the structures in the John Day Valley and in northwestern Oregon. In the former region these are sedimentary rocks known to geologists as the John Day series. Late in the same epoch or early in the Miocene, vast flows of lava, now known as the Columbia lava formation, began to well up from the earth. This was an age of volcanism, when the Cascade hills, later to become mountains, belched clouds of ashes that were carried eastward to take part in filling the great eastern Oregon lakes; when vents opened in hillsides to pour out gigantic rivers of molten rock that filled the lakes and valleys to the east and surrounded lofty mountain peaks with a sea of basalt; when the great plateau now encompassing most of Oregon from the west slope of the Cascades eastward was formed. The Blue and Wallowa Mountain ranges of today rise above the plateau, but the effect of their height is minimized by the thick strata of lava surrounding their bases. Geologists pronounce the formation to be one of the three greatest lava flows of the world. Twenty-five successive flows have been counted in the Deschutes Valley, and as many as twenty in the Columbia River canyon.
Changes other than volcanic were also taking place in the Miocene epoch. The Umpqua Valley was being elevated above sea level. The Calapooya Mountains, which had been rising late in the preceding epoch (as indicated by recovered shell fossils), were extending to join the slowly developing Coast Range, thus excluding the sea from what is now southwestern Oregon. Toward the middle of the epoch comparative quiet returned. The old animal life of the earlier epochs of the Tertiary period had perished, and new types succeeded. Forests blossomed in new glory. By the close of the epoch the Coast Range had formed a solid wall paralleling the Cascade hills, and the Willamette Valley had been elevated above the sea.
During the Pliocene epoch which followed, land was elevated over all the area of western United States. The Oregon coast extended many leagues farther west than it does today. A period of coastal depression followed, and land which once was mainland is now submerged far out at sea. Volcanic activity reappeared in the Cascades, and toward the end of the epoch there was great activity in mountain building both along the coast and in the Cascade region. It was then that the Cascades attained their great height, erected their superstructure of peaks and castles, and were crowned with snow. The barrier thus raised shut out from the interior the warm moisture-laden Qcean winds, and turned the climate colder. By the middle of the following, or Pleistocene, epoch, the glacial age had come on.
