Written by Emma Stuhl
This image shows three types of tectonic plate interactions and some of the resulting geologic formations. Image courtesy of the U.S. Geological Survey.
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In Vermont, 540 million years ago, separating tectonic plates steadily expanded the land under the sea. As Vermont lay under the Iapetus Ocean, the nearby Grenville Mountains began to erode. Water, wind, and ice cracked and fragmented the solid bedrock into fine particles, which washed and blew downhill. These sediments traveled down streams and into the ocean, where they settled out, along with the shells and dead bodies of small marine creatures. Over the ages, the loose sand and mud cemented into the stone that makes up the modern Green Mountains, and the calcium-rich shells and other bodily remains turned into the limestone and dolostone rocks that underlie much of New Haven. Today, these rocks provide ample calcium, an important plant nutrient, to the soils that cover them and to nearby plant roots.
In New Haven, the calcium-rich limestones and dolostones that underlie much of the town were warped in these massive geologic collisions. The extreme heat and pressure of these collisions folded the layers of bedrock and even metamorphosed some of the rocks, changing their physical and chemical structure. A layer of the calcium-rich bedrock recrystallized into marble, which arcs through town in a thin stripe. Some of the bedrock in town started as sand, rather than as the shells of marine organisms; this rock metamorphosed into the quartzite that now lines the eastern edge of town. Over the following millions of years, wind, rain, and ice scoured away the surface of the bedrock that New Haven sits upon, leaving behind the bedrock pattern we see today, a beautiful arc of different rock formations (see map below). In the map below, each color and three-letter code corresponds to a different rock type, which is explained in the legend at the bottom of the page. While there are many types of rock in New Haven, the majority are variants of limestone and dolostone.
This limestone bedrock was and is an economically important resource. In the 1800 and 1900’s, limestone was mined from the quarry and heated in the kiln on Lime Kiln Road to break it apart into lime for farmer’s fields and for plaster and masonry. The calcium in the bedrock also enriches the surficial materials and the soils that lie above it, slowly fertilizing the landscape.