How did this boulder get to Cape Cod?

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This is Doane Rock in Cape Cod, Massachusetts.  It’s a big rock, bigger than anything the wind or waves could carry.  So how did it get here?

  • Did it fall from a mountain nearby?
    • No, there are no mountains nearby
  • Did it erode from the rocks underneath and was left because it is stronger?
    • No, it is a different rock type from the type of rock it sits on.
  • Did a person with a big truck bring it there?
    • No, this has been sitting here for over 20,000 years.

So how did this boulder get to Cape Cod?

Glaciers!!

The Laurentide Ice Sheet covered most of Canada and the northern parts of the United States.  It carried rocks all the way from Canada out to the edges of the ice sheet and deposited these rocks into landforms called moraines and also dropped them randomly in the landscape (erratics).

But HOW did the Laurentide pick up this boulder and leave it in Cape Cod?

Ice sheets can freeze to the rocks and ground it’s flowing over, so the boulder was ‘plucked’ from beneath the ice sheet.  Now that the rock is frozen to the ice, it flows with the ice, and in this case, was carried many miles south to the edge of the ice sheet.  Cape Cod, Martha’s Vineyard, and Nantucket are landforms that mark the southern extent of the Laurentide during the last ice age!!  The boulder flowed in the ice to the edge of the ice sheet then melted out of the front edge and was deposited here, at the forearm of Cape Cod.

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