Rocky Mountain Front Field Trip
Lewis and Clark, traveling up the Missouri River, found the abrupt transition from prairie to
mountains at the present townsite of Cascade. Here a large igneous center--the Adel Mountains
volcanics--forms the front (or east side) of the Rocky Mountains. Square Butte and Cascade
Butte are laccoliths which formed when molten magma intruded between horizontal beds of
sedimentary rocks approximately 75 million years ago. Vertical dikes formed as magma rose up
through vertical cracks. Some dikes can be followed for miles. The dike swarm forms a
quasi-radial pattern in which dikes in the north-south direction are most prominent. The western
part of the volcanic field was cut by numerous low-angle faults. These rocks are now found in a
stack of imbricate thrust slices which formed sometime during the interval of 72 to 58 million
years ago. The faulted and folded sequence was later cut by dikes which intruded approximately
25 million years ago.
Primary topics: structures and rock types in a major igneous center, structures in the
fold-and-thrust belt
Route: Square Butte, Cascade Butte, Hardy, Craig, Gates-of-the-Mountains
Concepts: dikes, dike swarm, laccolith, volcanoclastic sediments, lineament, thrust fault,
stratigraphy from Proterozoic to Cretaceous, Belt Basin, "thin-skinned" tectonics.
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Last Updated December 1, 1998 by David Baker