A very rare rock was found at a construction site in Lake Oswego recently.

Crews performing excavation for the Lakeridge Middle School project turned up an ice age erratic boulder – so named because it was transported on glacial ice broken apart by the massive Missoula floods some 15,000 to 18,000 years ago, according to a local expert.

Jacob Parker, a site superintendent with general contractor Skanska USA Building, spotted the yellowish and greenish boulder – roughly three feet long, two feet wide and two feet thick – and instinctively knew there was something different about it.

“I walked through and examined all of the rocks we unearthed, and this boulder stood out as very unique in color and texture,” Parker said. “I quickly realized it may have some historical or geological significance.”

Parker and the Lake Oswego School District contacted Portland State University geology professor emeritus Scott Burns, who also serves as president of the Tualatin Ice Age Floods Foundation – a nonprofit that seeks to build public knowledge of local geology. Burns visited the jobsite to examine the find.

“I went over and took a look at it and I was very excited,” Burns said. “I said we need to have this because it’s so different and we’ve never seen an erratic like this one.”

After further research, the boulder was found comprised of rhyolite – a volcanic rock rich in silica that is extremely rare in the Cascade Range. Burns surmised that the rock likely arrived in Lake Oswego from Eastern Oregon or even farther away via an ice raft thousands of years ago. That could account for the many boulders that crews discovered at the project site.

“What happened there is they were rebuilding the middle school there and the reports said you will be able to dig down and there will be no boulder bigger than maybe a foot in diameter,” Burns said. “Well, they have boulders they were digging out that were five, six, seven feet across. And they have a huge field of these boulders that they were not expecting.”

Most of the boulders are composed of basaltic lava, which is typically much more fluid and travels much farther from its source than the highly viscous rhyolite.

“This is a major geological find,” Burns 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.”

The event led to an in-person Pacific Northwest geology lesson for students.

“We have a significant real-life learning opportunity for the students here at Lakeridge with this amazing history right under our feet,” Lakeridge Middle School Principal Kurt Schultz said. “Once students graduate from Lakeridge they are freshmen and will study geoscience. So, they’ll have firsthand experience with a piece of historical earth science found right at their school.”

Following a ceremony at the school project site, the rhyolite boulder was donated to the Tualatin Ice Age Floods Foundation and then transported to the Tualatin Heritage Center by Brian Clopton Excavating. The rock is now available for public viewing.

The construction project consisted of demolition of Bryant Elementary School and Lakeridge Junior High School, redevelopment of the site, and construction of a new middle school, sports fields and grounds – all while classes continued on site.

The new 138,325-square-foot school building, completed in fall 2020, consists of a square of interconnected, two-story buildings with a courtyard and a walkway in the center. There is a wood structural shell and decks with steel columns, whole columns fabricated from 29 trees removed from the site, and interior metal studs. The exterior facade consists of fiber cement panels, brick and concrete masonry units.

The school’s courtyard, incidentally, was patterned after the kolks (underwater vortices) that formed during the Missoula floods as water flowed into the Willamette Valley.

“Some of the Native American legends actually talk about floods,” Burns said. “The valley filled up from one side to the other side and everybody had to use their canoes to get away. And maybe that (the Missoula floods) was one of the last ones, because Native Americans were here at that time – 15,000 calendar years ago.”

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