Though the sun shone brightly, it was a cool and breezy Saturday morning when we gathered at the Portland Community College parking lot for the Spring Term Geology Field Trip. About twenty-four students and an inquisitive golden retriever joined with Professor Melinda Hutson to explore the northern Oregon Coast Range northwest of Portland. We car-pooled in ten vehicles that formed the convoy and headed across Portland to Highway 26 toward the coast. Though we had instructions, and two navigators, Shane and myself, to help our driver, Eva, we managed to drive past our off ramp somewhere in the tangle of Portland downtown overpasses.
Highway 26 tunnels through the east-slope of the Portland Hills from the Willamette Valley and rises to the top of the Sylvan heights before it begins a descent along the Cedar Hills Overpass into the Tualitin basin. These hills are made up of a mixture of basalts, conglomerations, and their resulting weathered clays. The Tualitin basin is more than fifteen miles wide in places and has many layers of Pliestoscene rock, sand, and gravel deposited by the hundred or so Lake Missoula Floods that washed down the Columbia Gorge. Below these deposits lay the plateau basalt flows that are part of the great Columbia Plateau Basalts (also called Columbia River Basalts) of the Miocene. These basalts were seen on the Coast Geology Field Trip where they were formed in pillow basalt formations indicating that they were deposited under water. In the cuts here along Highway 26 we can see the basalts are formed in columnar jointing and were not formed under water.
We stopped at 6 sites along this 134 mile loop Northeast of Portland, along and north of Highway 26 near the community of Vernonia, Oregon. The map below shows the location of each geological site we examined. Click on a site number and take the virtual field trip.
Created by STANLEY R. SANDERS, 2000