You know, this almost didn’t happen. Tonight, the cat decided she loved the notebook all my notes resided in, and removing my cat from the object of her affections can be fatal. I mean, does this really look like a feline inclined to relinquish the goods?
Fortunately for all involved, I was able to lure her away by opening the door to the porch. Now that summer’s here, she’s almost as addicted to the outdoors as I am. And so, at long last, I can present to you the first installment of our multi-part series on Oregon Geology. Come join me after the jump for the geologic journey to Astoria.
I’m going to have to wrap this in warnings and caveats. I’m not even a talented amateur geologist. There’s probably plenty I’ll get wrong, although I tried to be careful and only work with bits that I had good information on. And I’ll tell you when I’m not sure about what I’m seeing.
Such as now. You don’t want to know how many days I spent trying to identify this bit o’ basalt just across the Columbia River from Longview, WA:
As near as I can tell from geological maps and various books on Northwest geology written for the interested amateur, that is a bit of old seafloor, quite possibly part of the Gray’s River Volcanics. Despite the fact I was a doofus and did not zoom in to the full extent capable with my snazzy new camera, you can kinda sorta see what look suspiciously like possible pillows there, if you embiggen. I was strangely unwilling to risk a car speeding around the blind curve and squishing me, so I didn’t cross the highway for a closer look, a fact I’ve been cursing myself for ever since.
So, despite a photo that’s perhaps more appropriate to cryptogeology and my own appalling ignorance on the subject, what makes me think we’re looking at actual ocean floor? A few things. First, you’ll notice the weathering and color on this bit o’ basalt. Think back to the paltry post I did on eastern Washington. The Columbia River Basalts still have a raw, fresh quality to them, even after upwards of 15 million years and several gargantuan floods. You’ll see when we get to the Columbia River Gorge that even those parts of the Miocene flood basalts that spilled out into the wet west coast look pretty new. This one, on the other hand, appears affably middle-aged. The fact that the geological map of Oregon says this area’s covered in Eocene basalts clinches the case, although we’re proving by a preponderance of the evidence rather than beyond all reasonable doubt.
So what’s a bit of seafloor doing up here high and (quite often) dry? So glad you asked, because it’s an interesting story.
Drive the I-5 corridor from the Columbia River to Olympia, and you’ll notice a great many hills. These are the Willapa Hills, and between 55 to around 30 to 10 million years ago, depending on the spot, the whole area was underwater. The west coast we know and love didn’t exist yet. Instead, there was a rift spewing out new seabed in the ocean between the Pacific and North American plates, and a subduction zone somewhere to the East. Thanks to the subduction zone volcanoes thrown up by that plate tectonic action, the Western Cascades, we don’t know precisely where – their eruptions covered the area of contact under younger volcanics in most places. But as you head west, you can see ye olde seafloor, complete with its basalts and sedimentary cover.
Now, you normally don’t see seafloor on a continent – it’s heavy stuff, and sinks down beneath the lighter continental rocks at subduction zones. But in some cases, the subducting seafloor drags lighter sedimentary rock down with it. This is important, because there were two subduction zones in this area in them days. The little North Cascade subcontinent was busily docking with North America, while the Pacific Plate continued its journey west, and as things jammed up together, a second subduction zone came into being in very nearly its present location. Lighter sedimentary rocks riding the seafloor down into that trench “floated” the slab of seafloor that would become the Willapa Hills nearly two miles up, high and dry. If you ever want to demonstrate this in your bathtub, fill up with water, stick in a few handy rocks, and shove a bath pillow under them.
So there you go – seabed on dry land. A few tens of millions of years’ worth of stream erosion, and you have hills. Neat, eh?
Here’s a photo that’s sort of like a geological family-reunion snapshot:
Starting from the river, we have a Holocene alluvial terrace built up by the Columbia River; then the Pliocene non-marine rocks (conglomerate and wacke) laid down around the Ice Ages; then our old friends the Eocene volcanic rocks, which include not only the seabed, but some that erupted on land; Eocene-Oligocene volcanic rocks erupted by the subduction zone volcanoes (think lots and lots of andesite); and finally Pleistocene to Recent volcanic rocks (and when they say recent, they mean like 30 years if they’re a day – I trust you all recognize our old friend Mt. St. Helens). And yes, I know most of that stuff is in Washington and we’re supposed to be talking about Oregon, but I was standing in Oregon when I took the picture, so it counts, damn it.
Just think about this for a moment: when you study that picture, you’re looking at a history that covers over 50 million years. You are looking at island arcs that became part of the continent, old ocean floor, ancient eruptions, and a river and a volcano that are busy making more geology. That’s a pretty hefty hunk o’ history there.
Now, let’s look a bit west:
You will notice, just left of center, a shoulder of a hill with a flat top. That, my darlings, is our old friend the Columbia River Basalt. Several of those flows were opportunistic bastards who decided they wanted a seaside vacation, so they zipped right on down the Columbia River Valley several times, pushing the poor river ever further northward, and covering big strips of southwestern Washington and northwestern Oregon in burning hot basalt. It got so deep and intense it overflowed the valley in several places. Subsequent erosion, disapproving of such antics, scrubbed much of it away, but there’s plenty of patches left.
No doubt part of that erosion took place during the Ice Ages, when Glacial Lake Missoula floated its ice dam and sent flood after flood roaring across eastern Washington, down the Columbia River, and out to the Pacific, which was about forty miles further away than it is now, with so much water locked up in ice caps. We’re talking floods that traveled from Montana here, people, and they still had enough water left by the time they got here that Longview was drowned in at least 300 feet of water. And that river valley you’re looking at today was, at that time, a canyon that ran from Longview all the way out to the Pacific. The Columbia River has since then filled things in with sediment, creating a much more laid-back landscape, but just imagine: the area filled from canyon-bottom to hilltop with angry brown floodwater. Okay, so the canyon was only about forty feet deep round here, but still.
If you head down the Columbia River from here, you can see the over-steepened valley walls and faceted spurs left as the flood waters ripped at rock as it raced to the sea. And, since this area’s built not just on basalt but Holocene alluvial deposits, Pliocene debris, and Eocene marine sediments, it also suffers landslides. Between fire, water and slope failure, it just can’t catch a break.
Let’s follow the floods on down. Near Clatskanie, Oregon, they hit a narrow bit of the Columbia River which slowed them somewhat, but they were still deep and strong, probably around 275 feet. After those narrows, the valley widened where the bedrock changed from basalt to marine sandstone and mudstone. The floods flowed merrily down to Astoria and then forty miles out to sea, where we’ll meet with them again.
Today, Astoria’s a seaside town with a history. The town’s sited on the low hills erosion has carved from seafloor mudstones from the Miocene, laid down about 20 million years ago, underlain here and there by the basalts that erupted offshore. You’ll notice a cheerful reddish-orange hue to the mudstones where they’ve had a chance to weather, but they’re really brown. If you want to hunt fossils in Oregon, you’ve got a chance at a few here, though you’ll need a microscope for the majority. On your way in to town, you might notice sandstones that likely were deposited in an old valley of the Columbia River back in the Pliocene, when the climate was drier than today. And you’ll certainly run in to some Columbia River Basalts.
Let’s meet some fine examples of Astoria-area geology:
You’re standing on Coxcomb Hill, where a hot, probing finger of the Grande Ronde member of the Columbia River Basalt flows prodded its way into squishy coastal sediments. Those sediments lithified, but were too soft to survive, and erosion took them out, leaving the basalt behind. Across the river, the hill you see to the right is Scarboro Hill. Its core is old volcanic rock that got stripped from the Juan de Fuca Plate as it subducted beneath the lighter North American continent. The collision warped those rocks up like a card pinched between two fingers. It probably tells the Grande Ronde Basalt that it doesn’t know how easy kids have got it these days.
Now, sight northwest across the arched bit of the bridge to that long, low strip of hills. That’s Cape Disappointment. It’s the eroded remains of a Columbia River Basalt flow, most likely another bit of Grande Ronde Basalt. It’s now a pretty rocky cape with ten tons of fog and a reputation for killing ships – there’s a sandbar out there that likes to shift around, and has racked up an impressive 250 ships sunk. Don’t play Battleship against the Graveyard of the Pacific, my friends.
Now, sweep your gaze south from Cape Disappointment to an even lower strip of land. If you enlarge the picture, you’ll notice white froth around its tip, Pacific Ocean breakers. That is the Clatsop Sand Spit, and we’ll be getting to know it well in a moment.
For now, though, we’re going to remain on Coxcomb Hill, where the Astoria Column rises up like a very tall, pointy column, and take in the view. Here’s Scarboro Hill again, along with its buddy Bear River Ridge:
Bear River Ridge is what remains of a 2,700 foot thick sill of Grande Ronde Basalt that barged in on some unsuspecting sedimentary rocks and decided to stay. If you’re starting to get the feeling that you cannot escape the Grande Ronde Basalts out here, you are not mistaken.
In fact, why don’t we turn from north to south and have a look at Saddle Mountain:
To me, it looks like a mutant three-humped camel, but if other people saw saddles, fine. Whatever moves your mountain. Not that it’s moving it far in this case – Saddle Mountain’s a Coast Range interloper, a young upstart among the staid old hills of Eocene volcanics, its dark-brown basalts rising up in knobs that form the highest peaks in these here parts. In fact, most of the highest hills visible from here are part of that same flow, which buried an ancient river delta and caused general mayhem nearly sixteen million years ago. The lower ridges in the foreground, Eels Ridge and Lone Ridge, are relative young whippersnappers, 15.3 million year-old flows of the Frenchman Springs member of the Columbia River Basalts, which made their way down an old Columbia River channel, evicted the river, intruded the local sedimentary rocks, and refused to leave. Without them, Astoria might be part of Washington State, since the Columbia River’s used as the border.
After all that fire, it’ll be nice to see some water:
To the left, you’ll see the Youngs River flowing into (shocker) Youngs Bay, and the river on the right is the Lewis and Clark River. Between them, they’ve built up what looks to be a nice delta. Someday, if they’re very good, those muddy sediments will become sandstones and mudstones, and will tell future geologists the story of the two rivers that used to flow here. The low hills surrounding them are 40-30 million year-old sedimentary rocks, which I trust shall provide a good example to the new generation.
And what’s that off in the far distance? Is it, could it be, visible all the way from here….
Why, yes, yes it is Tillamook Head! Whoever would’ve expected to see more Grande Ronde basalt out here, right? We’ll have much more to say about it in our next installment of Oregon geology, but for now, imagine you’re an observer standing on – um, well, shit, that’s not safe – you’re a little birdie flying up above this landscape nearly sixteen million years ago, watching firey fingers of basalt nose their way into the sea, sending up clouds of steam and causing all sorts of mayhem. Anyone who says geology is boring needs to consider what the rocks are telling us. What they’re telling us just now is that this was a rather eventful place back in the Miocene. It makes Mt. St. Helens’ little upset look like a firecracker compared to the Space Shuttle taking off.
Let’s get off the hill that would’ve been a very uncomfortable place to stand several million years ago and head down to the mouth of the Columbia River, where we can see some points of interest, thanks to my intrepid companion’s mad skillz at shooting from the car:
From right to left: we see Scarboro Hill’s nose; the Long Beach Peninsula containing Cape Disappointment; Point Clatsop; and Clatsop Sand Spit, as we cross Youngs Bay. I told you we’d have quite a lot to say about Clatsop, and now that we can see a bit more of it, so we shall. First, however, note the color of the water. That, my darlings, is the sediment that will keep the beaches going for some time to come, and it will factor in when we begin to discuss what the Columbia River’s up to out here.
Clatsop extends all the way from Tillamook Head to here, not an inconsiderable distance. It’s a huge spit of sand, sediment dumped by the Columbia and its companions and sculpted by winds and waves. The Columbia River’s done all that in just the 8,500 years since the seas rose to their present stand. Its growth has been somewhat slowed by all the dams on the Columbia River, but it’s going to be with us for some time to come, together with its companion Long Beach in Washington. The two are part of the same system, sand trending south in summer, north in winter. Since the winter southwesterly winds prevail, the sand moves generally north.
Clatsop used to be home to great tracts of migrating dunes before some tidy-minded people decided to put a stop to their shenanigans in the 1930s and planted grass and shrubs all over them. Eventually, if things proceed as such things usually do, Clatsop’s shifting sands will become sandstone, preserving the layers of the dunes for the ages. Old beach ridges running parallel to the coast will lithify as well. Those ridges, marking old shorelines left (comparatively) high and dry as more sand got plastered to the beach seaward, are so straight you might think they were old railway embankments. They tell a story of what the sandspit used to be, and are likely created as breaker bars that, like a sort of Pinocchio, aspired to being a real beach, and eventually collected enough sand to do just that.
Now, you may have noticed with some bays that the sand spits end up very nearly taking over. In fact, they often do, and the area behind them fills up with sediment and becomes real estate. If you look up and down the coasts of Washington and Oregon, you’ll find many bays in the process of getting cut off from the sea, with sand bars nearly sealing them shut. All they’ve got left is an inlet, where the river just manages to cut through the bar. But not here, despite how the angle of this photo makes it look. The Columbia River’s a strong river, though, and the vigor of its flow has prevented sand bars from getting a proper toehold. It has not, however, managed to avoid the fate of most Northwestern rivers, which is to be nudged into a little northward bend at their mouths by migrating sand. Look at a map of the coast, and you’ll see it clearly.
We’ll end this missive on Oregon geology with the continuation of the Missoula Flood saga. When we left the Floods, they were busy pouring down the Columbia River through the narrows at Clatskanie. By the time they hit Astoria, they were very nearly down to sea level, but they still had a ways to go. Under the sea, they flowed through the Cascadia channel, through the Blanco Fracture Zone, and headed 250 miles further south until they dropped into the Escanaba Trough, a rift valley. Wait, you say – the Floods flowed underwater? Indeed they did, due to the sediment they carried. That load of debris, fine as it was, made them heavy enough to flow as turbidity currents. There’s only one word for the turbidite beds they left: megaturbidites. Even so far away from their point of origin, even after traveling submarine for hundreds of miles, they still left deposits up to 39 feet deep. And we’re talking sediments carried almost 500 miles from the mouth of the Columbia River.
Look back at that previous picture, at that faint brown tint to the water, and consider that the color is caused by suspended sediment from a river that’s carrying even less sediment than normal. Now consider what the water must have looked like when the Missoula Floods poured through carrying enough sediment to bury a house, flood after flood, five hundred miles away.
Floods of water, floods of basalt, subduction zones, mid-ocean ridges, and pile after pile of sediment, all piled up and jammed together, made this area what it is today. Even though most of that geology is covered by too much biology to be easily visible, it’s still astounding to look at, and incredible to contemplate. And we shall be doing far more of just that soon, because we are headed down the Oregon Coast, where raw, nekkid geology’s on glorious display.
Let’s take one last look from the top of the Column, and be on our way: