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. Sometimes the entrance to a den is partly closed by the occupant. Once in, he reaches out and claws the lower part full of earth, or rakes in trash and leaves. In most instances nothing is done to close the entrance. The snows drift back into the den, pile upward, and at last close the entrance most effectually. All the dens that I recall were upon northerly or easterly--the cooler--slopes. The snow as it fell would be likely to remain and close or blanket the entrance all winter long. Snow evidently enters into the grizzly's winter plans. Late one cold, snowless December I came upon a grizzly carrying spruce boughs into his den. Evidently he had used the den and found it cold. The den had a large opening; this he may have been intending to close. The rocky floor was already piled a foot deep with boughs. I have seen two other dens with floor-coverings; one of these was of pine twigs, and the other of coarse grass and kinnikinnick. But in most cases the bear sleeps upon the uncovered rocks or the naked earth. Snow is a factor in determining when a bear begins his winter sleep. If he is fat and food is scarce, an early, heavy snow is pretty certain to cause him to turn in early. If no snow comes and food is still to be had, the bear is likely to delay his hibernation.