THE GEOLOGY OF CRATER LAKE NATIONAL PARK
By JOSEPH SILAS DILLER.
LIMITS OF THE CASCADE RANGE.
The western limit of the great volcanic field is likewise the western border of the Cascade Range, which is made up at least largely, if not wholly, of volcanic material erupted from a belt of vents extending from northern California to central Washington. Lassen Peak marks the southern end of the Cascade Range, and Mount Rainier is near the northern end. Beyond these peaks the older rocks rise from beneath the range and form prominent mountains, the range itself occupying a depression in these older terranes.
A clearer conception of the development of the Cascade Range may be gained by considering the geography of the region during the later portion of the Cretaceous. At that time the coast of northern California, Oregon, and Washington was subsiding with reference to sea level, causing the sea to advance upon the land. In California the sea reached the western base of the Sierra Nevada and covered a large part, if not the whole, of the Klamath Mountains. In Washington it beat upon the western base of the range near the coast north of Mount Rainier, and in Oregon it extended far into the interior. Marine deposits of this period occur along the base of the Blue Mountains in eastern Oregon, as the Cascade Range did not then exist in Oregon to shut out the open sea from that region. East of the Klamath Mountains, as shown by the position and distribution of the Cretaceous rocks and fossils of marine origin, the open sea connected directly with the sea in the Sacramento Valley. The Cascade Range throughout a large part of its extent rests upon Cretaceous rocks, and is associated in Oregon and California with a depression in the older rocks between the Klamath Mountains on the one hand and the Blue Mountains and Sierra Nevada on the other. This depressed area beneath the lavas of the Cascade Range must not be regarded primarily as a region of subsidence, as its chief movement since tine Cretaceous has been upward, above the sea. The Klamath and Blue Mountains, as well as the Sierra Nevada, however, have been elevated so much more than the base of the Cascade Range that it would appear on the surface as a depression were it not filled with lava. The depression is so deep where the Cascade Range is cut across by the Klamath and Columbia rivers that the bottom of the lavas forming the bulk of the range is not reached. However, at the ends of the range the older rocks rise to form a more or less elevated base for those parts of the range, and at Mount Shasta. as well as on the divide between the Rogue and Umpqua rivers where an arch of the older rocks extends northeasterly from the Klamath Mountains toward the Blue Mountains of eastern Oregon, the Cascade Range gets so close to the western side of the depression that the lavas lap up over the arch of older rocks rising to the westward. At various points of the range granolitic rocks, such as gabbro and diorite, occur; but the deep erosion at these points may have reached the granolites corresponding to the lavas of the upper portion of the range.
There can be no reasonable doubt that fossiliferous Cretaceous rocks of marine origin are widely distributed beneath the Cascade Range from Lassen Peak to the Columbia, and that during the Chico epoch the whole area was beneath the sea. At the close of the Cretaceous, important changes occurred in the distribution of land and sea. Northern California, as well as southern Oregon, was raised above the sea excepting the Coast Range region north of Rogue River, which remained beneath the sea during the early part of the Tertiary. The marine deposits of the Eocene period in the vicinity of Roseburg run under the Cascade Range, but have not yet been found on the eastern side. The conglomerates of the Eocene, like those of the Cretaceous, contain many pebbles of igneous rocks, but they are of types common to the Klamath Mountains and are rare or unknown among the lavas exposed in the Cascade Range. During the Eocene there was vigorous volcanic activitya in the Coast Range of Oregon, but the record of such activity has not yet been found in the Cascade Range. That volcanoes were active along the range during the Eocene is rendered more probable, although not yet beyond question, by Dr. J. C. Merriam's discovery of Eocene volcanic deposits in the John Day region.b
There can be no doubt, however, that during the Miocenea the volcanoes of the Cascade Range were most active and the greater portion of the range was built up, although it is equally certain that volcanic activity continued in the same region at a number of points almost to the present time. While it may be surmised that the volcanoes of the Cascade Range are extinct, there are many solfataras, hot springs, and fumaroles, showing that volcanic energy is not yet wholly dissipated. All the peaks of the Cascade Range were once active volcanoes, and from them came most of the lava of the range. Each great volcano was surrounded within its province, at least during the later stages, by numerous smaller vents from which issued the lava that filled up the intervening spaces and built up the platform of the range.
All of the great volcanoes of the range probably had their beginning in the Miocene. Many of them, like Lassen Peak and Mount Shasta, continued their activity into the Glacial epoch, and have suffered much erosion since their last eruptions. In this manner important structural differences have been brought to light among the peaks about the headwaters of the Umpqua, Rogue, and Klamath rivers, and these may be noted as throwing some light upon the history of Mount Mazama.
Last Updated: 07-Mar-2006