Colander Cave

Coronation Island, like Dall and Noyes Islands, is located far from the Alaskan mainland and faces the open Pacific Ocean. It does not appear to have been overrun by large glaciers during the Ice Age, though local mountain glaciers may have been present. The island therefore holds promise for revealing the Ice Age animal history of coastal Alaska and testing the Coastal Migration Theory for the first entry of humans into North America. All of Coronation Island is a wilderness area within Tongass National Forest with no development whatsoever. Unlike many of the other islands in Southeast Alaska, it remains in pristine condition. The island is made largely of limestone, so caves containing fossil deposits are present there. The first expedition to search for caves on Coronation Island was made in the summer of 2001. Paleontologist Tim Heaton was joined by cavers Pete Smith and Steve Lewis for a four-day trip. Transportation to the island was provided by Cat Woods and Terry Fifield of Tongass National Forest, who conducted an assessment of a different part of the island during the same period. Several caves with significant paleontological deposits were located during the trip.

The western side of Coronation Island contains some large, flat areas of karst that show no evidence of having been glaciated. It was on one of these lowland areas that Colander Cave was discovered.

Colander Cave was discovered by Steve Lewis in a lowland karst area about 800 meters from the ocean and 200 meters above sea level (see photos above). Several sinkholes are interconnected, forming a small, steep basin (hence the name Colander). One of these sinkholes contains a 20-meter deep vertical pit with a large cone of rock debris at the bottom. From there, horizontal passages extend in two directions and have not been fully explored. On one side of the talus cone is a large room with a flat floor of dirt and rock (see photos below). A brown bear skeleton, a deer skeleton, and several bird bones were found exposed on this floor.

At left is a photo looking up the entrance pit of Colander Cave, which is only accessible by rope. The right photo shows the rock pile at the base of the pit and the flat floor (in the background) where the fossils were discovered. One femur of the bear was found in the lower right corner of this photo, separated from the rest of the skeleton.

The deer skeleton in Colander Cave is from an adult female. None of the bones appear to be broken, and the skeleton is tightly clustered. The deer must have fallen into the cave, moved away from the rock pile, and died in its present location. One bone was collected and radiocarbon dated at 3,300 years old. Deer are the only large mammals living on Coronation Island today, and their bones are frequently found in pits. This one is exceptionally preserved because it moved away from the immediate area of rockfall to a safe location before it died.

The deer skeleton is tightly concentrated, and the bones are less weathered and less scattered than the bear skeleton. The white skull is visible in the upper right corner of the left photo. The humerus in the center of the right photo was collected for radiocarbon dating.

The brown bear skeleton in Colander Cave is scattered about the floor of a large room with some elements partly buried in sediment and others exposed among rocks. The skull and many of the other bones are broken. The maxilla at the left and the humerus at the right were collected for study and for radiocarbon dating. The femur in the photo below was found far from the other bones on the other side of the rock pile below the entrance pit, but it appears to have come from the same individual.

No bears live on Coronation Island today, so the discovery of a fossil bear was significant. Coronation Island is the third island in Southeast Alaska (after Prince of Wales and Dall islands) where brown bear fossils have been found outside of the modern range of this species. This bear was radiocarbon dated to 11,630 years ago, which is consistent with the age of postglacial brown bears found on the other islands. Obviously brown bears were abundant throughout the Alexander Archipelago during the early postglacial period since so many of their fossils have been found. This suggests that brown bears may have survived the Last Glacial Maximum on coastal refugia in Southeast Alaska, and there is genetic evidence that supports this theory (see section on Research Results: Mammals). Coronation Island would be one of the most logical places for such a refugium.

The Colander Cave brown bear is a medium-sized, young adult individual. The limb bones have nearly obtained their full length, but most of the grown plates remain unfused and were found separated from the shafts (see photos above and at right). Like the deer, this bear obviously fell into the cave and died. Many of the bones are broken, either from the fall or from rocks falling on them. The bones are scattered about the flat floor, and one femur was found on the opposite side of the rock pile from the rest of the skeleton (see photo at right). It is uncertain how some of the bones arrived on the flat, dirt floor far from the rock pile, but there were no tracks or any other evidence of disturbance by animals. The bones may have been scattered by snow or ice during a cold period.

Three bird bones were found closely associated in the cave sediment close to one of the bear bones. One is the humerus of a song bird, one an ulna of a small auklet, and the other an unidentified tibiotarsus. The sediment in Colander Cave has not been excavated to determine its depth. All the sediment appears to predate the death of the brown bear, so it could contain fossil remains dating to the Last Glacial Maximum. Funds are currently being sought to conduct an excavation in this cave in hopes of finding an older bone deposit buried in the floor of the cave.

   
© 2002 by Timothy H. Heaton