Paper No. 147-5
Presentation Time: 9:10 AM
LAKERIDGE RHYOLITE ERRATIC BOULDER TRANSPORTED TO LAKE OSWEGO, OREGON BY THE MISSOULA FLOODS
In April of 2021 a 1000 kg greenish-tan rhyolite boulder was discovered during the construction of the Lakeridge Middle School in Lake Oswego, Oregon (just south of Portland). The discovery was made by the superintendent for Skanska Construction, Jacob Parker. It was found among a deposit of over 100 basalt boulders during the excavation of the new school. Rhyolite is not very rare on the western side of the Cascade Mountains, especially on the Willamette Valley floor. I suggest that it was most likely transported there by one or more of the 40 Missoula Floods that came through and carved out the valley where Lake Oswego lies today. The boulder probably originated in eastern Oregon or northeastern Washington. How did it get to the location? Was it ice-rafted on an iceberg or was it bed load that moved by bouncing along the river bottoms during the many megafloods. This boulder has now been moved to the nearby Tualatin Heritage Center where a collection of ice-rafted erratic boulders (granite, argillite) now lie. It is only the second rhyolite erratic boulder recorded related to the ice age floods in the last 100 years.