Beaverhead Range Beaverhead Mountains Became Part of the Boundary Are Part of an Immensely Thick Sequence of 1.4 Billion-Year-Old (Middle Between the Two Territories

Home , Bannock Pass, Battle of the Big Hole, Beaverhead Mountains, Lemhi Pass, Western Montana

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Butte Offi ce Billings Offi ce 1300 W. Park Street 1300 North 27th Street Butte, Montana 59701-8997 Billings, Montana 59101 406-496-4180 Montana Bureau of Mines and Geology 406-657-2938 THE BEAVERHEAD MOUNTAINS Pass into the Big Hole Valley to Bannack on Grasshopper Creek. It didn’t take long for that stream to be completely staked, so miners looked further afi eld. One by one the GEOLOGY streams on both sides of the Beaverhead Mountains were explored by miners, usually with little or nothing to show for their efforts except the names of many streams and peaks. These peaks of the northern Beaverhead Mountains (front photo) are the highest—up to 10,621 feet—and most rugged in the spectacular chain of Idaho became a territory on March 3, 1863. When Montana mountains that forms the Idaho–Montana border for over 100 miles, part of became a territory on May 26, 1864, the crest of the the Continental Divide (fi g. 1). The rocks exposed in the Beaverhead range Beaverhead Mountains became part of the boundary are part of an immensely thick sequence of 1.4 billion-year-old (middle between the two territories. The boundary in this area was Proterozoic) sediments that underlies most of the mountain ranges along not surveyed until 1906. the western border of Montana. In southwestern Montana many of the rock layers stand on end because of complex folding and faulting. By the 1870s the Shoshone had long since left the Montana valleys. Cattle ranchers had taken their place, and mounds of hay piled by “beaver slides” became a N prominent feature of the valley. Gold panning gave way IDAHO

MONTANA to “quartz” mines by the late 1870s, and some of these BELT BASIN mines grew into small communities: Midway, Monument, Moosehorn, and Pioneer. Outcrop limit of middle Proterozoic sedimentary rocks 1877 brought the “Nez Perce War” to the Beaverhead Mountains. Several bands of Nez Perce in western Idaho Figure 3. Looking south along the Continental Divide, layers of quartzite can be seen dipping, or rejected the call to report to a reservation and began a Missoula sloping to the right (west); the beds have been rotated from their original horizontal positions in a march eastward either to join their long-time friends the counterclockwise manner to vertical and then beyond, so that they are now overturned, with the Crow (Absarokee) or the Sioux, then in Canada. The Nez younger beds now lying beneath the older beds. The dark layer on the far right is an igneous intrusion Perce, totaling about 800 men, women, and children and Belt Supergroup Rocks Butte that intruded a fault; a close look reveals that it cuts across the quartzite beds at an acute angle. with about 2000 horses, crossed into Montana via Lolo The smooth gentle slopes on the west (right) sides of the mountains contrast with the rugged cliffs Pass. From there they headed south through the Bitterroot on the east. The difference is due to prevailing westerly winds that blew the snow onto the east side, Lemhi Group/ Valley, then entered the Big Hole Valley by Chief Joseph Yellow Jacket Fm where it then accumulated in much larger glaciers than on the west, resulting in much more cirque Beaverhead Range Pass. Thinking that they had eluded the soldiers who development on the east side. Salmon 100 MILES pursued them from Idaho, they made camp along the North Fork Big Hole River about 10 miles west of present- WYOMING HUMAN HISTORY day Wisdom, Montana. However, soldiers from Fort Shaw MONTANA (west of Great Falls, Montana) caught them by surprise, and the subsequent battle became known as the “Battle of the Big Hole.” There were grievous IDAHO For uncounted generations, the Shoshone peoples were the principal losses on both sides. Most of the Nez Perce escaped and headed south human occupants of the major valleys fl anking the Beaverhead Mountains. Figure 1. Location of the Belt sea and the much younger Beaverhead range. along the east fl ank of the Beaverhead Mountains to Bannock Pass, where These valleys now bear the names of Lemhi and Salmon in Idaho and Big they re-entered Idaho. From there they followed the Lemhi Valley along the Hole, Horse Prairie, Medicine Lodge, and Cabin Creek–Nicholia Creek west fl ank of the Beaverhead Mountains, then struck east until they entered The geological complexity of these rocks has been diffi cult for geologists to in Montana. The Lemhi and Salmon valleys, with their warmer climate in Yellowstone National Park. From there, they were pursued northward by the explain, but the story goes something like this: winter, salmon-fi lled rivers, and abundance of Bighorn sheep, were host U.S. Army and eventually surrendered just south of the Canadian border. to a year-round population. The opposite valleys in Montana were several 1. The Belt Sea. The sedimentary rocks that underlie these mountains were thousand feet higher, so saw mostly summer use, especially for their Mining activity on the Montana side of the Beaverhead Mountains nearly deposited about 1.4 billion years ago in a vast, shallow sea (fi g. 1). For extensive camas meadows. ceased by the 1890s, but saw some renewed activity during the 1930s and more than 50 million years, normal faults slowly dropped this basin down. It World War II. At present the main attraction these mountains have to offer is gradually fi lled with layers of mud, silt, sand, and limestone to a thickness The Beaverhead Mountains, like most mountains, are a signifi cant barrier to their recreational potential: hiking, camping, and spotting wildlife. of more than 45,000 feet (fi g. 2A). Geologists refer to this immensely thick travel. Along this 106-mile-long range, the mountains can be crossed most sequence of rock by various names: the Belt Supergroup in northern Idaho easily by only four principal passes: Chief Joseph, at the northern end of the and western Montana, the Purcell Supergroup in Canada, and the Lemhi range; Big Hole, about 10 miles farther south; Lemhi, about midway along Group in east-central Idaho. Geologists have long disputed whether the the range; and Bannock, about 14 miles southeast of Lemhi Pass. Except sedimentary rocks of the Beaverhead range belong to the Belt Supergroup for the passes, the mountains usually were free of human visitors. However, or the Lemhi Group, and whether the Belt Sea and the Lemhi Basin were Shoshone warriors-to-be sometimes sought these high mountains for their connected as one or were separate bodies of water. Because they appear to vision quests. WEST EAST have been so similar, for simplicity they will be discussed here as one basin. The fi rst recorded description of the Beaverhead Mountains comes from The sediments mostly record deposition in very shallow water or on the journals of Lewis and Clark, who crossed Lemhi Pass in 1805. Although Monument Freeman Peak Peak Homer Youngs immense, fl at alluvial plains that surrounded the sea. Sedimentary structures they described the adjacent valleys with their streams, animal life, and Peak such as ripple marks, mud cracks, salt casts, and raindrop imprints are vegetation, the mountains were noted merely for their diffi culty to cross. remarkably well preserved, as there were no burrowing animals to disturb Though historically important, the Expedition made little or no lasting Western strand Vertical beds Beaverhead Divide fault intruded by eastern quartzite the sediment this early in the earth’s history. Perhaps because this part of changes upon the people of the valleys. Freeman Thrust dark gabbro dike Eastern strand Beaverhead southwestern Montana was near the southeast edge of the Belt Sea (fi g. Divide fault 1), mostly sand was deposited here. As it was buried deeper and deeper In the 1820s, fur trappers (mostly French-Canadians and Iroquois) entered by younger sediments, it was fi rst compacted into sandstone and then these valleys and gave some of them their names, such as “Le Grand changed, or metamorphosed, by pressure and heat into quartzite. Trou,” or Big Hole (the word “trou” meant a wide valley surrounded by high mountains). Later trappers, mainly of Scotch extraction, brought parties 2. The ancestral Rocky Mountains. Beginning about 150 million years (brigades) of trappers into this area in search of beaver pelts and other furs. ago, tectonic forces began jamming crustal plates against the west coast of North America, shoving the rocks of the continent, including the Belt Warren Ferris, who traveled with a fur brigade, wrote a description of the sediments, and any younger rocks deposited above them, eastward. These Beaverhead Mountains in his 1831 journal. compressive forces fi rst buckled the rocks into gigantic folds (fi g. 2B), and then broke them along the reverse faults that make up the Beaverhead “We left Horse Prairie [Creek] on the last day of the month [August], Vertical beds western quartzite Divide fault system in the main photo. The faults can be seen to slope, or and crossing the mountain [near Bachelor Mountain or possibly up dip, steeply to the west (sidebar fi gure); the rocks west of the fault were Bloody Dick Creek] northwestward, descended into the Big Hole. pushed up and over the rocks east of the fault (fi g. 2C). The fault juxtaposes This is an extensive valley of sixty miles length, and fi fteen to twenty Most of western Montana, including the Beaverhead Mountains, is two different types of quartzite, but on both sides the original horizontal beds broad, bounded on every side by lofty, irregular and picturesque underlain by sedimentary rocks that were complexly folded and faulted have been rotated into near-vertical orientations. ranges of mountains, the bases of which are girded with dense during the formation of the Rocky Mountains. The main photo looks forests of fi r which in some places encroach upon the prairie domain. north and exemplifi es a cross section through the mountains, show- 3. The modern Rockies. About 55 million years ago, tectonic forces Above the pine region, the mountains present immense pointed ing several strands of the complex Beaverhead Divide fault system reversed, and the earth’s crust in this region began to extend, or pull apart masses of naked rock, hiding their giant heads among the clouds that separates two different types of sedimentary rock (photo above). again. In response, normal faults developed and began down-dropping the where the eye vainly strives to follow; and often even piercing The dark band that passes through the upper lake is igneous rock that generally north-trending valleys that characterize much of southwestern through the misty realm, where storm spirits hold their frolic revels, intruded the fault zone relatively recently, about 46 million years ago. It Montana and eastern Idaho, and uplifting the intervening mountain ranges, so that their gray peaks are often seen fl ashing and basking in the is approximately parallel to the sedimentary beds that have been tilted thereby forming the modern-day “basin and range” topography. The northern sun while the thickening vapours below are sending down torrents from their original horizontal position to near-vertical orientations by Beaverhead Mountains are fl anked by normal faults and the resultant of rain, and it may be belting their hoary forms with lightning lines of the forces that built the Rocky Mountains. The striking tree line angling valleys on both sides: the Big Hole on the east and the Salmon valley on the fi re, and beating their stolid breasts with blows and bolts of thunder, between the two lakes was trimmed by snow avalanches descending west (fi g. 3D). Erosion stripped thousands of feet of rocks from the mountain or darkening the atmosphere with heavy falls of snow and hail. The from the Continental Divide to the left. ranges, leaving only the old Belt rocks in the core of the Beaverhead caverns or gulfs—they are not vales—between these worlds of rock Mountains. Material shed during erosion partially fi lls both valleys. Normal are heaped with the snows of ages. faulting also occurred along the Beaverhead Divide fault, in contrast to the earlier reverse movement there. It was during this period of extension, 46 This valley is watered by innumerable willow fringed streams that million years ago, that the dark igneous intrusive bodies visible in the photos unite and form Wisdom [Big Hole] River, which fl ows a little east of were emplaced. Extension continues to this day in southwestern Montana, north, and, after leaving the valley, eastward, to its junction with the and many normal faults are still active. Jefferson distant eighty miles.”

4. Glaciation. These peaks owe their dramatic forms to sculpting by The fur trade essentially died out in the early 1840s. Nevertheless, those glaciers active during several ice ages over the past 2 million years. Snow, who had spent their years trapping, hunting, and living mostly by their own persisting through the summers, accumulated over hundreds of years and skills held on in these mountains. was compressed into ice. When the ice attained suffi cient thickness, it REFERENCES began to fl ow down the mountain valleys, carving the mountains into sharp In the early 1860s a new breed of adventurer entered the area: the miner. Evans, K.V., and Green, G.N., 2003, Geologic map of the Salmon National Forest peaks, plucking out the steep-walled cirques, excavating lake basins, and Gold in paying amounts was discovered in Idaho in 1861. When gold was plowing U-shaped valleys. Because the prevailing westerly winds resulted and vicinity, east-central Idaho: U.S. Geological Survey Geologic Investigations discovered near present-day Bannack, Montana in July 1862, miners Series Map I-2765, 19 p., scale 1:100,000. in copious amounts of snow blowing onto the eastern slopes, the east side from Idaho poured across the divides in the Beaverhead Mountains. They of the range displays the best glacial features (fi g. 3). Here, the glaciers also headed south from Montana Gold Creek mines through Deer Lodge Ferris, W.A., 1940, Life in the Rocky Mountains; a diary of wanderings on the extended well into the Big Hole valley, depositing moraines and outwash. sources of the rivers Missouri, Columbia, and Colorado from February, 1830, to Also, the northeast slopes of mountains are most sheltered from the sun, November, 1835; Denver, Col., Old West Pub. Co. East favoring glacial development. West sediments washing Lewis, R.S., Burmester, R.F., Stanford, L.R., Lonn, J.D., McFaddan, M.D., and into basin Othberg, K.L., 2009a, Geologic map of the Kitty Creek 7.5’ quadrangle, Lemhi County, Idaho, and Beaverhead County, Montana: Montana Bureau of Mines Beaverhead Divide Belt Basin fault system and Geology Open-File Report 582, scale 1:24,000.

Lewis, R.S., Burmester, R.F., Stanford, L.R., Lonn, J.D., McFadden, M.D., and Belt sediments Othberg, K.L., 2009b, Geologic map of the Bohannon Spring 7.5’ quadrangle, Lemhi County, Idaho, and Beaverhead County, Montana: Montana Bureau of Mines and Geology Open-File Report 583, scale 1:24,000.

Lonn, J.D., Burmester, R.F., Lewis, R.S., and Stanford, L.R., 2008, Geologic map crystalline of the Homer Youngs Peak quadrangle, Lemhi County, Idaho, and Beaverhead basement County, Montana: Idaho Geological Survey Digital Web Map 95 and Montana rocks Bureau of Mines and Geology Open-File Report 575, scale 1:24,000.

Lonn, J.D., Stanford, L.R., Burmester, R.F., Lewis, R.F., and McFaddan, M.D., 2009, A. Geologic map of the Goldstone Pass 7.5’ quadrangle, Lemhi County, Idaho, and C. Beaverhead County, Montana: Montana Bureau of Mines and Geology Open- File Report 584, scale 1:24,000. West East Beaverhead Ruppel, E.T., O’Neill, J.M., and Lopez, D.A., 1993, Geologic map of the Dillon 1º Mountains West x 2º quadrangle, Idaho and Montana: U.S. Geological Survey Miscellaneous East Investigations Series Map I-1803-H, scale 1:250,000. Big Hole Valley Salmon Valley younger sediments younger sediments ACKNOWLEDGMENTS All photos by Jeff Lonn, with permission. Text by Jeff Lonn and Bob Bergantino. Other ﬁ gures by Susan Smith. Editing and design by Susan Barth.

B. D. Figure 2. Cross sections showing geologic history of the Beaverhead Mountains. A. Belt basin ﬁ lled with sediment as it subsided 1.4 billion years ago. Arrows show tension. B. Crustal compression ~ 150 million years ago buckled the horizontal sedimentary rocks into huge folds during the formation of the ancestral Rockies. Arrows show compression. C. Continued compression caused reverse faults to form that broke the folds and shoved rocks from the west up and over rocks to the east. Arrows show compression. D. Crustal extension beginning about 55 million years ago formed normal faults that dropped the Salmon and Big Hole valleys down against the Beaverhead Mountains, and these valleys partially ﬁ lled with sediment. Arrows show tension.

Montana Bureau of Mines and Geology

Montana Tech of The University of Montana

Scope and Organization The Montana Bureau of Mines and Geology (MBMG) was established in 1919 as a non-regulatory public service and research agency for the State of Montana, to conduct and publish investigations of Montana geology, including mineral and fuel resources, geologic mapping, and groundwater quality and quantity. In accordance with the enabling act, the MBMG conducts research and provides information.

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