Friday, January 14, 2011

Architectural Geology of Boston: The Stone Walls of New England

A typical old, stone wall in Weston, Massachusetts, made up of gneissic and granitic glacial erratics.

“Something there is that doesn't love a wall,
That sends the frozen-ground-swell under it,
And spills the upper boulders in the sun;
And makes gaps even two can pass abreast.”
By Robert Frost, Mending Wall , 1914

It likely sounds improbable, but one of the things that got me interested in geology was the stone- walls in Central New York, where I’m from, and in New England, where I live now. A short drive outside the city is all one needs to see them. They’re everywhere, in varying states of repair or more likely disrepair. They crisscross through New England over an estimated 250,000 miles. They snake up and down through the woods, along the road, and through the countryside, and often are severed in two by a road or highway that has since been constructed.

More decorative than functional nowadays, they are a neglected remnant of a time and lifestyle long gone. Many of the region's stonewalls are reminders that the woods and forests that now surround them were once open pasture during the early part of the 1800's, when sheep-farming dominated the landscape. The forests have reclaimed the pastureland with the abandonment of sheep farms. The walls also have a story to tell, if you're willing to listen, that is geological.

Not having a formal education in geology, I noticed that the walls, or more accurately their stones, reflect the composition of the indigenous bedrock, the solid rock beneath the surface. It's a rather obvious observation, when you think about it. Colonial farmers and herders cleared their farmland and grazing pastures, which was a huge chore in glaciated New England, and marked their territory by using the large rocks strewn across the landscape. The legacy of their toil is built into their walls. Anyway, the walls got me started!

An old field wall in south-central New Hampshire made of indigenous, cobbles of granite.

Once I made the stone wall-bedrock association, I began to investigate the New England walls' compositions. I noticed that some walls were constructed of conglomerate, some limestone, and others were a curious igneous-metamorphic mix of granite and gneiss. Back in Central New York, the walls consist almost exclusively of limestone. The entire region was a foreland basin during the Devonian and a perfect environment for the marine biota responsible for the limey bedrock.

In the immediate western suburbs of Boston where I live, the walls of the Boston Basin are all conglomerate, deposited in a back-arc extensional environment within a magmatic arc-terrane during the Late Precambrian while tectonically in transit from the South Pole. I was totally hooked on geology when I found that one out! The walls in Massachusetts further to the west outside of the Basin are generally a mix of granite and gneiss, reflective of the accretionary history of the arcs and micro-terranes that formed New England during the mid-Paleozoic.  

A colonial-era wall composed of Roxbury Conglomerate within the Boston Basin.

In virtually every case, the walls (at least the old, original ones) found on the terrain represent a geological signature of the region or geological "terrane" from which they were derived. Take a drive through the Northeast or wherever you live, and you’ll see what I mean.

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