Historic Resource Study, Crater Lake National Park, Oregon, 1984
I. Formation of the Crater Lake Environment
A. Northern Plateau Area of Southern Oregon
The geology, altitude, and climate of the northern plateau area of southern Oregon forced specialized ecological adjustments on the part of the early aboriginal inhabitants. The physical feature most responsible for the specialization necessary for survival was the Cascade Mountain Range, a rugged continuance of the Sierra Nevadas north through California, Oregon, Washington, and sections of British Columbia. This chain originated perhaps forty million years ago from a weak north-south-trending seam in the earth’s crust. Through this fissure molten magma was ejected from the interior up through the inland sea that covered the region. After successive eons, this volcanic uplifting created a ponderous mountain chain rearing to an impressive height. During the next few million years, the creation of this mountain mass was followed by the formation of a series of huge, broad, shield volcanoes. These were ultimately replaced by the now familiar steep-sided volcanic cones stretching southward from Mount Garibaldi near Vancouver, British Columbia, and including Mounts Baker, Rainier, Adams, and St. Helens in Washington; Mounts Hood, Jefferson, Three Sisters, Mazama, and McLoughlin in Oregon; and California’s Mount Shasta and Lassen Peak.
The most significant effect of the Cascade Range and of its numerous high peaks was the creation of two distinct climatic zones in the present state of Oregon in which vegetation and animal life began taking on the singular characteristics unique to each one’s particular environment. As the Cascades deflected the moisture-laden winds rushing inland, lowering their temperatures and causing them to deposit their condensation on the lands adjacent to the ocean, it resulted in a lack of moisture in that sunny dry area immediately east of the mountains that is commonly referred to as a “rain shadow.” Of more consequence environmentally was the lack of rain in the very dry area of vast plains and desert flora on Oregon’s eastern plateau.