................................................... 405 Diagrammatic section of Square butte.............................................................. 407 Summary of field results..................................................................................... 407 Petrography of Square butte............................................................................... 407 Characteristics and minerals of the dark rock.......................................... 407 Megascopic and microscopic.................................................................. 407 Apatite..................................................................................................... 408 Olivine................................................................................................... 409 Biotite...................................................................................................... 409 Pyroxene..................................................................................................410 Orthoclase................................................................................................ 411 * Published by permission of the Director of the United States Geological Survey. L V I—Bum,. Gkol. 8 0 0 . Am., V ol. 6, 1894. (389) 390 WEED AND PIRSSON— HIGHWOOD MOUNTAINS OP MONTANA. Page Plagioclase.......................... 412 Nepheline............................ 412 Cancrinite............................ 413 Sodalite................................ 413 Natrolite.............................. 413 Chemical composition........ 413 Structure ând classification 414 Name shonkinite................. 415 White rock or sodalite-syenite 416 General petrology of Square butte. 417 Summary........................................... 422 P a r t I.—G eo lo g y o f t h e H ig h w o o d M o u n t a in s. SITU A TION. The group of mountains of which a brief account is given in this paper form one of the detached mountain groups lying east of the Rocky Moun tain Cordilleras and rising abruptly from the level plains of central Mon tana. A glance at the map of the state shows the Missouri river, born of the Jefferson, Madison and Gallatin, in the mountain-encircled Gallatin valley, cutting its way in a general northward course through the outer ranges of the Rocky mountains until it debouches on the great open plains of the northwest. From here it turns in a rude arc, flowing east ward over the cataracts which have given rise and name to the city of Great Falls, and past Fort Benton, the head of steamboat navigation. In this arc, between the Missouri and the Little Belts, a front range of the Rocky mountains, the Highwood mountains are enclosed, and they are about 30 miles east of Great Falls and 25 miles south of Fort Benton. From each of these cities the peaks of the range form the most conspic uous feature of the landscape and break the monotonous level of the plains. The meridian of 110° 30' west longitude and parallel of 44° 30' north latitude pass through their center. It is not proposed in this brief paper to present more than the salient features of the mountains as revealed by the field-work of the past summer, when the region was studied while mapping the geology of the Fort Ben ton sheet of the geologic atlas of the United States Geological Survey. Previous knowledge of the geology of this mountain group is confined to a few notes published by W. M. Davis and W. Lindgren, who were at that time members of the Northern Transcontinental Survey.* These observers recognized the fact that the mountains were largely formed of Cretaceous strata penetrated by intrusions of igneous rocks. Later papers »Tenth Census of the United States, vol. xv, p. 734. LOCATION, EXTENT AND TOPOGRAPHY. 391 by Lindgren * give the results of very careful and detailed petrographic studies of several rock specimens from this field. These studies show that from a petrologic standpoint the rocks of this district are of great and unusual interest. GEOGRAPHY AND TOPOGRAPHIC CHARACTERISTICS. The Highwood mountains form a distinct unit in the geography of Montana. The front of the Rocky mountains, formed of the Little Belt range, lie to the south of the group, with a broad, flat and open valley between, while the Judith and Big Snowy ranges fill the horizon to the northeast. Standing alone and rising abruptly from the flat Cretaceous plains, the mountains possess an imposing appearance hardly warranted by their height. The 300 square miles of highland forming the group is some 25 miles long from east to west, with a breadth, or north-and-south direction, of from 15 to 16 miles. The loftiest peaks, Highwood and Ar row, reach a height of 7,600 feet above tide, or 4,000 feet above the plains which sweep away in monotonous continuity from the foot-slopes. From most points of view the mountains are seen to consist of a gently contoured, rounded group of comparatively low peaks forming the north ern half of the group, rising gradually and culminating in the higher rugged crests which constitute the southern part of the cluster. A strik ing feature is a deep cut, drained by Arrow and Highwood creeks, which separates the mountains into east and west portions and forms a pass known as Highwood gap. While the main mass consists of an irregular collection of peaks connected by high ridges which make a topographic whole, there are also two buttes lying east of the mountains which are a part of the group. The easternmost is a flat-topped mountain mass, rising to an elevation of nearly 6,000 feet and forming the most salient feature of the landscape wherever visible. This is Square butte, an eminence as important geologically as it is prominent topographically. Between this and the main ridges of the mountains rises the dark pillared form of Palisade butte, the other of the two. Surrounding the range on every side is the open plains country, cut into picturesque badlands along the Arrow river, but elsewhere preserv ing that persistent horizontality so characteristic of the Great plains of the northwest. Rising abruptly from this level area, the mountains act as local condensers, so that the slopes are well watered; thunderstorms are common during the summer, and the higher peaks generally cloud- enveloped for a large part of the year. Although not of sufficient height to maintain permanent snow banks, the mountains give rise to several fine streams, which head in springs far up the narrow V-shaped trenches ♦ “ Eruptive Bocks from Montana.” Proc. California Acad. Sci., ser. 2, vol. iii, 1890, p. 39j also, Am. Jour. Sci., vol. 45, 1893, p. 286. 392 WEED AND PIRSSON— HIGHWOOD MOUNTAINS OF MONTANA. which everywhere score the higher elevations. This abundance of moisture favors agriculture, and the upland valleys and mountain

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