The Washington Cascade Range & Mt. St. Helens National Volcanic Monument: Road Log & Backcountry Geology

During the Middle Eocene, the Siletzia and Crescent terrains accreted basalt along the continental margin. Two large marine basins received sediments belonging to the Cowlitz Formation and Puget Group. The shoreline was positioned around Interstate 5 corridor or slightly east in Central Lewis County. Sedimentary deposits covered the Siletzia and Crescent oceanic basalts. The volcanic arc was positioned in Idaho and eastern Washington.The earliest Cascade Range volcanoes erupted in Western Washington producing high silica rhyolites and fragmental material during the Late Eocene time. Shield volcanoes erupted west of the present day Mount St. Heleens producing basalts.During Late Eocene, Oligocene, and Early Miocene, a large volcanic pulse produced a new volcanic arc east of the Late Eocene arc. Basalt and basaltic andesite yielded to andesite and dacite. Eruptions became more explosive. The Spirit Lake Pluton was injected near the end of this time period.During the Middle Miocene, volcanism in the Cascades slowed. Tectonic uplift eroded most of these volcanoes away. Dikes and sills became common. Flood basalt erupted from feeder dikes in eastern Washington, eastern Oregon, and Idaho. During Late Miocene, dikes and sills intruded north and south of Mount St. Helens. Gentle folding of rocks continued.During Pliocene through Pleistocene, volcanism in the Cascades picked up again. Glaciers covered much of the Cascade Range. For major glaciations are recorded. Mount St. Helens began erupting 40,000 years ago.Chapter 1 presents an introduction to the evolution of the Cascade Range. Chapter 2 presents an M.S. Thesis on the Cascadian Foothills. Chapter 3 discusses Southwestern Washington tectonic rotation. Chapter 4 begins with an introduction to stages and eruptive events of Mt. St. Helens. Chapter 5 begins the roadside geology of Mt. St. Helens National Volcanic Monument. Chapter 6 describes WA 503 road log geology and road logs to the south and east. Chapter 7 presents the final road log along US 12 from Randle to Salkum. Read more