The Madison Limestone of the Rocky Mountains and Great Plains is formally a geologic group that includes several formations, including the Lodgepole Formation and Mission Canyon Formation.
It was formed in those warm, shallow Mississippian seas that covered a lot of the interior of North America about 330 to 340 million years ago. The calcium carbonate deposited in those seas accumulated to as much as 2,000 feet thick in parts of Montana, where the Madison forms prominent gray cliffs. The Gates of the Mountains, named by Lewis and Clark for dramatic cliffs along the Missouri River east of Helena, Montana, are made of these Mississippian limestones.
|Madison Limestone at Gates of the Mountains|
You probably won’t be surprised to hear that the warm shallow water supported lots of life. The Lodgepole Formation especially has loads of fossils, ranging from crinoid stem columnals to brachiopods and small horn corals and much more. The Lodgepole Formation has nice bedding sometimes distinguished by a little silt or mud interbedded or mixed with the limestone. The beds are often two to four inches thick and sometimes show color differences that give the rock a distinct striped appearance – but don’t visualize bright colors, we’re talking about shades of tan and gray.
The Mission Canyon formation, younger than and on top of the Lodgepole Formation, is massive. That’s a technical term that means we seldom see the discrete beds like the ones in the Lodgepole, or for that matter in most layered sedimentary rocks. The Mission Canyon is just limestone. Lots of it. Microscopically, the Mission Canyon is called bioclastic – that means “life, broken” – and the rock is often made up mostly of tiny broken pieces of shells and crinoid stems and such, all cemented together by more calcite. It’s usually gray, and because it’s thick and doesn’t have planes of weakness, bedding planes, the Mission Canyon sometimes makes really prominent cliffs like those at the Gates of the Mountains.
Limestone is easily soluble in rainwater, which is normally slightly acidic because of the reaction between water and carbon dioxide in the atmosphere, which makes carbonic acid. Even in arid country like Montana, caves can develop. Lewis and Clark Caverns, a state park along the Jefferson River east of Whitehall, Montana, probably formed when Montana was a lot rainier than it is today, probably during the glacial periods of the past couple million years. Lewis and Clark were on the river just below the caverns, but they never saw them. The caves were discovered by non-Native Americans in 1882, and they were designated our second National Monument in 1908, but because the park service couldn’t manage it, the cave was transferred to the State of Montana in 1938 and became Montana’s first state park.
The porous, easily dissolved limestones of the Madison Group serve as important oil reservoirs in places like the Williston Basin of eastern Montana and western North Dakota, and elsewhere they are valuable groundwater aquifers.
—Richard I. Gibson
Photo of Madison Limestone at Gates of the Mountains by Richard I. Gibson.