Almost exactly five years ago I
posted
on a visit to Cottage Grove Ravine Regional Park. I was in the area and
thought I'd stop again. There's been a lot of work at the park since then, and
I wanted to check that the abandoned stream channel and dry waterfall were
still visible. Also, over the past five years I've become much better at
finding stromatolites, so I thought I'd try my luck.
Were the landforms still there?
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That's a dry waterfall, all right. The lowest interval seems
to have some weathered cross-bedding; perhaps a somewhat sandier bed in
the Prairie du Chien?
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I'd say yes, the old channel is still there.
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How about stromatolites?
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We have stromatolites!
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That's the stuff.
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Separation has occurred within a stromatolitic interval here.
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At the beginning, I was seeing them in loose blocks, which led to the question "where are
they coming from?" This was more complicated. The stromatolitic blocks were
much lighter-colored and less weathered, indicating fresh faces. There were no
obvious fresh faces on the surrounding walls, though. Had large stromatolitic
blocks broken up in the dry stream bed fairly recently? Or were these "plants", Prairie du Chien blocks that had been brought in for
landscaping and culvert riprap, then declared surplus and deposited here? The
walls were not helping.
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Features like the wavy lines give a general biotic impression, but I'd really want to see some light-colored patches
indicating fallen blocks to cinch a relationship.
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Fine lines are visible behind the dull gray and moss.
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Then I saw this enormous fallen block.
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Do you see them? They're near center. If not, hold on.
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There they were.
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How about now?
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A proverbial bull's eye.
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This block is too large and too well-mossed to have been placed here in the
past few years. Clearly at least some of the stromatolites are local.
I'm still wondering where the loose "clean" blocks came from, though.
