Monday, December 7, 2009

Not so Schisty

I'm grateful for the illuminating comments my father, geologist William Melson, left on the recent round of Great Falls photography. Below find his remarks regarding the image above, turns out what I took for schist was a gneiss. I post the comment here for the wannabe geologist in any of us, or anyone who simply enjoys talk of a good deep-sea slump, trench or submarine avalanche.
These blocky rocks are probably gneiss, which, like schist, is a metamorphic rock but not as platy or schisty. Some of the gneiss in the Potomac River Gorge is derived froma deep-sea slump deposit termed a turbidite. If I remember correctly, the original turbidites were deposited as submarine "avalanches" into a deep-sea trench during the Ordovician Period, around 450 million years ago. This trench was later folded and added to the North American plate. There, it became deeply buried within the plate, subjected to high temperatures and pressures which changed the formerly sand and mud of the turbidite in a gneiss.


Robin Morrison said...

I likes me a bit o' tectonic danish too.

dista: archaic honorific distinguishing generic 'Mister' from a more familiar version distinguishing misters one knows from misters one has just met.

amarilla said...

Glad you liked it. My father left other puff pastry, of a dense variety here: