Geological History of Crater Lake
The first point to fix our fascinated gaze is Wizard Island, lying nearly 2 miles away, near the western margin of the lake. Its irregular western edge and the steep but symmetrical truncated cone in the eastern portion are very suggestive of volcanic origin. We can not, however, indulge our first impulse to go at once to the island, for the various features of the rim are of greater importance in unraveling the earlier stages of its geological history.
The outer and inner slopes of the rim are in strong contrast; while the one is gentle, ranging in general from 10° to 15°, the other is abrupt and full of cliffs, as shown in figure 8. This difference is well expressed also by the contour map in figure 13. The vertical interval of the contours is 50 feet. Upon the inner slope the contours are crowded close together to show a slope so steep that one needs to travel but a little way to descend 50 feet, while upon the outer slope the contours are so far apart that to descend 50 feet one needs to travel a considerable portion of a mile. The outer slope at all points is away from the lake, and as the rim rises at least 1,000 feet above the general summit of the range, it is evidently the basal portion of a great hollow cone in which the lake is contained.