During the noon hour on July first, Park Headquarters at Crater Lake was engulfed in a cloud of dust. This dust originated from a rock slide on an adjacent talus slope. Slides of this sort may be expected on any talus slope soon after the disappearance of the winter's snow which covers them. The reason for rock movement on such a talus slope is made clear when one recalls the fact that water expands upon freezing. The water from the heavy early winter rains soon fills the small interspaces of the loose talus rock material and upon solidification forces the rock upward in a vertical manner from its horizontal plane. In the early spring when thawing begins the frozen water again condenses and the raised rock materials are lowered but not along the same verticals in which they were raised. This is due to the fact that their center of gravity has been changed in reference to their previous position before being raised by the frozen waters. As a result of this new position of gravity now extends a force along a new vertical which will cause the rock to fall toward the foot of the mountain. This downward movement of the rock from its first resting places may be slight. Nevertheless, it may have been sufficient to place a number of loose rocks in a precarious position so that a rock-slide could easily be started on the talus slope.