Tongass National Forest

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Geological History

Near the entrance of the cave, the staircase levels out into flat boardwalk and limestone looms above you in cliff face. You are now walking through karst.

What is karst?

Karst is a landform made of almost pure carbonate rock (limestone or marble) which has been eaten into by the weak carbonic acid solution created and carried by rain falling through the atmosphere. Here on northern Prince of Wales Island, the bedrock is 96%-pure limestone under a thin layer of acidic soil. Over the eons, the rain has flowed through this soil and eroded the limestone, creating this marvelous example of karst landscape.

Aided by the extremely acidic muskeg pools (2.4 to 5.5 on the pH scale), caves and other solution features develop four to eight times faster than they do elsewhere in North America.

Precipitation and acidic surface water began dissolving the rock. In time, the waters widened fractures, creating karst systems that extend from the alpine to the sea. These fractures carried water to where conduits were dissolved that transported the karst waters laterally. A small portion of these conduits are large enough for us to explore and even a smaller number are exposed at the surface or have been discovered. These are the caves of Prince of Wales Island.

Where acidic waters come in contact with limestone, vertical pits commonly form. Similar processes occur in the alpine, where glaciers, snowfields and rainfall have dissolved deep pits into the fractured limestone. El Cap Pit (left), is the deepest pit in limestone in the United States, with a free rappel of 598 feet and a total depth of 624 feet.

Where did it come from and how did it get here?

The limestone in this area was formed about 400 million years ago in shallow subtropical seas. There, ancient reefs created by stromatolite algae built up, and around them calcium carbonate shells and skeletons of marine creatures settled on the ocean floor. Over time, materials such as clay, silt, and gravels buried the carbonate deposits. The weight of these materials exerted pressure on the deposits and eventually turned them into stone. The hardened deposits were carried to Southeast Alaska on oceanic plates. Tectonic forces brought the Pacific and North American plates into collision, causing fracturing and folding along the plate margins. The limestone of northern Prince of Wales Island docked with what became Southeast Alaska about 170 million years ago.

Karst has developed throughout the 400 plus million year history of this limestone. The karst formations we see here, though, are more on the order of a couple of million years old, the majority being formed in the last 130 thousand years.

What other forces are at work here?

Glaciers are important in the geologic history of this area and the formation of this cave. The last glaciations reached their maximum extent between 22,000 and 15,000 years ago when glaciers reached a depth of 1,400 feet. The weight of the glaciers pushed down on the land. Worldwide, sea levels were much lower because so much water was held in ice. As the glaciers melted, the seas rose. Eventually, as glaciers retreated, the land rebounded from their weight. This “isostatic” rebound lifted the ancient shoreline, roughly 9,000 years old, approximately 40 feet above the present sea level. The base of this cliff was actually at sea level for several thousand years.

The most recent glaciations greatly modified some of the karst systems. As ice ages came and went, glacial activity collapsed and gouged some cave passageways, while filling others with sediment.

During the last 9,000 years, the climate has undergone may cycles but for the most part has become wetter. Peat lands developed across the landscape in poorly drained areas, forming dome and slope bogs, fens, and muskegs. The acidic runoff from these wetlands rapidly cut vertical shafts into the limestone, modifying pre-existing karst systems. Tectonic, isostatic, and climatic changes are recorded in the shape of the cave passages and the deposits preserved there.

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USDA Forest Service - Tongass National Forest Accessibility Statement
Last Modified: July 11, 2007