Wednesday, December 2, 2009

Rocks on the Shore of Lake Champlain Tell an Interesting Story

An early May paddle on Lake Champlain turned into an interesting geology field trip (for me at least ;-) ) when we passed this rocky shoreline north of Burlington, VT.  The rocks here record a significant event in the formation of North America.
To understand what happened here, imagine a deck of cards spread out on a table. Now imagine that you use your arms to bring the cards together into a pile. As the cards slide together some will end up on top of others, and the mass of cards will become shorter in the horizontal direction, but thicker in the vertical direction. In a similar way, colliding crustal plates (“drifting continents”) produce thick masses of earth’s crust pinched between them.  Around 440 million years ago, eastern North America was deformed as it collided with volcanic islands to the east as the ocean between North America and Europe was closing (and as the super continent of Pangea was assembling).  This event, called the Taconic Orogeny, resulted in the rocks of western New England piling up and forming the Taconic Mts. (which have been reduced from their former grandeur by 400 million years of erosion!)
So what's happening in this photo?

Essentially you're looking at the boundary between 2 of the cards you imagined above.  The rocks in the top half of the photo have been thrust westward (to the left) up and over the rocks at the bottom of the image along a "low angle thrust fault" (traced with a red line).  It's hard to judge just how far the top rocks have moved relative to the bottom rocks right here, but along a major thrust fault east of here, the displacement is estimated to be on the order of 50 miles! (See this Earth Science Picture of the Day).  And you can see that the rocks themselves have been squeezed, too - notice that the shortening and thickening is evident on a very small scale in the deformation of the light colored vein at "A".
You can see more pics of this outcrop, and pics of the entire paddling adventure if you have nothing better to do.
And here's a pretty nice cartoon of the Taconic Orogeny from oldest at the top to present day at the bottom.


Eric Klotzko said...

Awesome photos and article Kluge! What percentage of their original size to you estimate the Taconic Mtns are today vs. their peak elevations?

Anonymous said...

Nice post, Kluge!

Steve Kluge said...

Good question!
A small percentage. (cop out answer) I think it'd be accurate to say they are mostly gone today.