Geological History of Crater Lake
Crater Lake is deeply set in the summit of the Cascade Range, about 65 miles north of the California line. It may be reached, as shown in figure 1, by two routes, one from the Southern Pacific Railroad at Medford or Ashland on the west, and the other from the Southern Pacific at Klamath Falls or Big Spring, just beyond the limit of the map (fig. 1) on the east.
Ashland and Medford are in Rogue River Valley, which marks the line between the Klamath Mountains of the Coast Range on the west and the Cascade Range on the east. The journey from Medford by private conance 80 miles to Crater Lake affords a good opportunity to observe some of the most important features of this great pile of lavas. The Cascade Range in southern Oregon is a broad irregular platform, terminating rather abruptly in places upon its borders, especially to the westward, where the underlying Cretaceous and Tertiary sediments come to the surface. It is surmounted by volcanic cones and coulees (fig. 3), which are generally smooth, but sometimes rough and rugged. The cones vary greatly in size and are distributed without regularity. Each has been an active volcano. The fragments blown out by violent eruption have fallen upon the volcanic orifice from which they issued and built up cinder cones. From their bases have spread streams of lava (coulees), raising the general level of the country between the cones. From some vents by many eruptions, both explosive and effusive, large cones, like McLoughlin, Shasta, and Hood, have been built up. Were we to examine their internal structure, exposed in the walls of the canyons carved in their slopes, we should find them composed of overlapping layers of lava and volcanic conglomerate, a structure which is well illustrated in the rim of Crater Lake.