Puddles, Gravels, Pebbles, Cobbles, and Knowing Too Much

 

We had a couple of warm days this week, and three big liquid pools appeared in the ice.  Three of them, just offshore of the back yard.  None of the neighbors has these windows to lake; in fact, the guy two doors down was still driving his pickup out to his ice fishing house the day the pools in the ice appeared.

At first I figured they were probably just puddles on top of the ice.  But they got bigger and bigger, and bluer, and then there were four of them.  On a warm afternoon this week, I walked out onto the dock to see them better.  Of course the dock is pulled up onshore in the back yard, the end of it resting on the sandy strip we’ll call the beach this summer, but it’s a good six feet off the ground right now,  so it’s a better vantage point for viewing the pools in the ice.  Even from there I couldn’t quite tell.  So I  climbed down to get some pebbles. Or maybe they were cobbles.  Pebbles, cobbles whatever.  Stones, for the purpose of throwing.  One missed the mark, and stopped on top of the ice just before it hit the water.  I threw the next one harder and sure enough, it disappeared with an extremely satisfied ka-gloop, sending wide ripples out to the edges of the pool.  Definitely not just a puddle, a gen-yoo-ine hole in the ice.  That meant the end of walking the dog on the lake, as far as I’m concerned, though he hasn’t quite agreed with me yet and keeps wanting to head in that direction on our daily walk.   But the next morning, the dozen or so ice fishing houses we could see from here were gone, and the collective human wisdom will prevail.  After all, we are taller and can throw rocks and therefore see better the distinction between puddle and open water.

So I’m thinking probably have some springs in our 150-ft slice of lake, pumping warmer groundwater upward under the ice.  If I am right, it is a wonderful stroke of luck, that just offshore of our backyard some springwater will be flowing upward, keeping our stretch a little cooler and a little cleaner than it would otherwise be this July. 

If I’m not right, and instead what’s seeping up under the ice is leakage from our septic system, then something very different will be going on out there in July. Probably something very different, and unpleasantly green and soupy, will be happening by April.   

This is science at its best:  you see how I did that? I created multiple working hypotheses.  Testable in a few short months.  Clear water or green soup.  The trick will be to remain objective if one of the testable hypotheses turns into stinky and expensive  disaster.

Today is bright and cold and back down to the 20’s and a few ice fishermen are out again, but without their houses.  They walked out, and they’re just sitting on buckets on the ice.   The pools are shrinking today, and may ice up again before tonight’s storm hits.

I don’t know what will stand out for my readers in the preceding story (probably the stinky and expensive green soup), but for me what sticks is my inability to just call the stones I threw in the puddle “stones.”  I’ll tell you, there are some very wonderful things about knowing a thing or two or three about geology, but there are some irritations too and one of them is that I have to force myself to use the word “stones.” 

The things I picked up off the ground to toss into the pools of water are, technically speaking, pebbles, but big ones, almost big enough to be cobbles.  In the world of geology, and particularly in the world of sedimentology (the specialization of my training), the distinction lies at 64 mm.  Or if you prefer, 2.5 inches. Or, if you are reading this because you are one of the extra-lucky few people who know about Krumbein’s Phi Scale (maybe even because I taught it to you), right around φ = -6.[1]  The stone still sitting on the ice out back looks to be just about exactly that, as was the one that is now at the bottom of the lake.  So, you see, I am stuck not knowing quite what to call them.  Pebbles or Cobbles. 

Anyone who has spent too much time in school specializing in one thing or another runs into this trouble.  I imagine some astronomers are unable to see constellations as the rest of us can, knowing (as they do) that Alnilam, the center star in Orion’s belt,  is a blue-white supergiant, 1300 light-years away, much farther than Mintaka (900 light years) or Alnitak (800).  And that Alnitak itself is three stars, one of them a blue supergiant.  Already, see, there’s really no possibility of simply seeing these three stars in a straight line in the sky ever again.   You know too much.

My sister the Latin scholar posted on Facebook today something I did not know, but now do:  that the word “science” derives from a word meaning “to know.”  Now that I know that, it’s not too far a stretch to see why for many people the word “scientist” means “know-it-all.”  And not in a good way.

But the rewards of knowing a few things about science are usually worth these minor annoyances, annoyances like not being able to use a perfectly good word like “stone,” or not being able to see Orion’s belt as a two dimensional line ever again. 

In the case of my back yard, the pebbles, or cobbles, are part of a much bigger picture, a wonderful unlikely story.  A really big story actually, about a wide gravelly rush of melted  glacier that came charging through what is now the Cannon River Valley on its way to the Mississippi.  The big river flowed through steep rapids and drops, constantly tumbling and shifting all of this gravel from one side of the valley to the other, keeping any encroaching tundra vegetation to a minimum, and eroding away the edges of the white sandy hills at the valley’s edges. For a few hundred years, this river was probably most dramatic during the summer, when the ice was melting.  Glacial melting would have slowed or even stopped during the cold winter months.

Eventually, though, summer by summer, the glaciers that covered Minnesota with a mile-thick slab of ice finished melting.  Then, the wide, wild river calmed and narrowed to the trickle of its former self we call the Cannon.

How do I know it was fast, and steep, and powerful?  Because slow flat lazy rivers don’t move cobbles.  They move mud and silt, and during spring floods, maybe some sand.  Consider this 1902 photo of a gravel-bedded braided river in Alaska.  Imagine the mountains in the background are huge melting blocks of ice.  Now you're close to the picture I have in my head. 

http://commons.wikimedia.org/wiki/File:Alaska_Floodplain_1902.jpg

 If I can figure out how to post a Google Earth image to this blog, I’ll show you what the ground looks like along the road to our house – it’s a myriad of low ridges and sluiceways, running roughly parallel to the course of the Cannon, that made up the gravel bars and channels of the wide braid plain of the pre-Cannon riverway.

 Now the mile-wide gravel plain underlies the well-tamed towns of Stanton and Randolph, the Stanton airstrip, the Syngenta experimental fields, and the irrigated cornfields of the big farm on the corner by our house.  Irrigated because gravelly soil is well-drained soil -- too well drained to grow corn without extra water. *

So this is what science does, at least for me.  It takes me from the pebble I tossed off the dock to this image of an Alaskan river, and puts it right in my back yard, with a brief detail to Krumbeins logarithmic phi scale.  Not bad for an afternoon of blogging.  

It's a good thing, this knowing.  

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[1]Okay, if you truly want to know:   use this formula,

D = D₀2 ^{− φ}
where D is the diameter of the particle in question and D_o is a reference diameter of 1 mm. 

The value on the phi scale is the exponent of 2 that gets you to the diameter of interest times (-1).Why the multiplication by (-1)? It’s a convenience invented for the majority of sedimentologists, who deal more often with particles smaller than 1mm.  The gravel and conglomerate aficionados, like me, just put up with negative phi values, happy to know we are in the minority, and no one is trying to hone in on our macroscopic turf.  

*Also just a little too well drained for me to ignore the possibility that my septic drainfield may seeping warm water below the house’s foundation and out to springs under the lake. I'll keep you posted.  Check back at Stork Raving in three or four months.