By Mick Petrie
A small flotilla of canoes set off from Burlington’s North Beach and headed hundreds of years into the past. Jon Kim, a geologist for the State of Vermont, led this August field trip and asked the group to imagine the deep past as we paddled along the rocky shore.
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The shales and limestones deposited at the bottom for a hundred million years remain along the shores of Lake Champlain. The skipping stone you pick up at the beach is from this ancient sea.
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We looked across to the Adirondacks and back about 1.2 billion years. Below us flows a warm, shallow sea and behind us is only ocean; New England is not yet part of North America. This proto-continent is 20 degrees south of the equator, but the tectonic plate on which it sits is moving.
When tectonic plates collide, the rock is bent, folded and forced upward to make mountains. This is called orogeny. The Grenville orogeny was the first to form the basis of Vermont, yet much of what would become Vermont was at the bottom of the Iapetus Sea. The shales and limestones deposited at the bottom for a hundred million years remain along the shores of Lake Champlain.
The skipping stone you pick up at the beach is from this ancient sea.
Four hundred fifty million years ago, the Taconic Island Arc slammed into the Grenville super-continent. The Taconic orogeny created the Taconic and Green Mountains and also left behind one of the most spectacular thrust faults in the world. Clearly visible in the cliffs along the Burlington shore, an immense plate of 500 million year old pale yellow Dunham dolomite sits on top of 460 million year old dark gray Iberville shale. The older rock on top angles down into the earth and can be found miles beneath the Green Mountains in Waterbury.
More massive collisions and a nearly unimaginable span of time would pass before Vermont’s geography would resemble our present state, yet along the shoreline, you can glimpse the tumult and upheaval of the earth’s early history.
Thanks to Jon Kim and the GMC for a great trip.
