Adel Mountains Volcanic Field

[7] 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.

[10] The Adel Mountains Volcanic Field is a 3,280-foot (1,000 m) thick unit which lies unconformably on top of Cretaceous sedimentary rock of the Two Medicine Formation on the edge of the Great Falls Tectonic Zone.

[5] An "enormous number of dikes...emanate radially from its center", [6] [9][12] many of which are visually striking and vertical, and can be seen from the interstate highway and other roads in the area.

Based on fossil evidence, Lyons estimated in 1944 that the formation was late Cretaceous in origin, making it about 66 million years old.

[4][5] 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.

[21][22] The final scenes of the 1974 motion picture Thunderbolt and Lightfoot was filmed on and next to Interstate 15 as it passed through the Adel volcanics.

[25] A portion of the 1987 movie The Untouchables was shot at the Hardy Bridge in the Adel Mountains Volcanic Field, which doubled for the U.S.-Canada border.

[26] Motion picture star Gary Cooper was raised on the Seven Bar Nine Ranch, which was located in the Adel Mountains.

[27] The eruption that formed the Adel Mountains Volcanic Field is depicted in the third episode of the American TV series Dinosaur Planet.

Simplified geologic map of the Adel Mountain volcanic field.
Northwest view of Square Butte, a laccolith in the Adel Mountains Volcanic Field.
Adel Mountains Volcanic Field about 30 miles southwest of Great Falls, Montana, USA. The Missouri River can be seen in the lower right hand corner of the image. The Hardy Bridge is in the center of the image.