Brad Withrow-Robinson, Forestry & Natural Resources Extension Agent, Benton, Linn and Polk Counties. 

I was recently on a tour where we looked at how growing conditions, productivity and plant communities all change across the landscape according to elevation, soils, rainfall, aspect and other factors.  These are often included in the term “site productivity”.  These factors give important insights to the ability of a site to support different types of plants, and also how well they will grow there.  This capacity to produce biomass, or support tree growth is often expressed in the important forestry concept of site class as described in this article.

There is something odd about this rock….

We traveled from near the crest of the Coast Range back to the Valley floor to watch changes in site class and vegetation.  Our final stop was a rock sitting on a small hill beside a vineyard in Yamhill County, looking out across the Willamette Valley.

It is a large rock (about 90 tons), unrelated to any of the bedrock of the hill.  This rock helps tell a story of events during the last ice age that shaped the Willamette valley and its historic vegetation.  It influences the present, largely agricultural, vegetation as well.

Near the end of last ice age (between 10,000 and 15,000 years ago), a finger of the continental ice sheet moved down along the mountains near today’s Idaho/Montana boarder.  There it blocked the Clark Fork River with a massive wall of ice, which formed Glacial Missoula Lake.  The lake was almost 3000 square miles and up to 2000 feet deep, and held as much water as both lakes Erie and Ontario do today.

Eventually the lake built up to a point when the dam failed catastrophically, sending a huge surge of water rushing across the landscape of eastern Washington.  It raged towards the ocean, scouring out Washington’s coulees and channeled scablands, battered the Columbia Gorge (where it flowed nearly 1000 feet deep), and spread out into the Willamette, forming a large lake. This happened not once, but repeatedly, perhaps dozens of times over centuries.

These violently roiling waters carried dirt and debris scoured from Washington and the Gorge into the Willamette valley.  This heavy sediment load settled out as the water slowed and flowed up the Valley towards

Oaks from earlier wet savannas stand in modern grass seed field.

present day Eugene, dropping layers of soil on the valley floor now 100 feet deep or more.    Larger, heavier particles settled first in the lower valley.  Finer clay particles stayed suspended and were carried farther up the valley.  This is reflected in the soils of the Valley.  The mid and southern valley floor is dominated by heavy, poorly drained clay soils, reflected historically by the vast wet prairies and savannas (also promoted by frequent fires).  It is reflected today in the dominance of crops which are tolerant of wet ground, such as grass seed.  Of our trees, oak and ash like these soils, but conifers struggle.

So what does our rock at Erratic Rock State Natural Site  have to do with this?  Well, along with all the soil swept from Eastern Washington, the Missoula floodwaters carried icebergs, remnants of its glacial imprisonment in Montana.  Embedded in the icebergs were rocks from Canada and Montana. After being swept into the valley, the icebergs ran aground near the edge of the lake, and eventually dropped their payload as they melted, often a hundred feet or more above today’s valley floor.  The number and distribution of these glacial erratic rocks  helps illustrate the extent of the floods, which reached up to near 400 feet above sea level today.  That means Eugene would have been lakeside real estate during the floods 12000 years ago, and Corvallis and Albany would have been under a couple hundred feet of water.  Wow.

That puts the April 2019 floods into perspective,  doesn’t it?

This erratic rock was one of several used as pillars supporting an old barn in North Albany













This piece of grey granite was another unusual rock under the barn
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