Enigma Cave

Enigma Cave is located in a bay at the northwestern end of Dall Island and is only about 180 meters from the coast and 150 meters above sea level. It was discovered in the summer of 1993 during a Tongass Cave Project expedition, funded by Tongass National Forest, and is the largest known cave on Dall Island. Several short summer expeditions were made during early the 1990s to map this cave and search for additional caves in the region, but Dall Island is an enormous landmass that remains mostly unexplored for caves. The island is of special interest to Ice Age researchers because it appears to be in a location that was protected from large glaciers, especially along its western margin.

Two entrances to Enigma Cave and one to Slightly Enigmatic Cave occur near each other in an alcove at the base of a cliff. Cavers connected these two caves by excavating rock rubble from a blocked passage.

Enigma cave has four entrances: three at the base of a limestone cliff (see photos above) and one higher on the mountainside. The cave contains about 400 meters of passage, most of which consists of horizontal or gently sloping walkways and crawlways 1-3 meters in width. A strong wind blows through several of the passageways and has created some sickle-shaped stalactites on the cave ceiling (see photos below). The cave floor is rocky in some areas but contains sediment in others.

These photos illustrate walking passages near two closely-spaced entrances of Enigma Cave. Notice how wind in this area has formed curved, sickle-shaped stalactites on the cave ceiling.

The cavers that explored and mapped Enigma Cave in 1994 discovered a number of bone deposits throughout the cave. Otter bones were discovered in one location, and piles of fish bone from otter scat were found in several areas. A deer skeleton was recovered near one of the entrances. There was also evidence that bats were roosting in the cave.

The most significant discovery made in 1994 was a brown bear skeleton found in a small pit about 75 meters inside the cave. Brown bears to not live on Dall Island today (only black bears do), and this is the southernmost discovery of a fossil brown bear in Southeast Alaska. The skeleton was radiocarbon dated at 11,715 years old, making it one of the oldest postglacial bears from the region (some postglacial brown bear remains from El Capitan Cave and Bumper Cave are a little older).

At left is a photo of the Enigma Cave brown bear skull as it was found in the cave. The back of the skull can be seen sticking up toward the top of the photo, and the front of the skull is buried in sediment. The photo above is the same skull shortly after it was removed from the cave in 1995. Note that sediment still fills many depressions in the skull and has given it a black color.

Enigma Cave differs from the bear dens found at sites such as El Capitan, Bumper, and Kushtaka Caves. The cave is longer and wider near the entrance, and the bear bones are found farther inside the cave. At present it is not clear whether Enigma Cave was used repeatedly as a bear den. It appears that a bear became lost in the cave and fell into a small pit and died. Much of the skeleton was found and was collected in 1995 by Tongass National Forest geologist Jim Baichtal (see photos below). Some of the bones had fallen down a tight passage where sediment was slumping into a small stream and could not be recovered.

Fred Grady (left) collects bear bones in an upper passage of Enigma Cave. At the bottom of a nearby pit, called Sea Lords Rest, Kevin Allred (right) collects sediment laden with fish bones from otter scat. Additional brown bear bones (adult and juvenile) and an eagle skeleton were found at the bottom of Sea Lords Rest.

Paleontologists Timothy Heaton and Frederick Grady visited Enigma Cave in the summer of 2002 with a group of cavers. In a higher level of the cave, near an upper entrance, they found remains of at least two other brown bears, including a giant individual and a juvenile. These dated to the postglacial interval between 10,700 and 12,000 years ago. The juvenile and parts of the giant bear, along with a bald eagle skeleton, had collected at the bottom of a pit called Sea Lord's Rest. Because parts of the giant bear were scattered above the pit in a horizontal passage that probably served as a den (see photos above and below), this bear, at least, did not die from a fall.

This lower jaw (left) was one of several bones of a large brown bear found in an upper passage of Enigma Cave. Fred Grady (right) screens and searches cave sediments for small bones of rodents and fish at the beach camp. With his glasses removed, Fred's myopic eyes become a powerful binocular microscope!

Sediments in Enigma exist only in shallow local pockets. Like the bones recovered thus far, they are probably all of postglacial origin. Sediment samples were collected from several sites where fish bones (from otter scat) were abundant, and these are under study.

© 2002 by Timothy H. Heaton