Jill Severn's Gardening Column

The Earth beneath our feet

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Gary Ritchie, PhD, the author of Inside Plants, has written a Thurston County Historical Journal issue on the history of the ground on which we garden.

It is a fantastic tale. If not for all the science that supports it, no one would believe it. And it’s so complicated, many of us will not fully grasp more than its basic ideas.

Ritchie begins somewhere around 3.0 to 2.5 billion years ago, when single-celled organisms invented photosynthesis. That’s how plants create oxygen, which was sorely lacking on our young planet, which was then a mere 2 billion years old. All gardeners should be grateful for this, as should every creature that breathes or eats.

And here’s Ritchie’s summary of Thurston County’s more recent history:

“The history of Thurston County began 56 million years ago when the Yellowstone Hotspot exploded beneath the sea, disgorging a 480,000-cubic-mile lava plateau we have named Siletzia. Prior to that time, the area where Thurston County exists was under the Pacific Ocean.”

“Siletzia then accreted to the North American west coast about 49 million years ago, forming a new, massive continental shelf . . . This new zone gave rise to the Cascade Mountains and their splendid volcanic peaks, which stand against the sun as icons of our great and resplendent land.  As the continent continued to drift westward, the Hotspot again erupted 16 million years ago to form the vast Columbia Basalt Plateau. With its spectacular straight-walled coulees and cataracts, formed by Ice Age floods, it buried much of eastern Washington and Oregon under three miles of lava.”

Then there’s another leap across eons: “Having begun only 2.6 million years ago against the backdrop of such epic events, the Pleistocene Ice Age seems almost an afterthought. Nevertheless, it managed to bury Siletzia beneath great piles of glacial debris across much of Thurston County, transforming the landscape into the maze of lakes, prairies, moraines drumlins and eskers we call home.”

“Siletzia, once the dominant landform of the Pacific Northwest Coast, now teases us with her mostly hidden splendor, granting only fleeting glimpses of her unseen might: ancient rock shelves peeking out along Hood Canal, or roadcut cliffs in the Black Hills. She resides beneath our houses, churches and schools, Interstate 5, Capital Mall, and Tumwater Falls.”

“Siletzia, our Cascade Volcanoes, Columbia Basin Basalt Plateau, and Tumwater Falls – all gifts from the long-departed Yellowstone Hotspot. Gifts to behold.”

Why, you might ask, is the Yellowstone Hotspot now in a national park in Wyoming? It’s because the land has moved over it to the south and west – and it’s still moving:

“The rate of movement of the plate in the Yellowstone area from 16 Ma (million years ago) was about 2.4 inches a year, but for the past eight million years it has slowed to about 1.8 inches a year. Using that number, the Yellowstone Hotspot would arrive in Montana in about three million years and North Dakota in seventeen million years, leaving North Dakotans ample time to prepare.”

Eventually, Ritchie gets to the near-past – that is, a mere 18,300 years ago, when the most recent ice age reached Bellingham.

As recently as 1840, Ritchie writes that “geologists discounted the evidence for glaciation that was abundant in their native Scotland, believing it to have resulted from the Biblical flood.” Now scientists think there were ice ages about once every 100,000 years for the last 800,000 years.

The Vashon Ice sheet caused an ice dam that prevented Puget Sound waters from flowing north. Ritchie says “Fresh water flowing in from the Puyallup, Nisqually and Skokomish Rivers then backed up behind the ice dam,” and eventually began “emptying west through the Black Lake and into the Black and ultimately Chehalis Rivers, and then out to the Pacific Ocean via Gray’s Harbor.”

This explains a roadside sign I read years ago that claimed there was a time when the Chehalis River was bigger than the Columbia River is now.

By 12,000 years ago, the ice reached Tenino. At its height, it covered Thurston County at a maximum depth of 1,800 feet. Then it receded . . . and here we are.

Now as I’m weeding, I try to imagine my back yard covered by 1,800 feet of ice. I think about the land hosting my tomatoes plants being somewhere under the Pacific Ocean, all those millions of years ago.

I remind myself that the mountains we think of as timeless are relative newcomers on our earth, and we humans are even newer.

It’s enough to make a person feel small, temporary, and lucky to be alive after the ice has melted, in all this wonder and beauty.

Jill Severn writes from her home in Olympia, where she grows vegetables, flowers and a small flock of chickens. She loves conversation among gardeners. Start one by emailing her at  jill@theJOLTnews.com

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  • Drutty

    Thanks---fascinating & thought provoking~!

    Sunday, June 19, 2022 Report this

  • sunshine39

    I echo Drutty's comment. Thanks for giving us another viewpoint

    Tuesday, June 21, 2022 Report this