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Wellfleet Geology

July 19, 2017

 

 

During the last ice age, the giant Laurentide ice sheet marched south from Canada across New England, picking up, eroding, pulverizing (turning to sand) millions of tons of stone in its path.  21,000 years ago, the glacier reached its southernmost extent at Nantucket and Martha's vineyard.  At that time, sea levels may have been 300 feet lower than they were today, exposing miles of what is now the sea floor.  3,000 years later, the ice sheet had retreated to the Gulf of Maine.  In the meantime, meltwater carried loads of sand, gravel, and stone and deposited it in "outwash plains" that became the cape and islands.  Where large chunks of ice were left melting in the sand, Cape Cod's kettle ponds lie today.

 

Wellfleet is home to an abundance of stone deposited by glaciers during the most recent ice age.  The deposits consist of close to equal parts granite, felsic volcanic (extrusive) rock, and quartzite.  One older deposit that stretches south of Lecount Hollow up to Cahoon Hollow is primarily granite.  The green sections of the map, termed "eastham plain deposits" have a high percentage of volcanic rock.


Granite, like volcanic rock, forms from cooling magma, but cools underground, at a much slower rate, often yielding a more consistent grain which lends itself to building.  Quartzite is a metamorphic stone formed from quartz sandstone under enormous pressure.

 

Sources:

http://ngmdb.usgs.gov/Prodesc/proddesc_2057.htm

http://www.nature.nps.gov/geology/parks/caco/

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