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Montana, east, Big Ice Cave in the Pryor Mountains. Formed in Madison Limestone, Big Ice Cave stays cool enough to retain ice all year long. The cave floor is a slick sheet of ice and the pinnacles are formed by water dripping from the rock above. Some geologists refer to the Madison Limestone in Montana as geologic swiss cheese, full of holes and caves. Sales of this image benefit a non-profit organization. Global Health Equity Foundation is a philanthropic organization making tangible improvements toward global health equity. The Foundation combines research with advocacy and capacity-building projects.