The Shaping of the Willamette Valley

Rick Thompson, a member of the Lower Columbia Chapter of the Ice Age Floods Institute, is on a mission to find out how granite stones from Montana ended up scattered across farm fields in the Willamette Valley. 

On a recent trip to Washington County, Thompson said the stones have been around for quite a while.  

“Farmers usually plow or till them up and they’re often just sitting where the icebergs left them as they melted, Thompson said in a statement to KGW8. “The icebergs just floated around and then reached a certain area and sat there, melted and these rocks fell out.” 

The evidence of these icebergs is especially notable around the hiking trail along the Tualatin River in West Linn located at Fields Bridge Park, where three granite rocks that weigh a combined 46,000 pounds – or two tons – sit on the trail. 

A paved, wheelchair accessible trail designed by Ice Age Flood Institute, has several information kiosks that detail the events that happened 15,000 years ago. Thompson created one such map, showing the ancient 400-foot-deep Lake Allison that extended from Kalama, Washington to Eugene, Oregon. 

“I used a topographic map and traced the 400-foot depth level all the way down to Eugene,” Thompson said. “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.” 

Lake Alison was created from the flooding of Lake Missoula. 

Thompson said huge floods with water near the tops of the ridges on the Columbia Gorge had happened maybe as many as 100 times, carrying erratics, or giant boulders, into the Willamette Valley.  

Weighing in at 90 pounds, the largest erratic in the state is located at Erratic Rock State Park, near Sheridan. Not too far away at Left Coast Cellars Winery is where the second largest erratic in the state is located.  

Thompson said the reason erratics are so special is due to how far they are from their source.  

“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, Thompson said. 

Icebergs containing these rocks floated along Lake Allison, pushed west by strong winds. The melting of the icebergs and the receding of the water from Lake Allison shaped the valley, according to Thompson. 

The topsoil left after these events helped shape Oregon Agriculture as well. 

Thompson said, “It’s interesting because if the flood and erratic events had not happened, Oregon agriculture might never have developed either.” 

For more information, listen to Grant’s Getaways Podcast or read about the events in Grant’s Getaways book series.    

By: Hannah Ramsey