Spring Mill State Park (GM-34)
Spring Mill's historic impact is matched by the heritage of nature that the park preserves. Containing some virgin timber, the area is pockmarked by sinkholes, and nine named caves lie within the park. These features were formed, and are being modified by rainwater percolating through the soil down into the underlying limestone. The topography was formed during the past few million years, but the bedrock originated in seas that covered Indiana more than 300 million years ago.

Donaldson Cave system and Hamer Cave are the major caves in the park, and both were intimately related to the life of the early residents. Both provided water to power mills; saltpeter for gunpowder was taken from Donaldson; and villagers butchered meat and cooled perishable foods in both.

Our Hoosier State Beneath Us: Geomorphology