You saw, first hand, some of the multitude of gifts provided to Montana by the Madison Limestone. The Madison is a carbonate deposited in shallow water some 325 to 350 million years ago during the Mississippian Period. In much of central Montana, the top of the Madison horizon was exposed by erosion after initial deposition and a widely distributed karst topography developed before it was buried by Permian, Jurassic, and younger sediments. The paleokarst horizon provides a regional plumbing network for groundwater movement making the Madison horizon an aquifer of the first order.
In the Big Snowy Mountains, the Central Montana Uplift brings the Madison upward in a domal structure. Most if not all of the Central Montana "Island Ranges" such as the Highwoods, Big and Little Belts, Bear Paw, and Sweetgrass Hills, expose a core of younger igneous intrusive and extrusive (volcanic) rocks or of "basement" metamorphic rocks, but none are exposed in the center or crest of the Big Snowy Mountains. Instead, the Big Snowy Mountains have a wide and long cap of Madison Limestone all along their plateau-like crest. As you discovered, the crest exposes karst features like caves and caverns, including the Ice Cave. Lower down from the crest, large scale slumps which look like cirques but which were not created by alpine glaciation have pulled away from the domal surface and formed bowl-like headwater features. Many of the headwater features have high volume springs where the groundwater plumbing within the Madison intersects the bowl surface, with the Crystal Cascade being one of such springs.
Elsewhere in the region, the Madison Limestone is the source of the Big Spring in Lewistown, the canyon walls along the Smith River, Lewis & Clark Caverns, and countless "palisades" where folding has tilted the bedding plane such that long outcrops exposed by erosion subsequent to the folding have left walls of gray limestone for us to enjoy today. In semiarid climates such as Montana, limestones are ridge-formers, and with a thickness approaching 2,000', the Madison LImestone provides much of the non-igneous topographic high country, ridge, and hill country.
It's really great to see the Big Snowy Mountains through your eyes. I'm tempted to divert for a couple of days while in the region on our upcoming "Great Traverse" trip.
Foy