Saturday, August 3, 2013

The Long View: The Bones of Casco Bay

In his book, Roadside Geology of Maine, D.W Caldwell compares the islands of Casco Bay to the bones of a hand.  The view from the dock on Mackworth Island provides a view of these bones.  Like paleontologists we can use these bones to tell the stories of the past.

The view from Mackworth Island pier.  The rock that comprises farther islands
formed further in the past, because the land bows upwards.  The space between
islands tells us there was weaker rock there that has since eroded.
The bones were not always the long linear ridges you see before you.  In fact 470 million years ago you would have been looking at a flat landscape lain down by volcanos and the deposition of sediment.  A collision 400 million years ago changed all of that.  If you stood on the coast in Falmouth Foreside you'd be standing on the crest of a wave created by the collision.  The rocks between this apex and the outward islands were folded into a sine curve (a syncline to a geologist) as the microcontinent Avalonia tried to cuddle up closer to prehistoric North America.  The view from the pier provides a look over the trough of this wave and up to the next crest.

A representation of a syncline.  As you look across the bay imagine
standing on the flat edge of the gray near the anticline.  You
look forward past younger rock (brown) toward older (light tan)
While the wave still persists, it is not all it once was.  400 million years of water, ice and wind have shaved off a few pounds.  As you might expect the crests have lost the most weight.  This means the younger rocks on the peaks have been removed, allowing us a look at the older rocks below.  For this reason the farthest islands are the oldest rocks.  Effectively, as you look across Casco Bay, you look back in time.  So, what do you see?

It might be helpful to think of there being two rows of islands - The Great Diamond row and the Peaks Island row.  The back end of each of these islands is made of volcanic rock.  Volcanos form from melted rock and nature makes that happen by plunging older rock beneath Earth's surface.  This tends to happen when a continent (Avalonia) pushes its way across the Earth, submerging ocean bottom as it goes.  The two Atlanticward sides of these islands represent periods of time when Avalonia was encroaching on North America, but one didn't follow the other immediately.

These time periods when Avalonia was hauling across the world were punctuated by peaceful times of rest and erosion.  The front half of the Peaks Island Row, which includes Long Island is made up of the sediments that formed as the first film of volcanic rock broke down.  These sediments would have continued to pile up until volcanos started pumping out lava again.  When Avalonia resumed its movement, the volcanoes recommenced spewing, and the back half of Great Diamond and all of Little Diamond came into being.  When the conveyor belt stopped again, erosion and deposition started anew.  Some layers were tougher, some were weaker.  The strongest became Cow and Chebeauge
Islands and the front half of Great Diamond.  The weakest were torn apart by glaciers or washed away by streams.  Portland Harbor, which divides Portland and South Portland formed as the Fore River provided just this sort of differential erosion clearing schist and limestone from between banks of harder volcanic rock.

The bones of Casco Bay are what's left after 400 million years of erosion have cleaned off the flesh of weaker rock.  These remains tell the story of Avalonia's delivery to the shore of North America, and it can all be seen from Mackworth Island.

Caldwell, Dabney W. Roadside Geology of Maine. Missoula, MT: Mountain Pub., 1998. Print.

Hussey, Arthur M., and Henry N. Berry, IV. "Bedrock Geology of the Bath 1:100,000 Map Sheet, Coastal Maine." Maine Geological Survey: Bedrock Geology Bath 100K Report. Maine Geological Survey, 1 Feb. 2008. Web. 03 Aug. 2013. <>.

Marvinney, Robert G. "Simplified Bedrock Geologic Map of Maine." Map. Augusta, ME: Maine Geological Survey, 2002.

Osberg, Philip H., Hussey, Arthur M., II, and Boone, Gary M. (editors), 1985, Bedrock geologic map of Maine; Maine Geological Survey (Department of Conservation), scale 1:500,000

"Maine Geologic Map Data." Maine Geologic Map Data. 05 Apr. 2013. Web. 03 August 2013.


  1. Wonderful explanation for a layperson! I'm looking out at Cisco Bay as I type, wondering about how the islands came to be as I'm accustomed to the drumlin formation of the Boston Harbor Islands. This really helped - thank you.