Early day Montanans built their lives on a foundation of gold, silver and copper, many would say. But some geologists disagree. They would say Madison limestone is the state's foundation.
The Madison limestone's 1,000-foot thick base forms the largest artesian aquifer in the U.S., coursing beneath Montana east of the Rockies and across North and South Dakota, Wyoming, Alberta, Saskatchewan and the upper western portion of Manitoba. Its towering limestone features guided the travels of the most ancient people and more recently served as signposts for Lewis and Clark's journey. Today many of these same features are treasured Montana State Parks landmarks.
The natural springs and wells that irrigate eastern Montana's crops and support its wildlife draw water from the Madison's aquifer. A massive water-bearing layer of permeable rock, this aquifer is recharged by precipitation and surface water that is collected and then carried through the limestone.
In fact, water bubbling from the "cauldron" at Giant Springs State Park actually entered the limestone thousands of years earlier in the Little Belt Mountains 35 miles southeast of Great Falls. There, a 1,700-foot thick limestone formation creates Monarch Canyon and its surrounding ridges and cliffs. Cracks, tunnels and caves—where the limestone rises above ground—suck tremendous volumes of surface water deep into the earth. The water travels downslope through the limestone to emerge at Giant Springs at the rate of nearly 8 million gallons per hour, creating the largest freshwater springs in the world.
Bottled for drinking water, home to a state fish hatchery, and a cool lush environment for centuries of visitors, this spring is a resource more rare than vintage wine and far more difficult to duplicate.
The final, principal discharge area for the Madison aquifer is actually 600 miles to the northeast of Montana in the Canadian province of Manitoba, where it fills Lakes Winnipegosis and Manitoba.
Other Montana State Parks also owe their prominence to their Madison limestone features including Beaverhead State Park, Clark's Lookout, and the most spectacular display of limestone at Lewis and Clark Caverns State Park. At the Caverns, visitors can watch firsthand the timeless, underground dance of water and limestone and the intricate sculptures crafted one drip at a time over the eons.
Beaverhead Rock State Park commemorates the Lewis and Clark Expedition's tremendous relief to be back in ‘known’ territory, a place where the Captains expected to be able to acquire horses to continue their trek to the Pacific. An uplifted block of Madison limestone forms Beaverhead Rock and the narrows of the river. This limestone feature remains because it is more resistant to erosion than the surrounding sediments. Traveling southwestward on Route 41, the outcrop resembles a huge beaver swimming south across the road.
Five days past Beaverhead Rock, Clark climbed “a high Point of Limestone rocks” on the northwest outskirts of present-day Dillon to look for Shoshone horsemen. Clark took sightings on prominent features of the landscape and noted the course of the “Wisdom” River. This outcrop—now Clark’s Lookout State Park—is also an eroded remnant of Montana’s Madison limestone foundation.Gold, silver and copper played enormous roles in shaping the early day territory and later day State of Montana, while Madison limestone purifies our water and forms many of our familiar natural features. Both remind us of how interwoven our lives, communities, history and economic viability is with the geologic resources that surround us.