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Montana’s “Rock of Ages” 
Photo courtesy of Stan and Irina Guthrie
If Montanans ever decided to select a “state rock formation”, a pretty good argument could be made for the Madison Limestone. This “Madison Formation” can be seen in many of our state’s more recognizable landscapes (listed below). The photo above shows cliffs of Madison Limestone at the south entrance to the Gates of the Mountains area. The photo was taken from Upper Holter Lake (part of the Missouri River; 20 miles north of Helena). Here is a list of other places where you can see outcroppings of the same Madison Limestone.
1. The Bridger Mountains and the Horse Shoe Hills (Bozeman area)
2. The Little Rockies in north-central Montana
3. Lewis and Clark Caverns and Jefferson Canyon near Three Forks
4. The Sawtooth Range and Sun River Canyon between Helena and Glacier Park
5. The Little Belt Mountains, Sluice Boxes State Park southeast of Great Falls
6. Bighorn Canyon and the Pryor Mountains south of Billings
7. The mountains north and south of Lewistown (Judith and Snowy Mtns.)
8. The Sweet Grass Hills northeast of Shelby
9. The Castle Mountains southeast of White Sulphur Springs
What is a “formation”? . . .
To a geologist a “formation” (or “group”) is a thick layer (or series of layers) of a particular type of sedimentary rock covering a large geographic area. Some of Montana’s more famous formations include the Hell Creek Formation, the Judith River Formation, and the Two Medicine Formation, which have all yielded significant dinosaur fossils. The familiar “Rimrocks” of Billings and the White Cliffs east of Ft. Benton are both exposures of the Eagle Formation. The Madison Formation consists of sediment laid down during the Mississippian Period of the Paleozoic Era (roughly 350 million years ago). It was laid down over most of Montana, eastern Idaho, northern Wyoming, and the Dakotas. The Madison is between 1,000 and 2,000 feet thick throughout most of the Montana.
GEOLOGIC TIME SCALE
Reading the rocks . . .
One reason geologist get so excited about rock formations is that they provide clues about what the environment was like in that location when the sediments were being deposited. For example the Eagle Formation consists of sand deposited near the shore of a shallow inland sea, whereas parts of the Judith River Formation indicate the presence of deltas much like the one at the mouth of the Mississippi River. Based on the types of fossils contained in the Madison Formation, geologists think its sediments accumulated on the floor of a tropical shallow sea. The map below shows where geologists believe Montana was located during this time (the Mississippian Period).
 Map courtesy of Jim Schulz, Helena High Science Dept.
A deposit of calcite sediment . . .
Although it comes in many forms, all limestone is primarily made up of the mineral called calcite (calcium carbonate; CaCO3). This mineral is produced as marine organisms draw calcium carbonate out of the water in order to build shells or other hard parts. As these organisms die their soft tissues decay, but the shells, etc. made of calcite build up as sediment on the sea floor. Tropical waters also support abundant seaweeds that excrete calcite, which precipitates onto the seafloor as a light-colored lime mud, especially during periods of evaporation. Judging by the thickness of the Madison Limestone, this tropical marine environment persisted for many millions of years.
Terms: excrete, precipitate
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