Friday, November 8, 2013

Ginkgos and Rhythmites

Our vacation in the flood-ravaged landscapes of the Pacific Northwest came to an end where Interstate Highway 90 crosses Lake Wanapum on the Columbia River.  We had covered only about a third of the distance that the great Ice Age floods had traveled.  But we had no choice, we were out of time.
Route of the Ice Age Floods; red arrow marks the end of our vacation.  Map from the Ice Age Floods Institute; click on photo for closer view.
On the way home we managed to squeeze in two more quick geo-stops.  In hindsight, I see they nicely bracketed the range of time and events underlying the landscapes we had enjoyed so much.

Miocene Trees
From displays at Dry Falls Visitor Center.
It’s fascinating to imagine what a place was like far in the past, when our continent was somewhere else on Earth and the topography was not what it is today (e.g. no Cascade Range).  Of course the climate was very different, as was the vegetation.  Twenty thousand years ago there were lush forests in eastern Washington instead of today’s steppe/grasslands and wheat fields.

Then three million years later came the period of massive volcanism that would cover 64,000 square miles of the Pacific Northwest in lava flows (Columbia River Basalt Group).  It’s thought that some of these flows dammed lakes where dead trees accumulated, or perhaps lahars (mudflows) associated with volcanic eruptions deposited huge piles of dead trees.  In any case, the accumulations were then buried in basalt.  Instead of rotting, the wood was replaced with silica.  Millions of years later erosion exposed the petrified wood for travelers like us to enjoy, as we did during a short visit to Ginkgo Petrified Forest State Park near Vantage, Washington.
Some of the petrified logs look quite real, like this one with its "peeling bark" ...
Here's a nicely-preserved knothole:
We made only a brief stop and didn't take advantage of the trail through petrified logs of many species.  Forty kinds of trees grew in the Miocene Forests here, not just ginkgos.
Ginkgo Petrified Forest State Park --  46.969417° -119.982646°


Most of the basalt was produced 17 to 15 million years ago, but activity continued until about six million years ago.  Fast-forward five million years and one would find much of eastern Washington covered in dunes of loess -- fine windblown sediments.  Today the fertile loess-derived soils are highly-productive, and are covered in wheat and irrigated crops.  But among the wheat fields are weird intriguing dark rocky sparsely-vegetated landscapes called scablands.  Something happened here ...
Huge fallen chunks of basalt, ripped out and carried by floods.

Late Pleistocene Lakebeds

If you’ve been following this series of posts, you know that near the end of the last glacial advance there were floods of tremendous proportions that raced from an immense lake in northwest Montana all the way to the Pacific coast, arriving in perhaps just a few days.  That much water traveling at those speeds was hugely destructive.  The floods scraped off the blanket of loess over large areas and carved the underlying basalt as well, forming the scablands of eastern Washington.  That's where we spent most of our vacation.
Above, Dry Falls 15,000 years ago (Montana Natural History Center); below, Dry Falls on a hot September day in 2013.
Our final Pleistocene flood stop was a short distance south of Spokane at Campion Park on Latah Creek.  It didn’t really look like a park -- we saw no sign nor facilities.  But thanks to Bruce Bjornstad’s handy guidebook (see Resources below), we found it easily in spite of the road construction.  We pulled off, parked among the weeds and walked along the creek a short distance.
Photo by DM.
Across the creek from the park are Pleistocene rhythmites exposed in eroding slopes below nice houses.  Rhythmites are repeated deposits, in this case due to repeated flooding.  The current interpretation is that water from Glacial Lake Missoula flowed into another huge ice age lake -- Glacial Lake Columbia (see map near top of post).  The Latah Creek drainage was once an arm of the flooded lake. When the waters dropped and calmed after the flood went by, sediments settled out -- coarse first, then finer.  Likewise, the rhythmite bands are graded from coarse to fine from bottom to top.  Within the top part of some bands are much thinner layers interpreted as varves, annually-deposited rhythmites.   These at Latah Creek may represent summer runoff from melting glaciers.  JG Rigby recognized 20 separate flood events on Latah Creek (se Allen et al. 2009, Bjornstad 2006 below).
Note holes in rhythmites.  Something finds the old lake bed sediments useful ... ??
Campion Park,  47.607821° -117.379168°

What’s Next?

Of course this isn’t the end of the scablands story -- it’s just all that’s been written so far.  What will happen next?

Some say we’re currently in an interglacial period.  The climate will eventually cool and ice sheets will advance south again.  If so, sea level would drop, providing abundant seaside real estate.  But lobes of the great ice sheets may again impound immense lakes, and the dams may periodically fail.  It might be best to move south instead.  How about the scablands?  Will they expand, and be sculpted into even more fantastic forms?  Will Dry Falls flow again and continue to move upstream as kolks tear at the basalt?
The Dry Falls Visitor Center may have to be moved upstream if there are more Ice Age Floods.
Unfortunately, looking far into the future is a less-than-satisfying exercise.  My brief life on Earth, though informed by geology, keeps me from realistically imagining such worlds.  The scenes I try to envision are hardly vidid, just flat dull inanimate landscapes.

The near future is much easier to conjure up.  I foresee another trip ... we’ll cross the southern scablands, pass through the Columbia River Gorge, gawk at the Willamette Valley, and then happily find ourselves among the Miocene basalt headlands of the Oregon coast.

Resources for Travelers

Geo-tripping in Pleistocene flood country is wonderful in part because there are so many good resources to help travelers understand the landscapes.  In fact, there now is an Ice Age Floods National Geologic Trail.  It’s not a single route, but rather a collection of tours and stops from northwest Montana to the Pacific coast.  The IAFNGT is still under development.

Some useful websites (I'm sure there are many more)

Books and articles:

Allen, JE, Burns, M, and Burns, S.  2009.  Cataclysms on the Columbia.  Portland State University, Ooligan Press.  [I especially enjoyed MB’s contributions -- how geologists do their investigations, the saga of J Harlan Bretz and his radical flood scenario, and descriptions of what it might have been like to witness the events.]

Ault, D.  2001.  Glacial Lake Missoula and its Humongous Floods.  Missoula, MT:  Mountain Press Publ. Co.  [This book planted the seed that grew into a geo-trip through the scablands.  What a story -- and true!]

Baker, VR.  2009.  The channeled scabland:  a retrospective.  Ann Rev Earth Planet Sci 37:393-411.  PDF available.

Bjornstad, B.  2006.  On the trail of the ice age floods.  Sandpoint, ID:  Keokee Books.  [The second of two guides, this one on the northern part of the scablands.  Bjornstad provides detailed descriptions of trails, drives, and things to see along the way ... a great resource.]

Soennichsen, J.  2012.  Washington’s Channeled Scablands Guide.  Seattle:  Mountaineers Books.  [This proved to be a useful guide for first-time visitors, with recommended hikes, drives and camping].

On the ground

Early on we stopped at the Montana Natural History Center in Missoula and found maps and illustrations that gave us a good overall picture of where we were going and what had happened there.
Missoula, Montana 15,000 years ago.
At the Dry Falls Visitor Center, we took in the interpretive displays and bookstore, and enjoyed great scabland views.
Perch Lake in the coulee below Dry Falls.
The Montana Department of Transportation has put together a network of informative roadside geo-stops, and a website to go with it.  That’s a very cool Department of Transportation!
Vacationing geo-geek in Clark Fork Canyon, Montana.  Photo by DM.

This is the final post in a series about the Ice Age Mega-floods of the Pacific Northwest.


  1. I hate to say it, but you really didn't miss much in not walking through the Ginkgo petrified forest trail. Every decent piece on the trail is embedded in a concrete box topped by a metal grate, likely in an attempt to thwart potential pilferers. For photos and close-up examination opportunities, the visitor center area - which is really very nice - was a good choice.

    1. sorry to hear! I agree about the visitor center, and hope to go back when I have more time to look closely at the specimens, with information in hand.