Trees approaching a diameter of may be used to construct a dam, although the average is 10 to 30 cm. The length depends on the diameter of the tree and the size of the beaver. There are recorded cases of beavers felling logs of as much 45 m tall and 115 cm in diameter. Logs of this size are not intended to be used as structural members of the dam, but rather the bark is used for food, and sometimes to get to upper branches. It takes a beaver about 20 minutes to cut down a 15 cm wide aspen, by gnawing a groove around the trunk in an hourglass shape. A beaver's jaws are powerful enough to cut a 1.5 cm sapling in one bite.