OMN Willamette Valley Geology Presentation Part One: Ice Age Floods

A couple weeks ago, I gave a presentation on the geology of the Willamette Valley. I prefer to simply use slides and talk to the audience directly, rather than powerpointing them to death. My own personal feeling is that Powerpoint kills spontaneity, takes the focus off the speaker and content, and enforces a linearity of thinking I find offensive. On the other hand, the style I use means I can never give the same talk twice, nor necessarily be able to recapture certain moments, no matter how much I'd like to (and we did have a couple of those, which I'll bring up in later parts). On the other other hand, I'm free to come up with new "perfect moments" whenever inspiration hits.

So I've decided to attempt, not to recapture that talk, but to post the images I used, reiterate some of the important points, mention locations when appropriate, and include some links to relevant material. Most of the links will be from a post last week, but they'll be better organized by topic in this series.

The talk was organized into several segments, including Ice Age Floods, Stream Processes and Fluvial Landforms, Rock Materials of the Willamette Valley and Environs, and a brief overview of (what we were expecting to see at) Silver Falls State Park Geology. A final portion that I was not able to cover during the talk was on non-tectonic geologic hazards (Bob Lille covered earthquakes and volcanoes), with emphasis on mass movement and flooding. (Note: all the images will get much larger if you click on them, and you may find it more convenient to right click and open images in new tabs, so you don't need to reload the whole blog when you come back)

I didn't get hard links to each of these images, but the lat-long information is recorded in the data on the upper left of each one, so those interested should have no difficulty getting arbitrarily similar images from FlashEarth. Above is a satellite imagery map of Washington, Northern Oregon, Northern Idaho and Western Montana. Almost the entirety of the area affected by Glacial Lake Missoula and the Missoula floods is shown. Since I'm not entirely certain how far east the lake extended, it may have gone off the right edge of the image.

Zooming in to Eastern Washington, the dark streaks illustrate the "scab" in "scablands;" these are the channels followed and gouged out as the enormous torrent of water drained from Lake Missoula each time the ice dam collapsed. The consensus is that there were at least 40 of these floods, but there may have been more than twice as many. This would mean the repeat time was in the range of 50 to 100 years between floods.

 

This shows Crown Point, in the western portion of the Columbia River Gorge. Note that the point has no trees. Although it stands 800 feet above the Columbia River, it was overtopped by the floods as they emptied through the gorge, and stripped bare down to its CRB bedrock. I commented in an older post that this is, to me, the single most staggering and incomprehensible statistic I know of regarding these floods... to stand on Crown point, look across the gorge, and picture everything up to that point under water is mind-blowing.

Above is the I-84/I-205 interchange in NE Portland. Rocky Butte is the core, or neck, of an old volcano, and resisted erosion by the jet of water emerging from the gorge. To this day, the upstream moat is quite obvious to the east of Rocky Butte, where the turbulence created as the flood interacted with the solid rock  eroded out the eastern side. Streams of sediment were deposited to the the north and west, and to the south and west, where the current slowed and its competence was reduced. The deposit to the south is called Alameda Ridge, and I-84 to the lower left is following Sullivan's Gulch, a drainage channel that the floods created.

Northwestern Oregon and the Willamette Valley... I commented that the transition from green to tan, roughly the same as the transition from higher-elevation temperate rainforest to the oak-savannah biome of the valley floor, is probably a pretty good proxy for the area inundated by the floods. This is not exact, to be clear, but at this scale, it looks to me to be a good approximation.

Detail from an interpretive sign at the entrance to Erratic Rock State Wayside, which generally summarizes some of the points I've made thus far.

Text below the map shown above, making the point that much of the soil in the valley has developed from sediment deposited by these floods. While I know next to nothing about wine, let alone terroir, I'll point out that my understanding is that for Pinot Noir grapes, at least, the lateritic soils developed on highly weathered Columbia River Basalt produces a better flavor

The point of this slide is to illustrate the location of Erratic Rock. This sign points up the footpath, however, on an initial drive-by, it's easy to assume it's telling you to go around the corner ahead and turn right. The pullout is, in fact, right across the road from here, next to the garage on the left side of the photo.

Looking down the footpath up to Erratic Rock, Dana's car on the left, and the garage from the previous photo just visible under the overhanging tree near the end of the path. It's about a quarter mile walk in, with a climb I'd estimate to be about 150 feet.

And the rock itself...

Various sources claim that more than half of the rock's original bulk has been removed by collectors. While the rock type is quite unusual for Oregon- especially this part of Oregon- it's not that unusual in the grand scheme of things. Please don't collect samples here. You can find similar stuff in the Klamaths, in Idaho, and the Rocky Mountains of both the US and Canada. (See argillte, and particularly the section on the Belt supergroup, which is the likely source of this rock)

Looking up the hill, over a fragment broken off of the original block, in the mid-distance.

Turning around, and looking down the hill toward the valley floor. The rock itself isn't as impressive as its location on the upper shoulder of a pretty substantial hill. Note the crop: wine grapes!

Dana and Lockwood for scale- these are not little clasts! Photo from (though not by) Dana Hunter.This was from the last day of Dana's visit in July of this year, and if you click for the full-size photo, you can get a good sense of how terribly exhausted I was at this point. But happy.

And, in fact, these blocks are large enough to be seen (With decent optics, to be sure- I'm certain these are not visible to the naked eye.) from space.

The cross hairs are just to the right of the start of the foot path up to the rock- another slide to help clarify the location. This little parklet can be a pain in the butt to find. I'm sure I've driven by looking for it at least as many times as I've actually visited it. My general attitude is that this wayside isn't really worth a significant drive by itself, but if you're in the neighborhood, it's worth going out of your way a bit to visit. There are no facilities of any kind- no water, no restrooms.

This is another Missoula Erratic in Monroe, Oregon. I doubt it's in the exact site where it was originally deposited- it's carved with the date of the erection of the church with which it's associated. This and the following are from Dana's first visit, two years ago. (The upper cross hair is just onto this rock in this FlashEarth link)

Close up of the granitoid material making up the erratic in Monroe- there are some granitoid rocks in western Oregon, but they're uncommon, and you never see blocks this size out on the floor of the Willamette Valley. Lens cap is 52 mm in diameter.

South Viewpoint at Silver Falls State Park, Oregon, looking approximately west. This is well above the ~400 foot elevation reached by the floods- I think we're a bit over a thousand feet here- but much of the valley floor under the haze in the distance would have been inundated.