Saturday, September 10, 2011

Volcanic Ramblings Part 3: Salt Creek Falls

There's not too much of note to see on the drive down I-5 from the Corvallis exit to Route 58 just south of Eugene... at least little I'm not jaded to. So with an exception I'll tack on at the end, our first stop was at Salt Creek Falls. As the second tallest waterfall in Oregon, it's an enjoyable, lovely stop for anyone, even non-geotypes. I must say, Dana and Cujo359 got an earful of my griping about the $5 day-use fee for what used to be a nice, free roadside rest.

Dana has focused mostly on the glacial aspect of this spot in her post- and that is indeed an important part of the story- so I'll look more carefully at the bedrock geology. The Cascade Volcanic arc has an interesting evolutionary history. I have no idea if the pattern is unusual, but it youngs and narrows toward its eastern margin. In other words, when volcanism initiated about 35 mybp, following the docking of Siletzia (the Coast Range block), and subduction started outboard of that newly accreted terrane, it spread from the eastern side of the Willamette Valley to central Oregon. As time passed, though, the axis of active volcanism narrowed to what is now a fairly confined band along the eastern margin of what was originally a wide swath of activity. So the arc- at least in Oregon- can be divided into two portions: the Western (older) Cascades, and the High (younger) Cascades. The Western Cascades were more explosive overall, and produced a huge pile of pyroclastic rocks, volcanoclastic sediments and debris flows/lahars. They were also the ash source for the John Day formation in central to northeastern Oregon, famous for its preservation of Cenozoic mammals and other organisms.

According to a conceptual model developed by Ed Taylor, and apparently largely accepted on the basis of subsequent work by him, his students, and others, the origin of the modern Cascades began with the foundering of the ancestral "Plio-Cascades" 4-5 mybp into what is referred to as the "High Cascade Graben," as shown schematically above. The image that understandably comes to mind when one pictures these mountains are the big stratovolcanoes, but they are the exceptions, not the rule. Most of the High Cascade arc actually consists of a low ridge of basalt, andestic basalt, and basaltic andesite.

So what does all this have to do with Salt Creek Falls? When we stopped here with Sharkey, it was not for the pretty water feature, but because it was the boundary between the Western and High Cascades. The lower layer of rock, of Western Cascade affinity, is one I have not had an opportunity to look at closely, but judging from its fragmentary nature at a distance, I feel safe saying it's probably either lahar deposits or pyroclastic rocks of some kind- dark in color, so probably fairly mafic in composition. Overlying the fragmented rock is a thick intracanyon High Cascade mafic flow- Dana has found a source saying it's andesite, and I have no reason to say differently. I never could distinguish the mafic lavas without a petrological scope. So in a nutshell, we have a thick, competent, vertically-jointed rock overlying a soft, easily weathered and eroded rock.

This is a classic set-up for a waterfall.

Water erodes the softer rock and undercuts the overlying hard rock, which, because of its vertical jointing, slabs off to form a vertical face. While in most cases, erosion would incise and smooth the streambed, in this case, the physical arrangement of components acts to maintain a shear face. There are certainly other ways waterfalls can form, but I think of the "hard-over-soft" scenario as being the most common.

Well, isn't that sweet. Blooger has decided it controls the vertical and horizontal again. Sorry, I'm tired of fighting with this for now... maybe I can figure out how to fix it later.
Looking downstream, ~west, from the top of the falls.
This is perhaps not the best possible view to show it, but the landscape is pretty clearly the product of two distinct phases of erosion: a broad, glacially carved valley, followed by Holocene incision by Salt Creek.
The falls from the north rim...
...and from halfway down the trail.
Lovely columns.

Sunday, September 4, 2011

Sunday Funnies: Missing "A" Edition

The death of a computer is a horrible thing to watch. Two keys on my dementia-afflicted electronical difference engine no longer work: the "a" and the scroll down arrow. So how is it I have all the "a's" in these sentences? I copy one into the buffer, then paste it with the "Ctrl-V" command. It's convoluted, but it works, and one actually does get used to typing it that way. But god help me if another letter goes belly-up. Now, I find blooger has gone and "upgraded" its composition format- that is, taken a simple function that worked perfectly well and fixed it so that using it requires an advanced degree, and even then won't do what you want it to. Sigh. If my life didn't revolve around this fucking machine, I would take great joy in devising a fitting demise for it.

funny pictures history - The Frigidaire Works In Mysterious Ways, Its Wonders To Perform
see more Historic LOL
see more Lolcats and funny pictures,
demotivational posters - FORGET THE FORCE
see more Very Demotivational
demotivational posters - JOHN CLEESE
see more Very Demotivational
wtf photos videos - Here's Fig 3
see more WTF Pictures and WTF videos by Picture Is Unrelated
What Would Jack Do?
He Meant To Do That GIF - He Meant To Do That
see more Gifs
political pictures - uncle sam - That's So 1980s
see more Political Pictures
What Would Jack Do?
Sofa Pizza
Sofa Pizza
Fake Science
political pictures - guantanamo - Trollbama
see more Political Pictures
political pictures - paul ryan - Trixy Ryan
see more Political Pictures
Darius Whiteplume's Tumblr... two other hysterical historical markers at the link.
funny puns - Get 'Em While They're Trained
see more So Much Pun
political pictures - barack obama - True Slogans
see more Political Pictures
political pictures - the simpsons - republicans - Springfield's 2012 Candidates
see more Political Pictures
funny celebrity pictures - Kant!
see more Lol Celebs
 - Street Sharks, meet the new kid on the block
see more Gifs
Sober in a Nightclub
funny puns - Driving Children Insane Since Age Three
see more So Much Pun