About 450 million years ago, the first of two major mountain-building events occurred. Tectonic plates collided closing an ancient ocean and forming the Green Mountains. The ocean sediments on the continental edge were compressed and piled up by the force of the collision and formed the bedrock in Vermont. The deeper rocks responded to the high temperature of the Earth’s mantle and to the increased pressure by folding. These rocks underwent great compression transitioning to form crystalline structures. Some of the sediment melted and bubbled back up to the surface. Most of the rocks in Plainfield are azoic, meaning having no trace of life or organic remains. To look for fossils, one would need to travel to the Champlain Valley. On the other hand, the granite that abounds in Plainfield is not found on the west side of the Green Mountains.
Following the tectonic plate collisions, a long period of relative quiet followed, during which the newly formed mountains were eventually ground down by erosion. If the sediments that eroded off the Green Mountains were piled back up they would create mountains at least eight thousand feet high if not higher. During the period after the Green Mountains formed they were reduced to half this height by weathering. At this time the Winooski River was young and Vermont had a tropical climate. It was warm and swampy, with tropical fruit growing in the river valley.
Then, beginning around three million years ago a change in ocean currents led to cooling of the poles, which marked the beginning of the Glacial Period. During the last advance of ice out of Canada, the Laurentide Ice Sheet buried all of Vermont under 1 to 2 miles of solid ice lasting to as recently as 13,000 to 10,000 years ago. Glaciers covered New England and beyond. The glaciers scoured many meters of sediment from the mountainsides and laid down a massive layer of glacial till that became the parent material for many of the soil types in Vermont. Cape Cod and Long Island are made of material plowed up by the ice sheet and deposited in the ocean as a glacial moraine. Ten thousand years ago, as the glaciers retreated from Vermont, they left behind a barren landscape. Tiny tundra plants soon colonized the rubble, followed by hardy willows and alders. Woolly mammoths, mastodons, saber-toothed tigers, caribou, and wolves eventually roamed this rugged landscape.
The melting glaciers created new lakes and it is thought by Geologists that for a time the Winooski River may have flowed backwards to the Connecticut River, and that a lake 20 miles long covered the area where Plainfield and the neighboring towns now lie.