What is a cave? The National Speleological Society defines a cave as a natural cavity, recess, chamber, or series of chambers or galleries occurring beneath the surface of the earth, usually extending to total darkness, and large enough to permit human entrance. A cavern is a specific type of cave, naturally formed in soluble rock, with the ability to grow speleothems. Speleothems are stalactites (formations that grow from the ceiling) and stalagmites (formations that grow up from the ground)

Penn’s Cave is located in the topographic region of Pennsylvania known as the Ridge and Valley section of the Appalachian Highlands; within this section, the cavern is located in the physiographic division known as the Ridge and Valley Province. The Penn’s Cave area is a typical karst terrain, which is characterized by a large number of caves, springs, sinkholes, and surface streams that disappear underground.

Many millions of years ago, the rocks in this section were flat-lying. They were then compressed toward the northwest by immense pressure coming from the southeast. This pressure buckled the rocks into long linear folds called anticlines (upward buckled rocks) and synclines (downward buckled rocks). Erosion of these adjacent anticlines and synclines created long parallel ridges capped with resistant sandstone, such as the Nittany Mountain and Brush Mountain, and intermediate valleys, frequently floored with shale and limestone, such as Brush Valley and Penn’s Valley, where Penn’s Cave is located.