Portland State professor identifies 15,000-year-old boulder at site of new Lake Oswego middle school

A boulder unearthed by Skanska while excavating space for a new middle school in Lake Oswego has been identified as 15,000-year-old rhyolite — a volcanic rock not known this side of the Cascade Mountains.

The boulder — measuring three feet long, three feet wide and two feet high and weighing in at

approximately 2,000 pounds — was likely moved to the area by one of the great Missoula Floods from either eastern Oregon or Canada via an iceberg, according to Portland State University’s Scott Burns.

“This is a major geological find,” Burns, a professor of geology, said. “Of all of the hundreds of boulders moved to the Willamette Valley by the Missoula Floods, this is only the second rhyolite boulder recorded here.”

Burns said the boulder was likely deposited in Lake Oswego between 15,000 and 18,000 years ago.

“Most of the unearthed boulders are composed of basalt, but this large formation of rhyolite likely arrived here thousands of years ago,” he said.

This discovery isn’t the first historical unearthing with ties to Portland State. In the 1970s, a PSU student dug up half of a mastodon skeleton in Tualatin, which is now on display at the Tualatin Library alongside other ancient animal bones from mammoths, bison and a giant sloth.

The rhyolite boulder will be donated to the Tualatin Ice Age Tourism Foundation for display at the Tualatin Heritage Center.