In climbing along the bottom of a deep, narrow ravine one November day, I saw on the slope above me what appeared to be a carload or more of freshly dumped earth. My first thought was that a prospector was at work driving a tunnel; but upon examination it proved to be a recently finished but not yet occupied hibernating-den. The entrance was about three feet in diameter. Just inside the den was a trifle larger. It extended, nearly level, about twelve feet into the mountain-side. At the back it was six feet across and four feet high.
The size of the den varies and is apparently determined by the character of the soil in which it is made and also by the inclination of the bear making it. Most other dens measured were smaller than this one.
The grizzly may use the same den for several winters or have a new one each year. He may dig the den himself or take an old one that some other bear has used, or he may make use of one shaped by Nature--a cave or a rock-slide. I knew of one grizzly hibernating in a prospector's abandoned tunnel. Sometimes, like the black bear, he will dig a den on a steep mountain-side beneath the widely spreading roots of trees; sometimes beneath a large fallen log, close to the upturned roots which support it. In crossing the mountains one February I noticed a steamy vapor rising from a hole in the snow by the protruding roots of an overturned tree. I walked to the hole to investigate. The vapor was rank with the odor of a bear. Near my home on the slope of Long's Peak I have known grizzlies to den up beneath the snow-crushed, matted tree-growths at the timber-line, at an altitude of about eleven thousand feet.
Twice I have known bears to hibernate in enormous nests that were made of the long fibres of cedar bark. It must have taken days to construct one of these nests, as more than forty cedar trees had been more or less disrobed to supply material for it. It resembled the nests of trash that razor-back hogs in the South construct, though much larger. The bear, after piling it up, worked his way in near the bottom, somewhat after the fashion of a boy crawling into a haycock. Over this hibernating-nest the snow spread its blanket and probably afforded all the protection needed.