The Adel Mountains Volcanic Field (also known as the Adel Mountains, Adel Volcanics, and Adel Mountain Volcanics) is an ancient

volcanic field A volcanic field is an area of Earth's crust that is prone to localized volcanic activity. The type and number of volcanoes required to be called a "field" is not well-defined. Volcanic fields usually consist of clusters of up to 100 volcanoes ...

of heavily eroded 75-million-year-oldGunderson, Jay A. and Sheriff, Steven D. "A New Late Cretaceous Paleomagnetic Pole from the Adel Mountains, West Central Montana." ''Journal of Geophysical Research.'' 96:B1 (January 10, 1991).Harlan, Stephen S.; Geissman, John W.; Snee, Lawrence W.; and Reynolds, Richard L. "Late Cretaceous Remagnetization of Proterozoic Mafic Dikes, Southern Highland Mountains, Southwestern Montana: A Paleomagnetic and 40Ar/39Ar Study." ''Geological Society of America Bulletin.'' 108:6 (June 1996).

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...

s about 40 miles long and 20 miles wide (800 square miles, or 2,071 square kilometers)Merrill-Maker, Andrea. ''Montana Almanac.'' 2d ed. Guilford, Conn.: Globe Pequot, 2005. ''"Cliffs High and Steep" - The Adel Mountain Volcanics.'' Montana Department of Transportation. No date.
Accessed 2009-09-30.Sheriff, Steven D. and Gunderson, Jay A. "Age of the Adel Mountain Volcanic Field, West-Central Montana."
''Isochron/West: A Bulletin of Isotopic Geochronology.'' December 1990. in west-central

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...

about 30 miles southwest of the city of

Great Falls Great may refer to: Descriptions or measurements * Great, a relative measurement in physical space, see Size * Greatness, being divine, majestic, superior, majestic, or transcendent People * List of people known as "the Great" *Artel Great (born ...

. The area was named by

geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...

John Bartholomew Lyons,Lyons, J.B. "Igneous Rocks of the Northern Big Belt Range, Montana." ''Geological Society of American Bulletin.'' 55:4 (April 1944). who first described the general geology of the region in 1944.Beall, Joseph J. "Pseudo-Rhythmic Layering in the Square Butte Alkali-Gabbro Laccolith." ''American Mineralogist.'' 57:7-8 (July–August 1972). The Adel Mountains Volcanic Field is a significant and abundant source of

shonkinite Shonkinite is an intrusive igneous rock found in few places in the world. It is unique in having low silica, feldspathoid minerals, and large blocky crystals of black augite. It makes up much of the hard dark grey rock comprising certain mountains ...

, a very uncommon type of intrusive igneous rock found primarily in Montana,

Ontario Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central Ca ...

, and

Timor Timor is an island at the southern end of Maritime Southeast Asia, in the north of the Timor Sea. The island is East Timor–Indonesia border, divided between the sovereign states of East Timor on the eastern part and Indonesia on the western p ...

.Hyndman, Donald W., and Alt, David. "Radial Dikes, Laccoliths and Gelatin Models." ''Journal of Geology.'' (1987). Because of its geologic structure, the Adel Mountains Volcanic Field has drawn the attention of geologists for more than 100 years.Hearn, Jr., B. Carter. "The Homestead Kimberlite, Central Montana, USA: Mineralogy, Xenocrysts, and Upper-Mantle Xenoliths." In ''8th International Kimberlite Conference: The J. Barry Hawthorn Volume.'' Vol. 2. T. Stachel, ed. Maryland Heights, Mo.: Gulf Professional Publishing, 2004.

Description

The Adel Mountains Volcanic Field is a thick unit which lies

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

on top of

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

of the

Two Medicine Formation The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between and (million years ago), during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky M ...

on the edge of the

Great Falls Tectonic Zone The Great Falls Tectonic Zone is a major intracontinental shear zone between the Hearne craton and Wyoming craton Basement (geology), basement rock of the Archean Eon which form part of the North American continent.Boerner, D.E.; Craven, J.A.; Kur ...

. The

extrusive Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contra ...

rock is composed of

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

and conglomerate. The intrusive rock has formed numerous

dikes Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes ...

, sills, and plugs. An "enormous number of dikes...emanate radially from its center", many of which are visually striking and vertical, and can be seen from the

interstate highway The Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly known as the Interstate Highway System, is a network of controlled-access highways that forms part of the National Highway System in the United States. Th ...

and other roads in the area. Many of the dikes clearly fed laccoliths in the area. Two concentric rings of

laccolith A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apar ...

s lie in the northwest section of the field and from the center of the area. Square Butte, Shaw Butte (near the town of Ft. Shaw), and Cascade Butte (next to the town of Cascade) are three laccoliths belonging to the Adel field which are famous local landmarks. More than of rock which used to exist on top of Square Butte has subsequently eroded to reveal the igneous rock which forms the laccolith. Each laccolith was fed by a radial dike at its distal end where it rolled over from vertical to form the horizontal laccolith. Laccoliths spread from the end of their feeder dike towards the more-elongate axis of the parent volcano, in response to load-imposed stresses of the volcano. Adel Mountain is the highest peak in the volcanic field, at above sea level, and an unnamed peak in the field is the second-highest at above sea level.

Formation

The age of the Adel Mountain Volcanic Field has been re-estimated several times since 1944. Based on fossil evidence, Lyons estimated in 1944 that the formation was late Cretaceous in origin, making it about 66 million years old. This estimate was reinforced in the early 1970s. But separate analyses made in 1991 and 2004 re-estimated the rock as having formed in the late Cretaceous, about 81 to 71 million years ago. Most estimates today place the age at 75 million years. The Adel volcanics are part of the

Laramide orogeny The Laramide orogeny was a time period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the o ...

, a period of mountain building in western North America which began about 80 million years ago and ended about 35 million years ago. The age of the volcanism and

thrusting Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that sys ...

overlap, with some intrusions being affected by the thrusting while others were intruded into already formed thrust horses.

Magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

welled up from below for several million years, creating the Adel volcanics as well as the nearby

Highwood Mountains The Highwood Mountains are an island range (sub-range of the Rockies entirely surrounded by prairie) which cover approximately 4,659 km² (1,799 sq mi) of the Central Montana Alkalic Province in north central Montana in the U.S. They are in ...

and

Judith Mountains The Judith Mountains are located in central Montana in the Central Montana Alkalic Province in Fergus County, just to the northeast of Lewistown, Montana. Judith Peak is the highest summit of this range, with an elevation of . History To the s ...

. The crustal weakness associated with the Great Falls Tectonic Zone may have provided a route for the magma to reach the surface. Magma which never made it to the surface nonetheless created the many dikes, sills, plugs, and laccoliths which can be seen in the Adel volcanic field. Scientists have studied the Adel Mountains Volcanic Field for a number of reasons. Several teams of scientists have analyzed the

polar wander Polar wander is the motion of a pole in relation to some reference frame. It can be used, for example, to measure the degree to which Earth's magnetic poles have been observed to move relative to the Earth's rotation axis. It is also possible to use ...

path to study

continental drift Continental drift is the hypothesis that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The idea of continental drift has been subsumed into the science of pla ...

. Others have looked at the possibility of

petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...

or other

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...

s under the field, and concluded there is a strong possibility they exist there.

Development of the area

Although not permanently inhabited by Native Americans, several tribes camped in the Adel Mountains Volcanic Field on their way to and from bison hunting rounds on the nearby plains (such as the pishkun, or

buffalo jump A buffalo jump, or sometimes bison jump, is a cliff formation which Indigenous peoples of North America historically used to hunt and kill plains bison in mass quantities. The broader term game jump refers to a man-made jump or cliff used for hu ...

, at nearby

Ulm Ulm () is a city in the German state of Baden-Württemberg, situated on the river Danube on the border with Bavaria. The city, which has an estimated population of more than 126,000 (2018), forms an urban district of its own (german: link=no, ...

—the largest in the world). The

Lewis and Clark Expedition The Lewis and Clark Expedition, also known as the Corps of Discovery Expedition, was the United States expedition to cross the newly acquired western portion of the country after the Louisiana Purchase. The Corps of Discovery was a select gro ...

passed up the Missouri River through the area in July 1805, becoming the first white Americans to see the formations. Development of the area did not come until the 1930s. Cattle ranchers were the area's first white settlers. The primary route over the

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico in ...

to the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

, the

Mullan Road Mullan Road was the first wagon road to cross the Rocky Mountains to the Inland of the Pacific Northwest. It was built by U.S. Army troops under the command of Lt. John Mullan, between the spring of 1859 and summer 1860. It led from Fort Ben ...

, skirted the volcanic field to the north, although dirt

stagecoach A stagecoach is a four-wheeled public transport coach used to carry paying passengers and light packages on journeys long enough to need a change of horses. It is strongly sprung and generally drawn by four horses although some versions are draw ...

roads were built into the Missouri River Canyon in the volcanic field in the 1860s.Axline, Jon. ''Conveniences Sorely Needed: Montana's Historic Highway Bridges, 1860-1956.'' Helena, Mont.: Montana Historical Society, 2005. The road was upgraded and turned into a

toll road A toll road, also known as a turnpike or tollway, is a public or private road (almost always a controlled-access highway in the present day) for which a fee (or ''toll'') is assessed for passage. It is a form of road pricing typically implemented ...

in 1866, and had become a major route from Helena through Great Falls to Fort Benton by the 1870s. The

Montana Central Railway The Montana Central Railway was a railway company which operated in the American state of Montana from 1886 to 1907. It was constructed by James Jerome Hill's St. Paul, Minneapolis & Manitoba Railway, and became part of the Great Northern Railw ...

reached the region in 1887, which led to additional white settlement. A paved county road was laid down, and remained the sole access through the area for decades.Axline, Jon and Bradshaw, Glenda Clay. ''Montana's Historical Highway Markers.'' Rev. ed. Helena, Mont.: Montana Historical Society, 2008. In the 1920s, the

Montana Highway Department The Montana Department of Transportation (MDT) is a governmental agency in the U.S. state of Montana, responsible for numerous programs related to the construction, maintenance, and monitoring of Montana's transportation infrastructure and operati ...

began planning a major highway through the area as part of their effort to build

U.S. Highway 91 U.S. Route 91 or U.S. Highway 91 (US-91) is a north–south United States highway running from Brigham City, Utah, to Idaho Falls, Idaho, in the U.S. states of Idaho and Utah. Despite the "1" as the last digit in the number, US-91 is no lon ...

and provide a direct road connection between Helena and Great Falls. Contracts were let in 1929, the majority of construction occurred from 1931 to 1934, and the highway was completed in 1935. The Hardy Bridge, built over the Missouri River in 1931, was one of two major bridges built for Highway 91.

Interstate 15 Interstate 15 (I-15) is a major Interstate Highway System, Interstate Highway in the western United States, running through Southern California and the Intermountain West. I-15 begins near the Mexico–United States border, Mexican border i ...

was built largely on top of U.S. Highway 91 in 1967.

In popular culture

The final scenes of the 1974 motion picture ''

Thunderbolt and Lightfoot ''Thunderbolt and Lightfoot'' is a 1974 American crime comedy film written and directed by Michael Cimino and starring Clint Eastwood, Jeff Bridges, George Kennedy, and Geoffrey Lewis. Plot A young ne'er-do-well, Lightfoot steals a car. Elsewh ...

'' was filmed on and next to Interstate 15 as it passed through the Adel volcanics. A portion of the 1987 movie ''

The Untouchables Untouchables or The Untouchables may refer to: American history * Untouchables (law enforcement), a 1930s American law enforcement unit led by Eliot Ness * ''The Untouchables'' (book), an autobiography by Eliot Ness and Oscar Fraley * ''The U ...

'' was shot at the

Hardy Bridge Hardy Bridge is a Warren through truss, three-span, two-lane bridge in the western United States. It crosses the Missouri River and is located at milepost 6 on Old U.S. Route 91, about southwest of Cascade, Montana, which is southwest o ...

in the Adel Mountains Volcanic Field, which doubled for the U.S.-Canada border. Motion picture star

Gary Cooper Gary Cooper (born Frank James Cooper; May 7, 1901May 13, 1961) was an American actor known for his strong, quiet screen persona and understated acting style. He won the Academy Award for Best Actor twice and had a further three nominations, a ...

was raised on the Seven Bar Nine Ranch, which was located in the Adel Mountains.Parish, James Robert; Stanke, Don E.; and Pitts, Michael R. ''The All-Americans.'' New York: Arlington House, 1977. ; Eastman, John. ''Who Lived Where: A Biographical Guide to Homes and Museums.'' New York: Facts on File Publications, 1985. The eruption that formed the Adel Mountains Volcanic Field is depicted in the third episode of the American TV series '' Dinosaur Planet''.

References

External links

{{Montana Volcanic fields of Montana Landforms of Cascade County, Montana Cretaceous System of North America