Tuesday, June 1, 2010

Mega road trip - Day 1

Nothing much interesting between the Hudson Valley and Columbus, OH where I am tonight.  I've driven I-80 through Pennsylvania many, many times and it's always a long boring slog (you're actually happy to see the "Welcome to Ohio" sign).  Then again, I'm from the northeast so forested hills aren't too exciting to me.

There are a few interesting roadcuts on I-81 down past Scranton and Wilkes-Barre, however.  They are primarily folded sandstones and shales with interbedded anthracite coal seams clearly visible.  The Wyoming Valley along I-81 is a syncline - a downward warping fold (think of a canoe-shape - it's the labeled "Anthracite Valley Section" in the geologic map below). 

It used to be thought that the folding here was due to the compressional forces from the collision of Africa with North America to form the ancient supercontinent of Pangaea (this is what formed the rest of the Pennsylvania Valley-and-Ridge Province of folded rocks).

More recent seismic work has indicated that the syncline (and valley) actually sagged downward due to the removal of underlying salt beds (the same types of underground salt beds still mined in central and western NY to the north).  The salt (maybe 700 m or so) dissolved away from subsurface fluid flow during Appalachian mountain building from the above-mentioned collision.  It's a salt-collapse structure (I'll talk more about salt collapse if I post about Arches National Park next week!).

If you want to read more, see the paper by Harrison, Marshak, & McBride.

I always think it's neat to drive on highways, see high rockcuts on the side of the road, and think about how all that rock is just a few feet beneath our feet everywhere on the Earth's surface and extends downward for miles.  We only get to see an examine a miniscule portion of what's down there.

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