03 December, 2010

Bedrock Bonanza

You'll have to excuse me if I sound a bit distracted in this post - I'm writing it whilst watching Harry Potter and the Goblet of Fire again.  Why am I watching it twice?  Because that's the silly-arsed kind of thing writers do when one of the actors is reminding her vaguely of a character and she's trying to figure out who, why, and how she's going to take advantage of it.  Secondly, the rental expires in 19 hours.  Thirdly, when you don't feel like ponying up cash for a scientific paper, it's damned hard to research minor bedrock formations in the Seattle area.  And fourthly, researching this post made my brain bleed a bit.

It doesn't help that the internet keeps going out, either.  But, damn it, I promised you a missive on the bedrock in Seward Park, and that you shall have.

Feast your eyes upon this:

Feast Your Eyes Upon Actual Bedrock

Exposures of bedrock in Seattle are fairly rare.  Yes, I'm sure anybody who's tried to plant a garden round here has plenty to say about all the damned rocks, but the majority of stuff they're dealing with is glacial till and outwash.  It's full of rocks, but it's got a ways to go yet before it becomes actual bedrock.  As far as your actual bedrock, though, you're very nearly out of luck.

But thanks to a few earthquakes, you'll get some opportunities.  They gave Seattle a high - the Seattle-Bremerton High, in fact.  Travel along the high, and you'll catch out some sweet bedrock: the Puget Group in Renton, for instance.  But it's the Blakeley Formation we're concerned with here.  You can glimpse it in Seward Park.  It outcrops along with some Oligocene volcanic rocks at Boeing Field, by itself at Alki Point, and sweeps along the southern bit of Bainbridge Island all the way to Bremerton.  They give way after that to nice outcrops of the Crescent Formation from Bremerton on west - the foundations of the area, uplifted by earthquakes and excavated by erosion.

I'll tell you, after encountering very nearly nothing aside from glacial deposits, running into bedrock is pretty damned exciting.  Follow me after the fold, and I'll show you (drumroll please) the Blakeley Formation.

Blakeley Formation Up Close
That, my darlings, is a bit of rock that's somewhere between 26-37 million years old.  It looks pretty unremarkable until you realize it's been around far longer than our own dear branch of the Earth family tree, the Hominidae.  At least twice as old, in fact.

Makes the mundane rather magnificent, doesn't that just?

The exposures in Seward Park aren't the best, and most folks pass them right by without a second glance.  Understandable, considering:

Not Putting Itself Forward
It's a little shy in places.  But consider what it's whispering to us: the Blakeley Formation is 2,400 meters' worth of lost worlds.  Volcaniclastic sandstones tell us bits of it formed from sediments eroding off ancient volcanoes into an ocean.  Its conglomerates, sandstones and shales inform us that it's a submarine fan complex.  Its environment was a deep marginal ocean basin, like the ones we can see around southern California today.  Turbidity currents swept down the slope, powerful underwater avalanches that left graded beds of sediment in their wake, later to become the rocks beneath our feet.  Amazing, no?

The currents carried bits of land-based trees with them, leaving the sediments peppered with branches and leaves, which became fossilized along with the resident clams and snails.  Shore wasn't far, and layers of ash and pumice hint that the Cascade volcanoes were starting life just about then.  The subduction zone that gives us so much excitement today provided just as much or even more then.  Some of those turbidity currents, in fact, could very well have been caused by the shaking of enormous subduction zone quakes.

The bedding in Seward Park isn't as distinct as it is elsewhere, but you can see it in places:

Lengthwise View
The sandstones here are soft - soft enough to pluck right out of the bluff:

Bit o' Sandstone
And broken with a flick of the fingers:

And when you get your nose right up to it, this unprepossessing cliff:

Blakeley Formation Cliff
Becomes a thing of beauty:

Cliff Closeup
You know what else is remarkable about this place?  You weigh more here.  Y'see, you're on a gravity high.  Go north if you want to lose some weight: once you get off the bedrock of the Seattle-Bremerton High, you'll find yourself in one of the deepest gravity lows on Earth that we're aware of.  The faults have dropped that bit of Seattle so low that you actually do weigh a little less standing on the sediments there.  I find that utterly remarkable.

The Earth really is an amazing place, and for the geologically-inclined city dweller, there's plenty that shall enthrall you at Seward Park.  Hell, we haven't even gotten to the glacial erratics yet.


USGS: Bedrock Geology of Seattle (.pdf)

Northwest Geological Society Field Trip: "The Bedrock of Seattle" (.pdf)

Troost et al: "Geology of Seattle and the Seattle Area, Washington" (.pdf)

Friends of Seward Park: The Geology of Seward Park

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