Oregon’s Erratic Rock State Natural Site

One of Oregon’s smallest natural areas is Erratic Rock State Natural Site, located 12 km (seven miles) southwest of McMinnville on State Highway 18. The site is located 75 km (45 miles) southwest of Portland. In March, while in McMinnville on a wine-tasting excursion with friends, Jay and I visited the erratic rock.

What is an erratic rock?

During most of Earth’s geologic history, our planet has been free of ice, even at the poles. This condition is called a “greenhouse” state. However, at other times, Earth has had extensive ice cover; this condition is called an “icehouse” state. During the past ~2.5 million years, Earth has been in an icehouse state consisting of glacial and interglacial periods that cycle back and forth every 100,000 years or so. The last glacial maximum was about 18,000 years ago, when ice covered most of Canada and parts of the northern United States. During the past 18,000 years, Earth has transitioned to our current interglacial period. As the ice advanced and then retreated poleward, it left behind notable landscape features that tell us about its movements through time.

One informative feature is called a glacial erratic. This is a rock that is different than the underlying bedrock because it was carried by glacial ice far from where it originally formed. The name “erratic” is from the Latin word “errare”, meaning to make a mistake, or to be wrong. Of course, there is nothing “wrong” with these rocks; they are just “out of place”.

Floods that created erratics in the Pacific Northwest

Earth is in an “Ice Age” now—during both glacial and interglacial periods. However, in common usage, just the glacial periods, when ice advances, are typically referred to as “Ice Ages”. From about 18–13,000 years ago, as Earth was transitioning from a glacial period to our current interglacial period, glaciers were melting and surging downslope. In the process, the ice periodically dammed rivers to create lakes. When the ice dams broke, immense quantities of water rushed downstream in what is referred to as the “Ice Age Floods”. [I have to admit—”Ice Age floods” sounds more dramatic than “Glacial Period floods”!] The landscape of the Pacific Northwest was transformed by the floods. For example, most of Portland is built on sediments that were carried westward during flood surges. The Ice Age Floods Institute provides extensive information about the floods: https://iafi.org/iafi/about-the-ice-age-floods/.

Erratic Rock State Natural Site

An easy place to visit in northern Williamette Valley is Oregon’s Erratic Rock State Natural Site. Oregon’s state parks web site has a description: https://stateparks.oregon.gov/index.cfm?do=park.profile&parkId=96.

Future visits

There are many other places where effects of the Ice Age floods can be seen. For example, the “Channeled Scablands” are a part of Washington State where the landscape includes large erosional features created by the torrents of water that flowed through the region.

The Ice Age Floods National Geologic Trail (see yellow and red lines on map above) links places accessible to the general public. I’ll certainly be visiting some of these sites, and revealing more of these glacially-affected landscapes, in future posts.