Pacific Coast Ranges

Lore & Background

The Oregon Coast Range's oldest portions are over 60 million years old, with volcanic activity and a forearc basin being the primary mountain building processes. Volcanic activity approximately 66 million years ago in the Cretaceous Period created offshore islands beginning in the southern portion of the current range. These Roseburg volcanics were followed by the Siletz River Volcanics in the northern portions, and lastly a series of basalt flows from the Columbia River basalts also added to these formations. Much of the formations are the result of pillow basalt formations created when a hot basalt flow rapidly cooled upon meeting the salt water of the ocean. These deposits offshore were then pushed into the continental plate as a forearc basin rotating slowly over millions of years. This tectonic collision forced the basalt formations (and newer sedimentary rock formations that include marine terrace deposits) upward and created the coastal range. By the Early Oligocene period c. 30 million years ago the current coastline was in place and erosion has continued to shape the range, primarily through rivers cutting deep valleys through the igneous and sedimentary rocks.

Reader's Guide

The Oregon Coast Range creates a rain shadow effect for the Willamette Valley, which lies to the east of the Coast Range and west of the Cascade Range. This rain shadow creates a stable climate in the valley, with significantly less rain than the Oregon Coast. On the western side of the range, the same rain shadow causes more precipitation to fall, contributing to the numerous rivers that flow into the Pacific Ocean. A mild maritime climate prevails throughout the range with temperature and precipitation varying due to elevation and distance from the coastline. Characteristics of the climate include cool dry summers followed by mild and wet winters. The majority of precipitation accumulates in the form of rain, with snow during the winter months at the higher elevations, but no permanent snow pack. Annual precipitation differs from 60 inches (1,500 mm) in some parts to up to 120 in (3,000 mm), with the higher amounts coming in the higher elevations. The average high temperature in January is 36.3 °F (2.4 °C), and the average high in July is 61.9 °F (16.6 °C).

Did You Know?

• Marys Peak in the Central Coast Range is the highest peak at 4,097 ft (1,248 m).
• The Oregon Coast Range extends over 200 miles (320 km) from the Columbia River to the middle fork of the Coquille River.
• Annual precipitation in the range differs from 60 inches (1,500 mm) in some parts to up to 120 in (3,000 mm).
• The Roseburg volcanics, dating to approximately 66 million years ago, are the oldest volcanic formations in the range.
• The range creates a rain shadow that causes the Willamette Valley to have significantly less rain than the Oregon Coast.

Ancient Volcanic Origins

The oldest portions of the Oregon Coast Range are over 60 million years old. Volcanic activity approximately 66 million years ago in the Cretaceous Period created offshore islands beginning in the southern portion of the current range. These Roseburg volcanics were followed by the Siletz River Volcanics in the northern portions of the range, and lastly a series of basalt flows from the Columbia River basalts also added to these formations with some smaller flows in-between. Much of the formations are the result of pillow basalt formations created when a hot basalt flow rapidly cooled upon meeting the salt water of the ocean. These deposits offshore were then pushed into the continental plate as a forearc basin rotating slowly over millions of years. This tectonic collision forced the basalt formations (and newer sedimentary rock formations that include marine terrace deposits) upward and created the coastal range. Additional basalt flows originated from Eastern Oregon and added to the layers that were uplifted, as the newer Cascade Mountains had not yet been formed. By the Early Oligocene period c. 30 million years ago the current coastline was in place and erosion has continued to shape the range, primarily through rivers cutting deep valleys through the igneous and sedimentary rocks.

Climate and the Rain Shadow

A mild maritime climate prevails throughout the Oregon Coast Range with temperature and precipitation varying due to elevation and distance from the coastline. Characteristics of the climate include cool dry summers followed by mild and wet winters. The majority of precipitation accumulates in the form of rain, with snow during the winter months at the higher elevations, but no permanent snow pack. Annual precipitation differs from 60 inches (1,500 mm) in some parts to up to 120 in (3,000 mm), with the higher amounts coming in the higher elevations. The average high temperature in January is 36.3 °F (2.4 °C), and the average high in July is 61.9 °F (16.6 °C) with temperatures also varying by elevation. The further inland and the more southerly portions have a more Mediterranean climate that is more similar to the climate of the Willamette Valley. The Coast Range creates a rain shadow effect by forcing moisture laden clouds to rise by expelling moisture. This shields the Willamette Valley and causes a less maritime climate with hotter summers and less precipitation than the Oregon Coast.

Living Forests and Aquatic Life

The Oregon Coast Range is a heavily forested mountain range. Forests in the Northern section are considered to be some of the most productive timber land in the world. Trees include primarily Sitka spruce, western redcedar, Douglas-fir, and western hemlock. Other plants include huckleberry, salmonberry, salal, vine maple, Oregon grape, bracken fern, and thimble-berry among others. The mountains are home to a variety of wildlife including black bear, elk, deer, beaver, and many species of birds. Fish including salmon and trout, in addition to many other aquatic organisms inhabit the range's streams and rivers. Both the state and federal government manage forests throughout the Oregon Coast Range, where logging is a major industry in both private and government owned forests.

Three Distinct Sections

The Oregon Coast Range is divided into three separate sections: North, Central, and South. In the south is the oldest portion of the range, with formation beginning in the Paleocene era with the Roseburg volcanics, while the newest section is the northernmost portion, formed first with the Siletz River Volcanics. The Central and Northern sections contain more sedimentary rocks from the mud, silt, sand, and other volcanic debris than the lower Southern section. The Northern section, located in the northwest portion of Oregon, has peaks as high as 3,706 feet (1,130 m) for Rogers Peak. The Central section, located between the Salmon River and the Umpqua River, contains mountains as high as 4,097 ft (1,248 m) for Marys Peak. The Southern section, located between the middle fork of the Coquille River and the Umpqua River, contains mountains as high as 3,547 ft (1,081 m) for Bone Mountain.