Tuesday, October 2, 2012

OMN Willamette Valley Geology Presentation Part Three: Geology is All Around Us!

In the third part of my presentation on Willamette Valley Geology for The Oregon Master Naturalists Program, I explicitly developed one of my favorite themes: the idea that we're so surrounded by geology, we often don't even notice it. Like air, the only time most of us really pay attention to it is when it's missing. And that almost never happens.
I used the photo above and the following four to make a few points about rounding and sorting. This is not here in the valley, but at Cobble Beach on the south side of Yaquina Head, on the coast north of Newport, Oregon. The rock is a fairly monolithic basalt gravel, derived from weathering and erosion of the Columbia River Basalt that makes up the headland.
Transport of rock by water action, either in streams, or as in this case, by waves, will round the clasts, and smooth sharp edges and corners. Note camera lens cap for scale, which is 52 mm in diameter, or about 2 inches.
This is only a few paces away- same rock type, but vastly different grain size- more like coarse sand than the gravel in the previous photo. Why the difference?
The rocks on the right give a clue: where the shore is more protected, the wave energy is much lower. Larger fragments aren't transported in, and smaller fragments are concentrated. Where the shore is less protected, larger clasts can be moved in, and finer material is washed out, so the grain size is coarser.
This photo was simply to give a better sense of location; we're looking approximately west toward the Yaquina Head lighthouse. The stair down to this beach is just visible behind the cliff on the right midground.
The same rounding and sorting applies to stream gravel, which is one of three common types of aggregate used in the Willamette Valley Ecoregion. The two features to look for in identifying stream gravel are general roundedness (though broken, angular surfaces are not uncommon) and a wide variety of rock types. The rocks in stream gravels represent the types of rock found throughout the portions of that drainage upstream, but under-represent less durable, softer rock types, and over-represent harder, more durable rock types.
Another common gravel type is crushed diabase. Mixed sizes and angular surfaces allow this gravel type to pack tighter, making for a more durable surface. The down side is that this rock must be quarried, crushed, and often, transported greater distances, which makes for higher costs.
On closer inspection, you can see the characteristics that make crushed diabase distinctive: angular surfaces- little to no rounding- and a "salt and pepper" appearance. This is due to mineral grain sizes that are large enough to be distinct to the naked eye, but fine enough that the character of individual grains isn't terribly distinct.
Diabase is also frequently used as a construction or dimension stone, particularly in older buildings...
...and also as a landscaping stone. In particular, I often see it used to block off open areas from vehicle access.
This is a basalt quarry, I suspect in Columbia River Basalt, along I-5. Quarries like this are the source of the third major type of gravel used in this region: crushed basalt.
Crushed basalt gravel, in the "wild."
A crop of the above photo showing its characteristics: angular, flat tone, no crystals apparent to naked eye, and dark gray to black in color. With kids, I get them to learn basalt- by far and away our most common and abundant rock type in Oregon- as BUR, or Black Ugly Rock. With adults, I feel more comfortable admitting my own pet name for basalt is GDB- God-Damned Basalt. Not that I dislike it or anything, it's just so common around here it gets dull and repetitive.
I've noticed quite a few buildings in my neighborhood have gone with variations on the idea of xeriscaping, or dry, low water-cost landscaping. Some of these have gone with coarse stream gravel, either over the whole yard, or what the landscaper who created the above described as a "dry water feature." I took that to mean that it brings water to mind, but doesn't actually use any. I quite like it.
My favorite coffee shop. Is this really geology? You bet your sweet bippy. There's river gravel in the concrete- coarser in the street pavement, finer in the sidewalk. There's sand in the concrete blocks and mortar of the building itself. There's iron in the fence around the seating area, in the car and in the fire hydrant. There's copper in the power lines. There's sand, gravel and petroleum in the asphalt. There's sand in the window glass. And on, and on. Geology surrounds us, it's just that the materials are so much a part of our environment and day to day existence, we don't really perceive or think about it.
In fact, it's much quicker to pull out the items that *aren't* directly geological in nature. In this case, there are a few trees. The telephone pole and wooden porch are wood products, but in both cases I'm quite confident that preservatives made from geological products are infused into them.
It's *all* geology...
...even when it doesn't grab you by the neck and shake.
A photo of the sidewalk outside my favorite coffee shop, showing the wide variety of rocks present- though overwhelmingly volcanic in origin. While I chose this particular spot so I could get an agate and a piece of petrified wood in the same frame, there's nothing unusual for Willamette gravel here. See this post for more information.
This was a particularly auspicious find- I was walking around the block a few weeks ago, ostensibly to take photos of the various types of gravel, and I noticed this obsidian clast. It appears to be from recent damage- probably a car backing out of a parking spot and bumping the wall. Obsidian only rarely survives transport out of the Cascades and onto the valley floor.
In part two of this series, I remarked that Fisher Island was actually a part of Benton County, despite being on the Linn County side of the river, and that the river had cut off there. As you can see in this image, that area has been developed as one of our local sand and gravel operations.

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