We’ve had an eventful first day in Oregon so far. We saw the mouth of the mighty Columbia River at Astoria; watched the Columbia River Basalts plunge into the sea at Ecola State Park; and done a bit of desultory spelunking at the north side of Hug Point. The sun’s getting ready to sink into the sea past shattered sea stacks of brecciated basalt.
|Grande Ronde Silhouette|
And if we don’t get our arses round Austin Point and back before high tide, we’re going to be hugging something other than geology soon. It’s a mad scramble through a tangle of biology up to Highway 101 if you get stranded on the beach. So let’s get a move on.
The character of the place changes a bit down here. And it’s a good place to talk about basalt.
First off, if one’s just taking a casual look round, it’s easy to be deceived. What with all these dikes, stringers, sills, and so forth, it’s easy to think that these basalts are natives. In fact, for a while, that’s just what geologists thought. The source didn’t seem like it could be too far from here. After all, the way the Columbia River Basalts insinuated themselves into the local strata, it looked just like they’d arrived from below.
However, that’s not what gravity and chemistry tells us.
|A Finger o’ Basalt|
Check this dude out. If he were a local, he’d be rooted pretty far down, thousands of feet in fact. But gravity studies revealed that all that lovely, solid basalt stops just a few hundred feet beneath the surface. Weird, inexplicable – until you look east, and realize there’s a whole big plateau of this stuff covering appreciable parts of Oregon and Washington. But most of the basalt flows we’re used to travel only so far and then peter out: a mile, a dozen, sometimes a hundred. But nearly half a thousand? Read some of the old scientific papers on this area, and you’ll see that geologists struggled with the idea.
But then came chemistry. And chemical analysis said, “Yup, that’s sure Columbia River Basalt, that is. Eye-din-tee-cull.” It boggles the mind – we’ve had zero experience with state-spanning flows of flood basalts in recent history. If I’m remembering rightly, humans in historical time never have witnessed such a thing. But it happened here.
Down here to the south, the Grande Ronde looms more. And it’s a mass of fractures and fragments, welded together.
Make sure at some point to stand shoulder-to-shoulder with the point and look out to sea. It’s a wild and wonderful view.
Clamber over the riot of rocks, basalt rising up on both sides of you. Once you’re through the gap between head and stack, you’ll suddenly emerge from igneous to sedimentary again.
If you’re looking at those layers and thinking, “Hmm, tilty,” you’re not wrong – they’ve got a pronounced dip. Why this is, I have not yet discovered, but I imagine it has quite a bit to do with hanging about on the edge of a subduction zone.
Notice the basalt stack right to the right of it, there? That’s remnants of a big dike of Grande Ronde basalt that sheared off the Astoria Formation, and now protects it from the worst of the waves.
Down near the tide line, when you can find bits that aren’t all covered in crunchy-coated biology, you can see just how smooth and slick basalt can become when polished by sand-laden waves.
The spaces between the sea stacks and the headland form channels, and so we end up with these lovely smooth bits that look like they’ve been scoured out by a river. See how fractured and fragmented it is, even polished? When the basalt hit the coast and plunged into the cold Pacific waters, it cooled rapidly enough to shatter. In places, it actually boiled the water, and you can see red oxidized basalt where that happened.
Things look rather different where the water’s been at the sandstone.
Looks like the sea’s trying to tunnel through here, helped by pebbles.
I want to turn your attention now to the head, where you can see basalt and sediments getting really intimate.
See how the basalt threads its way through the sandstone, there? When it reached the coast, it sank into the mud and sand, butted in between layers, and wove itself into any space it could find or make. In places, it incorporated the sediments into itself. Told you this is XXX geology.
|Jaunty Basalt Cap|
If the Columbia River Basalt’s Grande Ronde formation hadn’t made it out to the coast, it’s quite possible, even probable, we wouldn’t be seeing these lovely cliffs. We certainly wouldn’t have the imposing heads and monolithic sea stacks. Sandstones and mudstones aren’t as inclined to hold up against wave action, as we’ll see when we get to Parte the Fifth.
In places, the intruding Grande Ronde squirted up through the sediments. Remnants still stand watch, tall and imposing.
Get closer to one of them. Let’s choose the one that’s tilting like a charming drunk.
Notice how chunky it is, some of it blocky and some more brecciated. This basalt didn’t have a long, quiet cool-down. If you’ve ever wondered why sea stacks look so rough, now you know it isn’t just the relentless pounding of the waves – it’s the explosive action of rapid quenching.
Come closer still.
Check that out. Chips, cracks, and even some rough conchoidal fractures off to the left, there. This is some seriously broken rock. It makes it a lot of fun to explore. Unfortunately, before we could get really immersed in it, we were getting immersed in something else. Stupid high tide.
One last, long, lingering look; one last fond pat for the rocks; one final moment to contemplate the power of good Mother Earth to create and destroy; and then it’s off to the Land o’ Lincoln (City), where we’ll meet the world’s shortest river.