History of Oregon/Formation and geology
Oregon is part of the Continental shelf of North America. The geological evidence suggests the western half Oregon was cemented on by the collision of the Siletzia plate, essentially a large volcanic island, with the original coast millions of years ago.
The western part of the state has a line of Volcanos and mountains that run North and South forming the Cascade Range. Another range of mountains along the coast also have signs of previous volcanism but are now dormant. The eastern part of the state is an semi-arid region of mostly flat-land that gradually increases in elevation as one goes East. The great Columbia River drains the waters of the region into the Pacific Ocean. A large portion of the state rotates in a clockwise direction according to GPS sensors.
Oregon was not affected by the massive Cordilleran Ice Sheets as Washington was, however the meltwater from the retreat created massive floods that surged down the Columbia River and reshaped the lower landscapes.