Sun River Canyon, Montana
Montana’s Sun River flows east across the “Disturbed Belt” of the Rocky Mountain Front. This zone of folded and thrust-faulted Paleozoic strata has climbed up on top of Cretaceous-aged deposits of the Western Interior Seaway. The Castle Reef Dolostone (Mississippian in age) weathers out in high-relief, while the shales of the Blackleaf Formation (Cretaceous) are more readily etched away and weather out as valleys. The result is a distinctive series of ridges, each a stratigraphic doppelganger of its neighbors, shingled up like imbricated pebbles in a fast-moving stream. All the ridges are oriented almost due north, indicating the underlying faults also strike north, and thus compression was from the west during the Sevier Orogeny (early Cretaceous).
Driving west up the Sun River Canyon is like a tectonically-induced Groundhog Day.
Enjoy this series of images of this wild place. I’ve included both Google Earth and Flash Earth views of the Canyon on its many thrust sheets and a series of Google Earth shots from slightly different angles, giving a sense of the three-dimensionality of this world-class fold and thrust belt. A final Google Earth shot shows you the view to the east, peering over top the stack of thrust sheets at the Great Plains beyond. Also: a photograph of the French Thrust, seen in outcrop (Castle Reef Dolostone atop black Cretaceous shale). Finally, I included a detail from the Geologic Map of the Choteau quadrangle by Dick Berg (2008), so you can waste an hour or so counting the number of thrust faults in this area.
Not enough? Try reading the many foundational studies by Melville Mudge in this area.