Imagine a when gigantic icebergs floated across the broad-shouldered Willamette Valley.

Rick Thompson is a detective of Oregon geologic history and he’s on trail of one of the region’s oldest mysteries: how hundreds of Montana granite stones ended up in farm fields in Oregon’s Willamette Valley.

“They’ve been in the ground a very long time,” noted Thompson on a recent field trip into western Washington County. “Farmers usually plow or till them up and they’re often just sitting where the icebergs left them as they melted.”

Icebergs in Oregon’s farm country?

“It’s true!” said Thompson, a member of the Lower Columbia Chapter of the Ice Age Floods Institute.

“The icebergs just floated around and then reached a certain area and sat there, melted and these rocks fell out, “ he added.

The evidence of icebergs is all around the metro area too; like the hiking trail at Fields Bridge Park along the Tualatin River in West Linn.  Three granite rocks totaling 46,000 pounds rest along the trail.

Thompson’s group, Ice Age Flood Institute, designed the paved, wheelchair-accessible trail, complete with several information kiosks along the riverside trail.  As you stroll you learn much about the remarkable events that occurred 15,000 years ago.

In fact, one kiosk offers a colorful map that Thompson created of the Willamette Valley that shows off the ancient “Lake Allison;” a 400-foot deep lake that stretched from Kalama, Washington to Eugene, Oregon.

The ancient lake was created when the massive Lake Missoula burst thru an ice dam and flooded the region.

“I used a topographic map and traced the four hundred foot depth level all the way down to Eugene. I drew all the nooks and crannies where the valleys would have filled with water and then I went back and put in all the major cities, towns and highways so people can have a sense and appreciation for how much water there was in the valley.”

Thompson is a self-proclaimed “flood nut” and said that the huge floods roared through the Columbia River Gorge with water lapping at the ridge tops.  The floods carried giant granite boulders called “Erratics” deep into the Willamette Valley across Lake Allison.

In fact, not long ago we flew with Roger Anderson and his Vista Air Balloon Adventures over the valley floor and saw the lasting impressions of the flood events.  It was a breath-taking ride to be sure, but it was also quite revealing for we could easily make out Lake Allison’s marks on the valley floor.

We could see the rise and fall of the river and lake bottom that was created during the ice age floods.

In addition, you don’t have to travel far to see and touch ancient history too.

There’s a huge erratic rock near Sheridan, Oregon at Erratic Rock State Park.  At 90 tons, it’s the biggest in the state and you can visit and touch the rock anytime.

Not far from that site is another huge erratic that you can visit and perhaps enjoy a glass of pinot too.

Pull into Left Coast Cellars Winery and see the second largest erratic in Oregon.

What makes Erratics so special?

“Oh, the distance from it’s source,” noted Thompson. “Plus, it’s all granite and to imagine the size of the iceberg that carried a 90 ton rock so far from its source is just amazing.”

The icebergs floated across Lake Allison for a time and most were pushed west by prevailing winds.  When the water dropped and the bergs melted, the granite chunks were left behind – like a ring around the bathtub.

“It affected the entire northwest and shaped the Willamette Valley,” said Thompson.

Moreover, the Lake Missoula Floods eventually brought pioneers to Oregon in a roundabout way.

It’s true! You see, the floods or rock, ice and other debris scoured the Eastern Washington landscape of all its rich topsoil and then deposited it in the Willamette Valley.  It was the same rich topsoil from which early Oregon pioneers built a thriving agricultural economy in the mid-19th century.

Thompson speculates, “It’s interesting because if the flood and erratic events not happened, Oregon agriculture might never have developed either.”  It is such a powerful and compelling story that nearby Tualatin, Oregon has embraced it too.

Last December, Yvonne Addington, President of the Tualatin Historical Society, helped arrange the delivery of two giant erratics that are now displayed at the Tualatin Heritage Center.

She said that local folks are betting the erratic story is something people will want to see and know better.  Put simply, she believes that “if you display the ancient rocks, people will come.”

“We have a strong interest in the ice age here,” said Addington. “A local man discovered a Mastodon skeleton in 1962 (it is displayed in the Tualatin City Library) and that has led into erratics conveying the power of nature that shaped of our community. It’s something that visitors and residents can enjoy and it has a special quality that no other city really offers.”

Back out in Washington County, Thompson continues to track down more erratics across farmland as he develops an “Ice Age Trail.  He wants travelers to someday journey the region and learn more about the powerful forces that shaped the Oregon we know today.


about author Grant McOmie

Grant McOmie is a Pacific Northwest broadcast journalist, teacher and author who writes and produces stories and special programs about the people, places, outdoor activities and environmental issues of the Pacific Northwest. A fifth generation Oregon native, Grant’s roots run deepest in the central Oregon region near Prineville and Redmond where his family continues to live.

In this Grant’s Getaway

Flag as Incorrect

Is any of the information on this page incorrect?

These maps and directions are for planning purposes only. You may find that construction projects, traffic, or other events may cause road conditions to differ from the map results. For travel options, weather and road conditions, visit, call 511 (in Oregon only), 800.977.6368 or 503.588.2941.

Share your thoughts Comments

Have something to say? Your Comment

  1. ERIK JOHNSON says…

    …….a great article that’ll enlighten many more of the diversity of our western geology. I studied many years ago thru Dr. Dave Korwin at Mt. Hood College, and when our geology class did a fieldtrip to the peak of St. Helens on my BD of 3/27/80 and the mountain poofed a little explosive burp and blew some ash, I had been smitten with studying the geological lore of our world where ever I am. Thank you, and keep up the good work…..

    Written on March 16th, 2012 / Flag this Comment
  2. Chris says…

    Any connection with the agates in the Agate Desert at the foot of Table Rock near Medford?

    Written on March 16th, 2012 / Flag this Comment
  3. Grant McOmie says…

    Thanks Erik! I had a similar experience in the 70’s w/a Geology Prof and as a result, developed “new eyes” to look at our region –

    Chris I do not know if there is a relationship between the Lake Missoula Floods and those two sites – you might send a note to Rick Thompson – his link is in the text of our story. He’ll know sure.


    Written on March 17th, 2012 / Flag this Comment
  4. Grace Gustafson says…

    These erratics are so interesting. The grandkids and I will be making a few trips to see them. Being a Christian I believe that these rocks came from Montana but only with the flood in the Old Testament. Perhaps there were glaciers when the great flood receded. In SD we also had erratics but didn’t know at the time that’s what they are called. Thank you for all the insight and information.

    Written on May 17th, 2012 / Flag this Comment
Win a Pendleton Blanket


Subscribe to the Travel Oregon email newsletter and be entered to win a commemorative Crater Lake Pendleton Blanket.

Click here for terms and conditions.

You're almost there!
Click the link in the email we just sent you to confirm your subscription.

Hmm, something went wrong, please try later.