Shonkin Sag laccolith; A differentiated alkaline intrusion, Highwood Mountains, Montana
W. P. Nash, Lawrence M. Monson, 1987. "Shonkin Sag laccolith; A differentiated alkaline intrusion, Highwood Mountains, Montana", Rocky Mountain Section of the Geological Society of America, Stanley S. Beus
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The Shonkin Sag laccolith is on the northeast flank of the Highwood Mountains in Sec. 22–27, T.2 1N., R.11 E., in Choteau County, Montana. The laccolith lies entirely within the Geraldine, Montana, 1:24,000 Quadrangle. It is approximately 44 mi (71 km) east of Great Falls and 54 mi (87 km) northwest of Lewiston. The nearest town is Geraldine, about 3 mi (5 km) to the north (Fig. 1). The best access is through Geraldine, which lies on Montana 80,27 mi (43 km) south of Fort Benton or 38 mi (61 km) north of Stanford. The shortest route from Great Falls is by way of U.S. 87 to Geyser, a distance of 47 mi (76 km). From Geyser, a gravel road can be used to reach the Buck Ranch access, 27.3 mi (43.9 km) to the north. This route contains many excellent outcrops and views of other alkalic igneous bodies, including Square Butte and Round Butte. Outcrops of the Shonkin Sag laccolith can be seen on the Buck Ranch, which is entered 3.7 mi (6.0 km) south of Geraldine (Fig. 2). Follow the ranch access 1.5 mi (2.4 km) to the ranch buildings. The western outcrop is adjacent to this road. Trails east of the ranch yard lead to the top of the laccolith and permit easy examination of an entire cross section by traversing down the canyon draining to the southwest. This cross section can be viewed from the Flat Creek Road, which intersects Montana 80, 0.5 mi(0.8 km) north of the small town of Square Butte
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Rocky Mountain Section of the Geological Society of America
One of six volumes generated by each GSA section for the Decade of North American Geology (DNAG) project, this Centennial Field Guide contains descriptions of 100 sites or site clusters representing outstanding geologic locations in northern Arizona, Colorado, Idaho, Montana, North Dakota, New Mexico, South Dakota, Utah, Wyoming, and Alberta.