The hills date from the late
Cenozoic era, and range up to over . 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.
U.S. Route 26 (the Sunset Highway) is the principal transportation link through the hills, traveling through the
Vista Ridge Tunnels, 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. Since 1998, the
west side MAX Light Rail has run roughly parallel to US 26 through the hills, including a section
tunneled deep underground. ==Radio broadcasters==