The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Foundations of Mount Mazama
The Western Cascade Volcanic Series
Climate and Flora
During the long interval between Upper Eocene and Upper Miocene times, when the Western Cascade series was accumulating, the climate gradually became cooler. The subtropical vegetation which flourished in the John Day Basin in Eocene time retreated westward and southward, and persisted during the Miocene in only a few favored places, where it was mixed with a temperate flora. The Oregon forests of the Miocene, writes Chaney,12 were “like those of today in the valleys of Michigan and Ohio, and in the Redwood Belt of California; they were essentially like those which had lived in the uplands during the Eocene.” The climate was mild and moist. Yet even the prolonged volcanic activity of Eocene, Oligocene, and Miocene times cannot have built a high and continuous range separating western from eastern Oregon, for redwoods continued to grow east of the present Cascades, showing that cool, moist sea winds still penetrated far inland as late as Upper Miocene times. Living there with the redwoods, according to Chaney, were black and live oaks, box elders, madroños, plane trees, and poplars. West of the present Cascades, some of the subtropical vegetation persisted in sheltered valleys.
It was not until the High Cascade volcanoes began to rise and the Western Cascade region had been elevated by folding that a mountain barrier shut off eastern Oregon from the supply of rain. Not until then did the redwoods disappear from east of the Cascades, nor until then did the floras on opposite sides of the range begin to show the striking differences we find today.
We may thus conclude that the region about Crater Lake, which had been a low subtropical plain dotted with volcanic cones at the close of the Eocene, had become by the close of the Miocene a fairly high volcanic plateau, clothed with temperate forests, through which flowed many large rivers and on which lay innumerable lakes formed by dams of lava. The coast line lay approximately where it is today.