Above: I took this drone photo of myself (blue shirt) during a recent hike in the Little Rockies of north-central Montana.
These cliffs are made of nearly vertical layers of Madison limestone, formed from sediment that was deposited during the Mississippian Period 320-360 million years ago. Thick deposits of corals, shells, and other forms of calcium carbonate accumulated on the floor of a shallow tropical sea when this part of Earth’s crust was much closer to the equator. The Madison limestone makes a major contribution to the scenery of Montana - The Gates of the Mountains, the Rocky Mountain Front, Lewis and Clark Caverns, Sluice Boxes, and Bighorn Canyon are all made of (entirely or partially) Madison limestone.
The rest of the story.
The sediment was deposited in horizontal layers, eventually became rock, and was covered by younger layers, which also became rock (sandstones and shales). Then about about 60 million years ago magma worked its way toward the surface, causing the layers to be domed upward. The magma hardened, becoming the igneous rock found at the core of the mountain range. The doming occurred in an area about 15-20 miles in diameter. Over time, most of the limestone and other layers above the igneous intrusion eroded away, leaving only the steeply-tilted perimeter of the limestone dome that forms the cliffs shown in the photo and other similar outcrops around the perimeter of the Little Rockies.
For more about the cliffs, go to Bigskywalker.com