Tuesday, September 28, 2010

I Love Deadlines

"I like the whooshing sound they make as they fly by." It's especially gratifying when it's a deadline I set myself... in this case, the deadline for submissions for AW 27, important geological experiences. Like some other editions of the Wedge, I expected the difficult part would be to pick just one experience- after all, no single experience is enough: there's so much to know and do that "years" is a narrow timeline for feeling anything like competent with respect to geology.  "Decades" is a better approximation, and even then, there are important aspects of the science with which each of us will never feel all that comfortable. How to choose one experience out of the hundreds of hours of class and lab time? Thousands of hours reading? Months of accumulated time pondering over (and silently cussing at) outcrops?

So it was with a feeling of surprise that I knew almost immediately which particular day was the one that stood out in my memory.  A bit of background is in order: I referred to myself as the eternal undergraduate. I started at OSU in spring of 1980, and graduated in June of 1988.  There were simply no classes left in geology that I was allowed to take, and I had sat in on a number of graduate classes.  There were plenty of classes in other departments and disciplines that I'd like to have taken, but it was time to move on.  The main thing that enabled this, though, was that 20-30 years ago, it was possible to work one's way through school, pay as you went, and come out debt free.  That is not the case any more.

I worked in Clinical Pathology in Veterinary Medicine from the beginning of 1981 until summer of 1983, then I landed a job as a lab and field worker with a forest soils research project, where I ended up working until late 1990- even after I graduated, they found ways to keep me on for a while. One of the projects we worked on for years involved looking at the soils and soil water chemistry at Cascade Head. Below is a map of the area, though it doesn't show old 101, which lies a bit east of the new highway. (Screen capture from page 5 of Research Publications of the Cascade Head Experimental Forest and Scenic Area, 1.4 Mb PDF)
(As an aside, if you're ever going through- or even near- Otis, The Otis Cafe is secretly famous for the quality, quantity and price of its food... it's one of those "Don't tell anyone, but you should know..." sorts of places.) Cascade Head is an Eocene volcanic center, and the basalt- partially submarine, partially subaerial- is interfingered with nearshore marine sediment.  And everything is weathered. With the exception of the shore itself, it is no exaggeration to say that the soil and saprolith are 10-15 meters thick.

Except.  Except...

In our soil pits, from which we collected samples and water that had leached through the top meter of the soil, we kept finding these clusters of basalt cobbles, about fist-sized. They too were deeply weathered, but under a centimeter or two of clay, quite fresh.  Judging by the weathered surface, they were pretty angular; if they had been transported, it wasn't far.  Without thinking much about it, I simply assumed they had moved in by soil creep. It turned out later that creep wasn't a tenable mechanism, because there wasn't enough uphill uphill from our site, and no basalt outcrop either.

However, the primary investigator had his own thoughts... he was first and foremost a soil chemist, and these cobble clusters were affecting ground water flow in ways he didn't understand.  They also made getting the lysimeters (the devices that captured soil water) into place a real pain, and they were also very sporadic- some pits had none, others had to be abandoned.  In short, they were throwing an extra variable into some research that was already more complex than anyone liked, and apparently, he really didn't appreciate this.

So it was with a mixture of excitement and trepidation that I greeted PI's announcement that we were going to figure out once and for all what was going on with the cobble clusters.  I didn't have my degree yet, but there was certainly no one else who knew any geology, and I knew immediately that this was show time.  It wasn't a situation where my job was on the line- I was a good worker both in the field and lab, and I knew they knew it.  But my geologic credibility was on the line, and that was very scary to me.

I had read a master's thesis by Melanie Barnes, (M.S. 1981 The geology of Cascade Head, an Eocene volcanic center in the central Oregon coast range), but I went back through it paying special attention to discussion of the sedimentary facies, which didn't help much. So when the big day came, I still had no real idea what might be going on- though I had dropped the idea of creep. Not only was that physically unlikely for the reasons mentioned earlier, but it didn't sit well with the fact the cobbles came in discrete clusters, not randomly scattered.

So basically, we spent the late morning and much of the afternoon simply driving around, looking for outcrops. Since the bedrock is so deeply weathered, that was more difficult than might be imagined.  As Dana learned last week, in the western part of the Pacific Northwest, often the best geology clues and cues can come from subtle color changes in soils and rock weathered in place... but those colors can also be very confusing and misleading.  In effect, we spent most of the day carefully examining silty mud.  I was at wit's end.

Finally, we came around a corner and there it was:
And even though this is terribly simplified and schematic, and the color differences much more stark than the actual outcrop (also, much less obscured by botany), it really was that clear to me: we were seeing lag cobbles in cut-and-fill structures. In the cartoon above, the dark circles represent the basalt cobbles. Smaller clasts, pebbles, had simply gone completely to clay, and were only noticeable as different colored spots- in others words, not really noticeable at all in the context of our soil pits. In the Eocene estuaries, tidal flats and near shore, drainages from the land, particularly during times of high flow, would incise channels and move the cobbles along as part of their bed load. Later those channels would be filled in as flow decreased or as meandering cut off one location after another.  And the clusters of cobbles would be left to confound future scientists.

I've looked around a bit and haven't found any really great photos of cut-and-fill structures, though this one at Andrew Alden's gallery is good, and you can see how the coarser clasts get concentrated at the bottom.  Then earlier today, as luck would have it, Suvrat posted a couple of really nice photos- see pictures 3 and 4.

Now why was this so important to me?  It required me to figure out something on my own when it counted- this wasn't a class exercise with a right answer, and someone who knew it to check my work afterward.  It wasn't just me on my lonesome pondering imponderables. It was work for some people who knew no geology at all, who had no clue what was going on, and to whom the answer really mattered.  And I was able to come through and provide an answer, an explanation, that worked with what we knew, and helped explain some of the things we'd seen.  Sometimes the cobble clusters seemed to channel groundwater flow.  Sometimes they seemed to go back in the soil profile, sometimes they seemed to go across, from the perspective of working in the pits.  They seemed to start or end very rapidly on their edges, but at the same time, they had a vaguely linear presentation.  The cut and fill model worked with what we knew about the area, and what I knew about geology; it fit, and it explained.  It was immensely satisfying.

And though I'll never live to know enough geology to satisfy me, it was the first time that I really understood I knew enough to solve at least some problems that needed solving.

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