West slope of the mountains from Beaverton looking north
|Elevation||1,609 ft (490 m)|
|Topo map||USGS Linnton|
The Tualatin Mountains (also known as the West Hills or Southwest Hills of Portland) are a range on the western border of Multnomah County, Oregon, United States. A spur of the Northern Oregon Coast Range, they separate the Tualatin Basin of Washington County, Oregon, from the Portland Basin of western Multnomah County and Clark County, Washington.
The highest peak in the range is Dixie Mountain at 1,609 feet (490 m). Other notable peaks include Cornell Mountain at 1,270 feet (390m), Council Crest at 1,073 feet (327 m), and Pittock Hill, location of the Pittock Mansion.
The hills date from the late Cenozoic era, and range up to over 1,000 feet (300 m). Composed mainly of basalt, the mountains were formed by several flows of the Grande Ronde basalt flows that were part of the larger Columbia River basalts. 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 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 km2) Forest Park. The landscape, inside and outside the park, is predominantly forested.
U.S. Route 26 (the Sunset Highway) is the principal transportation link through the hills, traveling through the Vista Ridge Tunnel, Tanner Creek Canyon, and over the crest of Sylvan Hill. This route through the hills connecting the agricultural Tualatin Basin to the navigable Willamette River was developed as a plank road in the 19th century. The Great Plank Road (Canyon/Jefferson Road) was a major factor in the early growth of the city of Portland.