Tualatin Mountains
Encyclopedia
The Tualatin Mountains are a range of hills on the western border of Multnomah County
Multnomah County, Oregon
Multnomah County is one of 36 counties in the U.S. state of Oregon. Though smallest in area, it is the most populous as its county seat, Portland, is the state's largest city...

, Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...

, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. A spur of the Northern Oregon Coast Range
Northern Oregon Coast Range
The Northern Oregon Coast Range is the northern section of the Oregon Coast Range, in the Pacific Coast Ranges physiographic region, located in the northwest portion of the state of Oregon, United States. This section of the mountain range, part of the Pacific Coast Ranges, contains peaks as high...

, they separate the Tualatin Basin
Tualatin Valley
The Tualatin Valley is a farming and suburban region southwest of Portland, Oregon in the United States. The valley is formed by the meandering Tualatin River, a tributary of the Willamette River at the northwest corner of the Willamette Valley, east of the Northern Oregon Coast Range...

 of Washington County, Oregon
Washington County, Oregon
- Major highways :* Interstate 5* Interstate 205* U.S. Route 26* Oregon Route 6* Oregon Route 8* Oregon Route 10* Oregon Route 47* Oregon Route 99W* Oregon Route 210* Oregon Route 217* Oregon Route 219-Demographics:...

 from the Portland Basin of western Multnomah County and Clark County
Clark County, Washington
Clark County is a county located in the southwestern part of the U.S. state of Washington, across the Columbia River from Portland, Oregon.Clark County was the first county of Washington, named after William Clark of the Lewis and Clark Expedition...

, Washington.

The highest peak in the range is Cornell Mountain at 1270 feet (387.1 m). Other notable peaks include Council Crest
Council Crest Park
Council Crest Park is a city park in southwest Portland in the U.S. state of Oregon. Amenities include paved and unpaved paths, a dog off-leash area, picnic tables, public art, a vista point, and a wedding site that can be reserved. The park, operated by Portland Parks & Recreation, is open...

 at 1073 feet (327.1 m), the highest point in the city of Portland, and Pittock Hill, location of the Pittock Mansion
Pittock Mansion
The Pittock Mansion is a French Renaissance-style "château" in the West Hills of Portland, Oregon, USA, originally built as a private home for The Oregonian publisher Henry Pittock and his wife, Georgiana. It is a 22 room estate built of Tenino Sandstone situated on that is now owned by the...

.

The hills date from the late Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

 era, and range up to over 1000 ft (305 m). Composed mainly of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

, the mountains were formed by several flows of the Grande Ronde basalt flows that were part of the larger Columbia River basalts
Columbia River Basalt Group
The Columbia River Basalt Group is a large igneous province that lies across parts of the Western United States. It is found in the U.S. states of Washington, Oregon, Idaho, Nevada, and California...

. Human settlement goes back 10,000 years to the area's earliest known residents, the Chinook people.

Despite steep slopes, periodic landslides, and multiple earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

 faults, many residences have been built in the Tualatin Mountains, though much of the northern portion is undeveloped land within the 5,000 acre (20.23 km²) Forest Park
Forest Park (Portland)
Forest Park is a public municipal park in the Tualatin Mountains west of downtown Portland, Oregon, United States. Stretching for more than on hillsides overlooking the Willamette River, it is one of the country's largest urban forest reserves...

. The landscape, inside and outside the park, is predominantly forested.

U.S. Route 26 (the Sunset Highway) is the principal transportation link across the hills, traveling through the Tanner Creek Canyon and over the pass at Sylvan. This route through the hills
Canyon Road
Canyon Road is a road connecting Beaverton and Portland, Oregon, United States. It was the first road between the Tualatin Valley and Portland and contributed significantly to Portland becoming the area's major deep water port, and subsequent early growth of the city.The total modern length is...

 connecting the agricultural Tualatin Basin to the navigable Willamette River
Willamette River
The Willamette River is a major tributary of the Columbia River, accounting for 12 to 15 percent of the Columbia's flow. The Willamette's main stem is long, lying entirely in northwestern Oregon in the United States...

 was developed as a plank road
Plank road
A plank road or puncheon is a dirt path or road covered with a series of planks, similar to the wooden sidewalks one would see in a Western movie. Plank roads were very popular in Ontario, the U.S. Northeast and U.S. Midwest in the first half of the 19th century...

 in the 19th century. The Great Plank Road (Canyon/Jefferson Road) was a major factor in the early growth of the city of Portland.

Since 1998, the west side
MAX Blue Line
The MAX Blue Line is a 33 mile light rail line in the MAX Light Rail system in the Portland metropolitan area in the U.S. state of Oregon. Operated by TriMet, the line runs between Hillsboro and Gresham, via downtown Portland...

 MAX Light Rail has run roughly parallel to US 26 through the hills, including a section tunneled deep underground
Robertson Tunnel
The Robertson Tunnel is a tunnel through the west hills of Portland, Oregon, U.S. for the TriMet public transit MAX Light Rail system. The tunnel is 3.1 miles long and consists of twin 21 foot diameter tunnels. There is one station within the tunnel at Washington Park, the third deepest such...

.
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