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”.

Here is a large erratic (Jay for scale) in Torres del Paine National Park, southern Chile. The lake in the background was formed by glaciers and the rock was carried by glacial ice and dropped when the ice at this location melted. Because ice is so viscous, it can carry extremely large rocks over distances of hundreds of km (even hundreds of miles).

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/.

Here is a map of the areas in the Pacific Northwest (blue color) that were impacted by the Ice Age Floods. The white-colored area at the top of the map shows the maximum southerly extent of the ice during the last glacial maximum. Incredibly, most of the flooding in Idaho, Washington, and Oregon was a result of ice dams in Montana! Ice created dams and a body of water called Glacial Lake Missoula. [The floods are sometimes referred to as the “Missoula floods”.] When these ice dams broke, catastrophic quantities of water flowed downstream, reaching all of the way west to Portland and south to the southern end of the Willamette Valley in Oregon. The National Park Service has a web site with this map and information about where to see features associated with these enormous floods: https://www.nps.gov/iafl/planyourvisit/index.htm.

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.

Here is the path to the natural site located southwest of McMinnville on State Highway 18. To us, it was fun to see Delfina vineyard where grapes were grown for the wines we had tasted just the day before! The short path leads up-slope to the erratic rock perched on the hilltop.
Here I am standing on top of the erratic rock. The metamorphic rock is unlike the underlying volcanic rocks of the region. Instead, this rock originated in the Northern Rocky Mountains and was carried to the Williamette Valley by the Ice Age floods (see map above). This rock is the largest erratic in the region. Try to imagine the valley below filled with water to the level of this hilltop. In the water were icebergs that entrained rock pieces, floated from Montana to Oregon, then dropped the rock pieces when the iceberg melted. This occurred only about 13,000 years ago, just yesterday in geologic time.

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.

This photo of Palouse Falls is from a Washington state web site: http://www.sevenwondersofwashingtonstate.com/the-channeled-scablands.html. The amount of water that currently flows over the Palouse Falls is much too small to have caused such extensive erosion of the basalt bedrock. Rather, this canyon was created by the Ice Age (or Missoula) floods. At this location the flood waters probably filled the canyon to the brim with a raging torrent—sufficient flow to deeply erode into the basaltic rock. 

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.

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  1. Dale Smith on June 15, 2023 at 9:52 pm

    I suspect you will be discussing the Willamette Meteorite!

    • Landscapes Revealed on June 15, 2023 at 10:07 pm

      Wow! I did not know that a large meteorite was ice rafted to Oregon during the Missoula floods. Looks like I’ll need to go to NYC to see it, although there is apparently a replica in Eugene. Thanks for the info.

  2. Leslie Dawson on June 16, 2023 at 6:29 am

    Didn’t these floods bring the soil to the Willamette Valley that the grapes love?

  3. Keeley Kirkendall on June 16, 2023 at 6:58 am

    You provide such a wonderful education. Thanks

    • Landscapes Revealed on June 16, 2023 at 9:03 pm

      Thanks Keeley!

  4. Gloria Mooney on June 19, 2023 at 11:13 am

    I’d like more info on the beautiful rolling hills —?

    • Landscapes Revealed on June 19, 2023 at 1:30 pm

      I think the hills are mainly a result of the underlying volcanic rocks. Some types are more resistant to erosion than other types. The young flood deposits have smoothed out some of the low-lying areas.

  5. Earle Sloan on June 23, 2023 at 10:27 am

    Thanks, Karen. I enjoyed your post. Keep them coming! (and enjoy the wine along the way)

    Earle Sloan

    • Landscapes Revealed on June 23, 2023 at 3:04 pm

      Thanks for reading Earle!

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