#50 - Wall of Limestone Surrounds the Little Rockies

Photo courtesy of the Bureau of Land Management

Tropical Montana . . .
This aerial photo shows the southern edge of the Little Rockies in north-central Montana. An open-pit gold mine once owned by Pegasus Gold is visible near the center of the photo. The light-colored cliffs in the foreground are made of Madison Limestone, a sedimentary rock laid down during the Mississippian Period about 340 million years ago. The sediment accumulated in thick horizontal layers on the floor of a shallow tropical sea. Although sedimentary rock is formed in horizontal layers, the cliffs are made of layers of limestone that have been tilted into a vertical position. The vertical wall of limestone wall can be seen around most of the perimeter of the Little Rockies. The photo below shows a portion of the wall on the north side of the mountains - opposite the side shown in the top photo.

Click on the drone photo above to enlarge. You may be able to see me (bright blue shirt). Magma got pushy . . .
The presence of this wall is due to the same geologic processes that deposited the gold in this area. Both involved magma. About 60 million years ago, magma worked its way up to the Madison Limestone causing it to be domed upward. Eventually the magma hardened, becoming a type of igneous rock (syenite porphyry: red on the diagram below). The dome was about 15-20 miles in diameter. Over time, most of the limestone above the igneous rock was eroded away, leaving only the vertically tilted edge of the limestone dome, which forms the cliffs shown in the photo. The cross-section diagram below shows the tilted Madison limestone (light blue) that forms the cliffs. The core of igneous rock (syenite porphyry) is the red area on the diagram. (Image courtesy of the Department of Energy)

Below: The wall, which is the bottom edge of the dome, is all that remains. In the diagram below, the Madison limestone is the light blue area that encircles the igneous core (red) of the Little Rocky Mountains. Courtesy of the Montana Bureau of Mines and Geology

Term: limestone

For much more about the wall, CLICK HERE to access my blog and photo tour.