Montana Earth Science Picture of the Week

This aerial photo shows the southern edge of the Little Rockies in northcentral 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 350 million years ago.

Geologists believe that 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 limestone wall can be seen around most of the perimeter of the Little Rockies.

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. 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.

Above: This cross-section diagram shows the tilted limestone that forms the cliffs and the core of igneous rock (porphyry). Courtesy of the Billings Geological Society

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) . Courtesy of the Montana Bureau of Mines and Geology

More about the Madison Limestone . . .
Limestone is primarily made up of the mineral called calcite. Seaweeds in the shallow sea probably precipitated much of the Calcite. Shells (also made of calcite) accumulated on the sea floor, adding to the thickness. The lower part of the Madison Limestone is called the Lodgepole Formation, named after a small town on the northern edge of the Little Rockies. The upper part, called the Mission Formation, is named for Mission Canyon on the west side of the Little Rockies. Each formation is about 1,000 feet thick.

Where else will you see the Madison Limestone? . . .
The same prominent limestone can be found in other part of Montana, including the Sawtooth Range between Helena and Glacier Park, Lewis and Clark Caverns, the Gates of the Mountains north of Helena, in the Castle Mountains near White Sulphur Springs, in the Judith Mountains, in Bighorn Canyon, in the Sweet Grass Hills north of Shelby, and in the Little Belt Mountains south of Great Falls.

Terms: Mississipian Period, limestone