Montana Earth Science Picture of the Week

This photo, taken about 20 miles southeast of Great Falls (near Belt), shows an unusual circular hole known as “the Belt Meteor Crater.” Despite its name, the “crater”, which measures 100 feet across and 35 feet deep, was not made by a meteorite slamming into the surface . . . In fact it has nothing to do with rocks from space! Actually, the hole is a type of formation that geologists refer to as a “sinkhole,” and it was caused by changes that took place in rocks that lie beneath the surface.

Like Swiss cheese . . .
There are plenty of sinkholes in and around the Little Belt Mountains of central Montana. This is because a thick (up to 1700 feet) formation called the Madison limestone underlies the area. The Madison limestone it is made up of microscopic shells and other forms of calcite deposited when shallow tropical seas covered much of Montana during the Mississippian Period 350 million years ago. As water soaks down through soils above the limestone the water becomes slightly acidic. Then as it works its way down through cracks in the limestone, it dissolves away the calcite- rich rock, forming caves. The Lewis and Clark Caverns in the Madison limestone near Three Forks were formed this way. As a result of this process, the Madison limestone has so many caves that it is often described as a “geological Swiss cheese.”

Eventually these caves collapse, and that’s what happened to form the meteor crater and other sinkholes. In the case of the meteor crater, a fairly large cave must have formed in the limestone not far beneath the prairie surface. Eventually layers of sandstone above the cave collapsed onto the cavern floor to form the sinkhole.

A deep blood kettle . . .
According to Dr. David Baker, the geologist who guided me to the Belt Meteor Crater, it once served as a buffalo jump, or “pishkun,” for Native Americans as evidenced by bison bones and arrowheads on the floor of the hole. “Pishkun” is Blackfeet for “deep blood kettle.” Recently scientists visited the sinkhole to collect bison bones that will be carbon-dated to determine how long ago Indians used it. They also found an arrowhead(s) made of obsidian. Scientists can determine where the obsidian came from by comparing its mineral composition with obsidian outcrops in the region. This will provide insights about Native American trade routes.

Below: The Monarch Sink . . .
An even larger sinkhole, called the Monarch Sink, is located several miles south of Belt. It is a circular hole that measures about 75 feet deep and 150 feet across (too wide to fit in the photo). Unlike the Belt Meteor Crater, limestone at the Monarch Sink does not have layers of sandstone above it. The huge rocks scattered around me on the snow-covered floor are pieces of Madison limestone that made up the top of a very large cave before it collapsed. The huge wall behind me is also Madison limestone.

Photo by Dr. David Baker
Little Belt Consulting Services

NOTE: Both the Belt Meteor Crater and the Monarch Sink are located on private property. Please do not trespass.

Terms: dissolution, Mississipian Period, limestone, sinkhole