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Adel Mountains Volcanic Field
The Adel Mountains Volcanic Field (also known as the Adel Mountains, Adel Volcanics, and Adel Mountain Volcanics) is an ancient volcanic field of heavily eroded 75-million-year-old〔〔Gunderson, 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 rocks 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. ISBN 0-7627-3655-0〕〔(''"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 about 30 miles southwest of the city of Great Falls. The area was named by geologist 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, a very uncommon type of intrusive igneous rock found primarily in Montana, Ontario, and Timor.〔〔〔Roberts, Eric M. and Hendrix, Marc S. "Taphonomy of a Petrified Forest in the Two Medicine Formation (Campanian), Northwest Montana: Implications for Palinspastic Restoration of the Boulder Batholith and Elkhorn Mountains Volcanics." ''Palaios.'' 15:5 (October 2000).〕〔yndman, 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. ISBN 0-444-51777-4〕
==Description==
The Adel Mountains Volcanic Field is a 3,280 feet thick (1,000 meters) unit which lies unconformably on top of Cretaceous sedimentary rock of the Two Medicine Formation on the edge of the Great Falls Tectonic Zone.〔〔〔Sears, James W.; Harms, Tekla A.; and Evenchick, C.A. ''Whence the Mountains?: Inquiries Into the Evolution of Orogenic Systems: A Volume in Honor of Raymond A. Price.'' Boulder, Colo.: Geological Society of America, 2007. ISBN 0-8137-2433-3〕 The extrusive rock is composed of breccia and conglomerate.〔 The intrusive rock has formed numerous dikes, sills, and plugs.〔 An "enormous number of dikes...emanate radially from its center",〔Sheriff and Gunderson, "Age of the Adel Mountain Volcanic Field, West-Central Montana," ''Isochron/West: A Bulletin of Isotopic Geochronology,'' December 1990, p. 21.〕 many of which are visually striking and vertical, and can be seen from the interstate highway and other roads in the area.〔〔 Many of the dikes clearly fed laccoliths in the area.〔
Two concentric rings of laccoliths lie in the northwest section of the field 11 miles (18 kilometers) and 14 miles (23 kilometers) 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 2,000 feet of rock which used to exist on top of Square Butte has subsequently eroded to reveal the igneous rock which forms the laccolith.〔
Adel Mountain is the highest peak in the volcanic field, at 7,093 feet (2,162 meters) above sea level, and an unnamed peak in the field is the second-highest at 6,845 feet (2,086 meters) above sea level.〔(Daumiller, Gerry. "Montana's Tallest Peaks by Mountain Range." Natural Resource Information System. Montana State Library. Revised September 2007. ) Accessed 2009-09-30.〕
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