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 Earth's crust, crust that is prone to localized volcano, 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 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 ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

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 landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

about 30 miles southwest of the city of Great Falls. The area was named by

geologist A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...

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 the southernmost Provinces and territories of Canada, province of Canada. Located in Central Canada, Ontario is the Population of Canada by province and territory, country's most populous province. As of the 2021 Canadian census, it ...

, and

Timor Timor (, , ) is an island at the southern end of Maritime Southeast Asia, in the north of the Timor Sea. The island is Indonesia–Timor-Leste border, divided between the sovereign states of Timor-Leste in the eastern part and Indonesia in the ...

.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 o ...

on top of

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

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 82.4 Ma and 74.4 Ma, during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky Mountai ...

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 contr ...

rock is composed of

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

and conglomerate. The intrusive rock has formed numerous

dikes Dyke or dike may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), formations of magma or sediment that cut through and across the layering of adjacent rocks * Dike (mythology), ''Dikē'', the Greek goddess ...

, 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, or the Eisenhower Interstate System, is a network of controlled-access highways that forms part of the National Hi ...

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 apart ...

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 Cascade, or Cascading may refer to: Science and technology Science * Air shower (physics), a cascade (particle shower) of subatomic particles and ionized nuclei ** Particle shower, a cascade of secondary particles produced as the result of a high ...

) 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, 80 to 70 million years ago, and ended 55 to 35 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 syst ...

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

horses The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 milli ...

Magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

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, Montana, Fergus County, just to the northeast of Lewistown, Montana. Judith Peak is the highest summit of this range, with an elevation ...

. 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 a highly supported scientific theory, originating in the early 20th century, that Earth's continents move or drift relative to each other over geologic time. The theory of continental drift has since been validated and inc ...

. Others have looked at the possibility of

petroleum Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring un ...

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 Hydrophobe, hydrophobic; their odor is usually fain ...

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 A bison (: bison) is a large bovine in the genus ''Bison'' (from Greek, meaning 'wild ox') within the tribe Bovini. Two extant taxon, extant and numerous extinction, extinct species are recognised. Of the two surviving species, the American ...

hunting rounds on the nearby plains (such as the pishkun, or

buffalo jump A buffalo jump, or sometimes bison jump, is a cliff formation that 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 hunt ...

, at nearby

Ulm Ulm () is the sixth-largest city of the southwestern German state of Baden-Württemberg, and with around 129,000 inhabitants, it is Germany's 60th-largest city. Ulm is located on the eastern edges of the Swabian Jura mountain range, on the up ...

—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 great-circle distance, straight-line distance from the northernmost part of Western Can ...

to the

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

, the

Mullan Road Mullan Road was the first covered wagon, wagon road to cross the Rocky Mountains to the Inland Empire (Pacific Northwest), Inland of the Pacific Northwest. It was built by United States Army, U.S. Army troops under the command of Lt. John Mullan ( ...

, skirted the volcanic field to the north, although dirt

stagecoach A stagecoach (also: stage coach, stage, road coach, ) 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 ...

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 for which a fee (or ''Toll (fee), toll'') is assessed for passage. It is a form of road pricing typically implemented to help recoup the costs of road construction and ...

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 Rail ...

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 began planning a major highway through the area as part of their effort to build U.S. Highway 91 and provide a direct road connection between

Helena Helena may refer to: People *Helena (given name), a given name (including a list of people and characters with the name) *Katri Helena (born 1945), Finnish singer * Saint Helena (disambiguation), this includes places Places Greece * 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 in the Western United States, running through Southern California and the Intermountain West. I-15 begins near the Mexican border in San Diego County and stretches north to Alberta, Ca ...

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

Climate

In popular culture

The final scenes of the

1974 Major events in 1974 include the aftermath of the 1973 oil crisis and the resignation of United States President Richard Nixon following the Watergate scandal. In the Middle East, the aftermath of the 1973 Yom Kippur War determined politics; ...

motion picture ''

Thunderbolt and Lightfoot ''Thunderbolt and Lightfoot'' is a 1974 American crime comedy film written and directed by Michael Cimino in his directorial debut, and starring Clint Eastwood, Jeff Bridges, George Kennedy and Geoffrey Lewis. The film follows John "Thunderbol ...

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

1987 Events January * January 1 – Bolivia reintroduces the Boliviano currency. * January 2 – Chadian–Libyan conflict – Battle of Fada: The Military of Chad, Chadian army destroys a Libyan armoured brigade. * January 3 – Afghan leader ...

movie ''

The Untouchables Untouchable or Untouchables may refer to: People * Untouchability, the practice of socially ostracizing a minority group of very low social status * Untouchables, word for the Dalits or Scheduled Castes of India * Untouchables (law enforcement), ...

'' 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, silent screen persona and understated acting style. He won the Academy Award for Best Actor twice and had a further three nominations, ...

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