Bumper Cave


A Forest Service helicopter transports personnel and gear to a subalpine meadow one kilometer from Bumper Cave (left), and a temporary camp is constructed with a spectacular view of Calder Mountain (above). The cave is located through the forest beyond the weatherport.

In 1993, while the Tongass Cave Project was exploring cave systems in the subalpine area east of Calder Mountain, another cave filled with bear bones was discovered. This cave was almost identical in size and configuration to the Hibernaculum in El Capitan Cave, and it had obviously been used by bears as a den. In 1994, with small grants from the University of South Dakota and the National Speleological Society, and with extensive logistical support from Tongass National Forest, Dr. Heaton excavated this cave over the course of ten days. He was assisted Dave Love, and also received help from other cavers and from Cat Woods and Terry Fifield of the Forest Service. Because of its remote location, the crew and gear were flown in and out by helicopter (see photos above).

A tarp was rigged over the entrance of Bumper Cave to provide shelter from the rain (see photo at right). The entrance crawlway led to a low horizontal room filled with rocks and dirt with weathered bear bones scattered around. A second tight, muddy crawlway led to a long sloping room with better preserved bear remains. At the end of the passage, at the bottom of a two meter dropoff, was a small stream and a low room containing a nearly complete brown bear skeleton (see photos below). Some parts of the skeleton had fallen into a "drain hole" where the tiny stream disappeared, but most were exposed and had excellent preservation. All the major bones were recovered with the curious exception of the lower jaws. The skeleton appeared to be of a female, and next to her were the partly buried skeletons of two small juvenile bears in the same stage of development. It appeared likely that a mother had died in the back of her den with her cubs.

A grid system was established for the cave, and bones were mapped and photographed then removed. They were washed in a stream at camp and dried and cataloged in the weatherport. Sediments were collected from the more productive areas for screening. To wash the sediments, 300 meters of fire hose were rigged from the nearest stream to a location about half a kilometer from the cave. This provided high pressure water for washing sediments through box screens and mosquito net bags. A total of 113 gallon-size bags of sediment were packed to the washing station and processed. Much of the sediment was pure clay and was very hard to break down. As at El Capitan Cave, the deeper, inorganic sediments were not fossiliferous.

As at El Capitan Cave, the oldest, most complete, and best preserved bear skeletons and bones were found deepest in the den passage, whereas bones nearer the entrance were much more scattered and fragmentary. At least twelve individuals were recognized from Bumper Cave based on duplicated parts and different stages of development, and all of them best matched brown bear (Ursus arctos). Only a single claw was a better match for black bear (Ursus americanus). The complete skeleton from the back of the cave was radiocarbon dated to 11,645 years old. A bone from a larger individual deep in the cave dated to 11,225 years old. A tooth and jaw from the entrance room were dated at 10,970 and 7,205 years old. The latter date is the youngest date obtained so far on a brown bear from the southern Alexander Archipelago and may be close to the date of this species' disappearance. Most of the bears seemed to be females or juveniles (not a single baculum was found), but one tooth and a few vertebrae from the entrance room came from giant individuals (probably males).


At left is the brown bear skull after it was removed from the cave. Above is a sliver of a caribou metacarpal--the first remnant of of caribou from Prince of Wales Island.

The only mammal beside bear that was recognized during the excavation was long-tailed vole (Microtus longicaudus), whose teeth were found in great numbers during the screening process. This incredibly low diversity for a well-preserved cave deposit could only be explained by a combination of factors. First, the cave is located in a northern region, where diversity tends to be low compared to lower latitudes. Second, the cave is located on an island which has substantial colonization barriers. Third, the cave is located at high elevation, and most of the island's mammals are closely associated with the coast and cannot travel from the shore to such a remote location.

A year following the excavation when bone fragments were being identified at the Smithsonian Institution, it was noticed that a sliver of long bone belonged not to bear, but to caribou (Rangifer tarandus, see photo above)! This represented yet another species not living on Prince of Wales Island today. Forest Service geologist Jim Baichtal had suggested that caribou might have lived in the Alexander Archipelago because a historic population of this ungulate was known from the Queen Charlotte Islands to the south in the early 1900s. The development of the rainforest appears to have caused the demise of this species on both islands. Since this discovery at Bumper Cave, more complete caribou bones (and an antler) have been found in two other caves (in the back of El Capitan Cave and in On Your Knees Cave).

   
© 2002 by Timothy H. Heaton