Wednesday, August 15, 2012

Real Info

South Burlington, Vermont

I'm delighted.  I went to the Lake Champlain Echo Center yesterday.   It is basically a kid's museum, but there is a resource room.  The resource room lady, Laura, was very knowledgeable and very helpful, and she has resources and references at hand.   I learned a lot from Laura.

  • The features in the lake like Ferris Rock are bedrock, not boulders as I suspected.  That is made clear by looking at the detailed bathymetry data.  See the picture.  Laura notes that there are some basalt intrusions into the otherwise softer sandstone and limestone bedrock that might be more resistant to erosion.
    Bathymetry data showing Ferris Rock

  • Further close examination of the bathymetry and elevation data show that, contrary to my impression, the lake is not a gorge, but rather a water covered portion of the U-shaped glacial valley.  However, there's lots of faults and thrusting, making it plain that the valley predates the glaciers by a long time.
  • What is the nature of the lake's basin? Valley? Canyon?  She says there is no official word.  However --
  • Natural slippage of this 1 foot block on Valcour may be analogous to formation of the whole valley.
    • Geologists say that it does not appear to be a river valley, or a river formed canyon like the Grand Canyon.
    • It does appear to be a rift valley.  More specifically, it appears to be slippage of a block when neighboring blocks are pulled apart.   The picture below is an illustration from Valcour Island.   The slipped block in the picture is only 1 foot wide, but imagine a slipped block the size of the entire Champlain Valley.  Laura had a technical word for this that I forgot.
    • The Champlain Thrust, within the city limits of Burlington shows older rock thrust over younger rock in a fault.
    • The palisades down by Split Rock and across from Porter bay are also clearly thrust upward from a fault.
  • Laura reminded me that the western shore of the lake was once the coast of Laurentia (the pre-continent to North America) and certainly the location of kind of plate subduction that creates trenches like the Mariana Trench.   The green mountains were perhaps formed at a much later tectonic plate collision.   The region is so ancient, that most direct evidence of the history has been wiped out by repeated stretching and shrinking of the ocean floor.

    The point is that the complete answer to my questions involve a complex combination of glacial, volcanic, and fault thrust geologies.

    I can't speak for you but for me, this is fascinating information.  This region is anything but boring.

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