Six miles west of McMinnville just off of Hwy 18 sits a 90-ton rock, the Bellevue Erratic, that was floated as much as 500 miles in an iceberg by way of the Columbia River during the Ice Age Floods.
The largest iceberg erratic found in the Willamette Valley, it’s originally from the Northern Rocky Mountains.
When the iceberg in which it was encased melted, the rock was left behind at the 300 foot elevation level.
A short uphill hike leads visitors to the Erratic Rock State Natural Site, where they can look out across the vast landscape and imagine the huge amount of water that filled the Willamette Valley during the Ice Age Floods.
The Bellevue erratic rests upon a gentle hill 150 ft above the floor of Oregon’s bucolic Willamette Valley.
The erratic floated in on an iceberg during a Missoula mega-flood when floodwaters backed up to 400 ft elevation behind the Kalama Gap choke-point, forming temporary Lake Allison.
This is akin to Wallula Gap forming terminal Lake Lewis and Lake Condon on the southern/downstream side of the Gap, and the erratics found at ~1200ft asl there.
The erratic Bellevue boulder consists of banded argillite, which can be traced back 400 miles or more to the 1.5 BILLION-year-old Belt Supergroup that lay beneath the ice dam for glacial Lake Missoula in northern Idaho!
This is the source of pinkish Missoula flood sediments, as well.
The discovery that erratic rocks were found at or below the 400 foot elevation in the Willamette Valley indicated that the water inundated this region from Portland down to Eugene up to 400 feet above present day sea level!
The Portland region owes its rich agriculture, beautiful geography, and many handsome erratics to a series of massive Ice Age Floods that burst from an ice dam, the last of them about 15,000 years ago with a few smaller Glacial Lake Columbia mega-flood pulses finishing off around 14 kya.
