Tongass National Forest

Karst and Caves

Some 850 square miles of the Tongass are underlain by carbonate bedrock. With the geologic and climatic setting of Southeastern Alaska, dramatic karst landscapes and their associated extensive cave systems have formed.

Karst

Karst is created when rain falls onto soluble bedrock such as limestone. On its journey to earth, the rain picks up and dissolves carbon dioxide from the air. This forms weak carbonic acid, which slowly dissolves the surfaces of the limestone, including any cracks in it. In time, the cracks enlarge as the bedrock continues to dissolve until an underground drainage system begins to develop. More water passes through the rock, accelerating the formation of underground karst features such as the disappearing/reappearing streams and caves found on Prince of Wales Island.

The rugged karst terrain is often dense and difficult. The roughness of the karst topography and the vegetation make access a challenge, and many caves are dangerous to enter and explore.

Caves

It is estimated that there are thousands of caves in the Tongass National Forest. Some 650 have been inventoried, most on northern Prince of Wales Island and several westerly islands.

These caves are a virtually untapped source of information about the formation of the islands, past environments, changing animal populations, and human uses of the caves.

Only since 1987 have the caves been formally explored and mapped. The results of these activities are finds shedding dramatic light on the prehistoric and historic ecology of the region. Bone and soil deposits within the caves document the history of the region. The mineral deposits coating the walls of some caves hold a wealth of long-term historic ecological data including vegetation in the area and climatic changes over thousands of years. MORE ABOUT CAVE RESEARCH ON PRINCE OF WALES ISLAND

Caves in the Tongass are wet and undeveloped. Anyone entering them must have the appropriate equipment and be alert to the potential hazards and the fragile condition of cave features.

The Forest Service offers tours of the longest known cave in Alaska, El Capitan Cave on Prince of Wales Island, as well as interpretation of two other karst and cave sites. Please check with the Craig or Thorne Bay district offices for information on available tours, caving safety, regulations, and ethics.