Oregon Geology Bonus Features: Geologic Core

There's a little something wonderful buried deep under the ground at Portland's Washington Park.  If you go to the MAX light rail station:

MAX Light Rail Station

And step into the elevator:

Not the TARDIS, but it'll do as a time machine

You'll notice that you're traveling into the past.  Deep into the past.  Millions and millions of years, in fact.

We don't often think about that, when we're going underground.  But dig down below the soil, into the rock, and you're on your way to another age.  Here, two hundred and seventy feet of vertical travel takes you sixteen million years into the past, back to the time when the Columbia River basalts were busy paving vast portions of Washington and Oregon.  Its a world that bears little resemblance to our own.  And it's all laid out for you to see.




All that history, mounted in tubes on the wall between platforms:

Core East

Core West

All of this would have been much better if my mad photography skillz hadn't been downright rude at the time, but it'll do for a journey into a tumultuous past. Quite a few cores were bored through the Tualatin Mountains in preparation for boring the three mile long rail tunnel; this one is the most complete.  Displaying it for the public was a stroke of genius.  It's time people saw the geology beneath their feet and celebrated a little science.

Allow the Core Sample Timeline to introduce itself

Let's begin at the beginning, nearly sixteen million years ago.  As you begin walking the timeline (see slideshow below for the full experience), you're following the Winter Water Member of the Grande Ronde basalt formation.  This is the oldest bit of the Grande Ronde on display, and it's thick - goes on along the wall for at least 15 feet.  Something interesting about the Winter Water Member: it's one of the few you can recognize on sight.  It, unlike other bits of the Columbia River Basalt Group, has plagioclase phenocrysts visible to the naked eye.  I wish I'd known that when I was there, because then I might have found some in the core sample.

Following that, you'll come upon the contact between the Winter Water and Sentinel Bluffs Members of the Grande Ronde.  Keep walking.  You've got nearly thirty feet worth of basalt to go before anything else happens.  And one thing that should be impressing itself upon you by now: that's a fuck of a lot of basalt.  And it ain't even half over yet.  Portland wouldn't have been the most comfortable city to live in for a while there.

But then, there was an interval.  The Grande Ronde stopped flowing.  All silent on the volcanic front.

Closeup of the Vantage Horizon

You've reached the Vantage Horizon.  About 15.6 million years ago, three hundred thousand years of peace reigned.  Soils had time to form.  Plants, trees and animals thrived.  Humans might have found it a nice place to live, except for the fact they hadn't evolved yet. 

Seeing those sudden paleosols after so much basalt is a shock and a delight.  But don't get too used to it.  Take a step, and bang! you're back in basalt.  We're in the Wanapum Formation now.  Wave goodbye to palesols and hello to nature's pavement.  Flow after flow after flow of flood basalts spilled over this site.  You are doomed to walk among it FOREVER!  Okay, only for about forty feet or so, but still.  That's a hell of a lot of basalt, and we're talking about almost a million years of molten rock.  If your mind hasn't boggled, you've either spent too much time in deep time or you haven't got an imagination.

Closeup of Wanapum Basalt

Then, after all that nearly-endless basalt, the Wanapum Basalt comes smack up against the Portland Hills Silt.

Portland Hills Silt

There was a very long period of erosion before the Portland Hills Silt was deposited.  We left the Wanapum Basalt about 14 million years ago.  Now we're standing in front of something that, at its oldest, is around 700,000 years.  That's the thing with geology: periods of high excitement are followed by big blanks.  It's like Mother Nature delights in ripping pages out of the history book.  So here we are, and all we can really say by looking at this core sample is that there was basalt, then ?, then loess.

More loess or loess is more?

Lots and lots of loess.  Lots.  Apparently, there were glaciers, and winds sweeping over outwash plains, and the loess accumulated to enormous depths.  You'll walk along about thirty feet of loess before anything else happens.  Then you'll see that the place heats up a bit.

Contact between Boring Lava and Portland Hills Silt

Don't be fooled by the name.  The Boring Volcanic Field is anything but.  Starting about 2 million years ago, tectonic stresses started pulling the Portland Basin apart, and the lava came: shield volcanoes and cinder cones, mostly, with some eruptions salting the area with ash.  So things had gotten a bit exciting again, and still could today, considering no one's quite sure whether the Boring Volcanic Field's truly extinct or not.

The absolute neatest thing about it, though, is that reversals in the polarity of the earth's magnetic field are clearly marked on the core:

Reverse the polarity!

I'm sorry if this makes me a total geek, but that just delights me to no end.  I mean, the bloody magnetic field flipped, north became south, and we can see it recorded in the rock!  Okay, we need instruments to see it, but still.  It's bloody freaking awesome is what it is.

And you can see bits where the Portland Hills Silt and the Boring Volcanic Field battled it out for supremacy.

Portland Hills Silt with a bit o' Boring Volcanic Field rock.  Nice vesticles!

Ice!  Silt!  Volcanoes!  Fight!

And then, and then (and this is my favorite thing outside of the polarity reversals), the Floods came:

Missoula Floods closeup

Oh, yeah.  Right there.  Right there, you can see where the floods came roaring through, wall after wall of water, burying this region of Oregon up to 400 feet deep in roiling, icy, sediment-laden, gargantuan jökulhlaup.  Brilliant.

Moi et Missoula

 After that, lots more silt, etc.

It really is amazing to see so much geology in a tube like that, displayed with such loving artistry.  If you get the chance, you should go see it for yourself.  The scale of the thing can't be captured by a camera.  But I've done my best, and here present you with a slide show of the whole thing, start to finish.



You've just walked nearly sixteen million years.  How amazing is that?

Here endeth Oregon Geology Phase I.  Never fear.  Phase II cometh, after an interval in which we enjoy Mount Rainier, the Olympics, and quite possibly some Arizona geology.  Believe me, I have no shortage of wonderful sights to show you.  Stay tuned!

Some useful references:

Portland, Oregon, geology by tram, train, and foot (.pdf)

The petrology and stratigraphy of the Portland Hills Silt - a Pacific Northwest loess (.pdf)

The Boring Lava Field, Portland, Oregon

Nomenclature of the Columbia River Basalt Group (from Reidel and others, 2002) (.pdf)