Madison Limestone in the Little Rockies of Montana
May 17, 2021
Photographer: Rod Benson
Summary Author: Rod Benson
These light-colored cliffs are made of nearly vertical layers of Madison limestone, formed from sediment deposited during the Mississippian Period about 340 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 hiker in the blue shirt provides a sense of scale.
The sediment was deposited in horizontal layers, but about 60 million years ago magma worked its way up toward the surface, causing the Madison Limestone to be domed upward. Eventually, the magma hardened, becoming igneous rock (syenite porphyry), which forms the core of the mountain range. The doming occurred in an area about 15-20 miles (24-32 km in diameter. Over time, most of the limestone above the igneous intrusion eroded away, leaving only the steeply- tilted edge of the limestone dome that forms the cliffs shown in the photo. Many other similar outcrops can be found around the perimeter of the Little Rockies. Photo taken on March 19, 2021.