Geology and New Caves in the Custer-Gallatin National Forest
By Florence Miller, Physical Science Technician, Custer-Gallatin National Forest
The Pryor Mountains are a unique isolated mountain range in south central Montana abundant in biological diversity, with ecosystems ranging from desert shrub to sub-alpine forests. The geology of the Pryor Mountains is characterized by a stack of 400-million-year-old sedimentary rocks which overlie 2–4-billion-year-old igneous and metamorphic basement rocks of the Wyoming Craton. During the Laramide Orogeny, subduction of the Farallon Plate off the Pacific coast resulted in far inland mountain building, creating the Pryor Mountains. A combination of uplift and erosion of a thick stack of sedimentary rocks eventually exposed the Madison Limestone in the Pryor Mountains. The Madison Limestone represents 600 feet of cherty, fossiliferous carbonates deposited during the Mississippian in a warm shallow ocean environment on the northern portions of the Wyoming Shelf.
Unconformably overlying the Madison Limestone is the Amsden Formation, 200 feet of shale and limestone deposited in both nearshore terrestrial and offshore marine environments in the late Mississippian to early Pennsylvanian. As a result, the upper Madison is bounded by regional solution collapse zones and paleokarst breccia deposits. The vast majority of solutional caves in the Pryor Mountains are in the upper Madison Limestone within this paleokarst layer near the Madison Limestone/Amsden Formation contact.
Since 2018, an interagency team working with the US Forest Service and Bureau of Land Management has been inventorying caves in the Pryor Mountains to enhance agency management, monitoring, and protection. The caves in the Pryor Mountains are an important biologic resource for bats, insects, and other animals, providing water and shelter in an otherwise arid environment.
Three caves of note, “Scat’s the Way Cave”, “Octopus Den Cave”, and “Joan’s Home Cave”, were found within the paleokarst layer in the summer of 2022. The caves contain an assortment of impressive geologic formations including boxwork, cave pearls, coatings, columns, coralloids, crusts, draperies, flowstone, moonmilk, rims, soda straws, spar, stalactites and stalagmites. These caves are unique when water is present, containing moist earth, dripping water, and small pools, offering a hidden oasis in an arid environment. The caves are also biologically rich, containing large woodrat middens, animal scat, roost stains, bat guano, insects, bones, and animal dens. Woodrat middens are of particular interest as they contain piles of plant fragments, fecal pellets, and collections of other local material and are embedded in “amberat” (crystallized urine) that can exist for thousands of years, providing an evolutionary and historical record of the past.
Our first find of the season, “Scats the Way Cave”, has 130 feet of passage and is distinguished by flowstone, draperies, stalactites, stalagmites, large middens, and pools of water. The largest find so far this year is “Octopus Den Cave” with 210 feet of known passage and several leads still to be explored. The cave’s largest room is 80 feet by 60 feet and is 14 feet tall. The cave is well decorated and contains large slabs of moonmilk, an animal trail, den, and scratch marks. Our most recent find is “Joan’s Home Cave” which has 91 inches of passage and features uncommonly deep pools (6-12 inches) for Pryor Mountain caves and pristine moonmilk throughout.