High and Western Cascades
Crater Lake, the remains of Mount Mazama
Arising near Crater Lake, the Rogue River flows from the geologically young High Cascades through the somewhat older Western Cascades and then through the more ancient Klamath Mountains. The High Cascades are composed of volcanic rock produced at intervals from about 7.6 million years ago through geologically recent events such as the catastrophic eruption of Mount Mazama in about 5700 BCE. The volcano hurled 12 to 15 cubic miles (50 to 63 km3) of ash into the air, covering much of the western U.S. and Canada with airfall deposits. The volcano's subsequent collapse formed the caldera of Crater Lake.
Older and more deeply eroded, the Western Cascades are a range of volcanoes lying west of and merging with the High Cascades. They consist of partly altered volcanic rock from vents in both volcanic provinces, including varied lavas and ash tuffs ranging in age from 0 to 40 million years. As the Cascades rose, the Rogue maintained its flow to the ocean by down-cutting, which created steep narrow gorges and rapids in many places. Bear Creek, a Rogue tributary that flows south to north, marks the boundary between the Western Cascades to the east and the Klamath Mountains to the west.
Much more ancient than the upstream mountains are the exotic terranes of the Klamath Mountains to the west. Not until plate tectonics separated North America from Europe and North Africa and pushed it westward did the continent acquire, bit by bit, what became the Pacific Northwest, including Oregon. The Klamath Mountains consist of multiple terranes—former volcanic islands and coral reefs and bits of subduction zones, mantle, and seafloor—that merged offshore over vast stretches of time before colliding with North America as a single block about 150 to 130 million years ago. Much of the Rogue River watershed, including the Rogue River canyon, the Kalmiopsis Wilderness, the Illinois River basin, and Mount Ashland, are composed of exotic terranes.
Among the oldest rocks in Oregon, some of the formations in these terranes date to the Triassic, nearly 250 million years ago. Between 165 and 170 million years ago, in the Jurassic, faulting consolidated the Klamath terranes offshore during what geologists call the Siskiyou orogeny. This three- to five-million-year episode of intense tectonic activity pushed sedimentary rocks deep enough into the mantle to melt them and then forced them to the surface as granitic plutons. Belts of plutons, which contain gold and other precious metals, run through the Klamaths and include the Ashland pluton, the Grayback batholith east of Oregon Caves National Monument, the Grants Pass pluton, the Gold Hill pluton, the Jacksonville pluton, and others. Miners have worked rich deposits of gold, silver, copper, nickel, and other metals in several districts of the Klamaths. Placer mining in the mid-19th century soon led to lode mining for gold. Aside from a mine in eastern Oregon, the Greenback Mine along Grave Creek, a Rogue tributary, was the most productive gold mine in Oregon.
Serpentine, a rock type found along the Illinois River
In Curry County, the lower Rogue passes through the Galice Formation, metamorphosed shale, and other rocks formed when a small oceanic basin in the merging Klamath terranes was thrust over other Klamath rocks about 155 million years ago. The lowest part of the seafloor of the Josephine Basin, as this ancient sea came to be called, rests on top of the Kalmiopsis Wilderness, where it is known as the Josephine ophiolite. Some of its rocks are peridotite, reddish-brown when exposed to oxygen but very dark green inside. According to geologist Ellen Morris Bishop, "These odd tawny peridotites in the Kalmiopsis Wilderness are among the world’s best examples of rocks that form the mantle." Metamorphosed peridotite appears as serpentine along the west side of the Illinois River. Chemically unsuited for growing plants, widespread serpentinite in the Klamaths supports sparse vegetation in parts of the watershed. The Josephine peridotite was a source of valuable chromium ore, mined in the region between 1917 and 1960.
At the mouth of the Rogue River, along the coast of Curry County, is the Otter Point Formation, a mélange of metamorphosed sedimentary rocks such as shales, sandstones, and chert. Although the rocks formed in the Jurassic, evidence suggests that they faulted north as part of the Gold Beach Terrane after the Klamaths merged with North America. Oregon's only dinosaur fragments, those of a hadrosaur or duck-billed dinosaur, were found here. In the mid-1960s, a geologist also discovered the beak and teeth of an ichthyosaur in the Otter Point Formation. In 2018, a geologist from the University of Oregon found a toe bone of a plant-eating dinosaur near Mitchell in the east-central part of the state where the coast lay 100 million years ago. This discovery has also been billed as the first dinosaur fossil find in Oregon.