was a great upheaval in the region. The low-lying land and adjacent sea bed were thrust up by forces below the earth's surface, and about the site of Baker became what were probably Oregon's first mountains; while the shallower sea bed, with its lime shales and volcanic rocks, became the Powder River Mountains.
At the opening of the Cretaceous period (sixty-five to one hundred and ten million years ago), sea surrounded the Klamath Mountains, flowing in from California over the site of Mount Shasta to what is now Douglas County and thence to the main ocean by a passage near the mouth of the Coquille River.
The close of the Cretaceous period saw the Blue and Klamath regions, with their accretions, separated by a sea dike that had been slowly rising out of the ocean bed from Lower California to the Aleutian Islands. The elevation of this barrier, the Sierra Nevada Range in California and the Cascade Range in Oregon and Washington, divided the State into two geologically, geographically, and climatically dissimilar parts. It made the region to the west a marine province, in which geologic changes were brought about by agencies existing in and emanating from the sea; it made the region to the east a continental province, the development of which was bound up with the large land mass of the continent.
Rising slowly, the dike shut out the sea from the interior and created three great drainage areas: one to the south, which in time became the Colorado River; one to the north, which in a later age formed the Columbia River Basin; and a third, in what is now southeastern Oregon, whose outlets were cut off and whose waters disappeared through evaporation. At the close of the Cretaceous period, the sea retreated and never again advanced farther than the present axis of the Cascades.
At the dawn of the Tertiary period or age of mammals, fifty million years ago, eastern Oregon was a region of lakes. The Blue Mountains and the Cascade hills were green with forests and beautiful with large flowering shrubs. Magnolia, cinnamon, and fig trees flourished. Sycamore, dogwood, and oak appeared. The Oregon grape, now the state flower, grew densely in the hills. Sequoias towered to imposing heights.
The earliest, or Eocene, epoch of the Tertiary period is represented by the first upthrust of the Coast Range, by the Monroe, Corvallis, and Albany hills, and by the Chehalem and Tillamook coal beds. The development of coal, however, was greatest along the Coos Bay coast. New land was forming in the next epoch, the Oligocene, as shown by
