If you could go back 500 million years in northwestern Connecticut, you would be standing at the eastern edge of the Proto-North American continent and along the shoreline of a tropical ocean. What a difference!
Today the diversity of plants in this area is astonishing, largely because of the bedrock below. When most of us look at plants, we seldom think about what is under them or how it affects what grows. Indeed, many gardeners neglect even to test the soil that nurtures their flowers and vegetables. What is under the ground, however, plays a vital role in what can grow above the ground.
Back to that tropical sea. How do scientists know that’s what was once there? Geologists say that “the present is the key to the past.” Rocks today are clues to how Earth looked long ago. To determine the geology of Earth’s ancient surface (called paleogeography) and how continents moved through time, geologists study the rock and fossil records. They use techniques like paleomagnetism to examine ancient volcanoes, and more.
The Proto-North American continent, called Laurentia, was then on the equator, hence the tropical climate and life. The ancient, tropical sea floor was different from the tropical coral reefs we know today. When this ancient life died and settled onto sediments on the sea floor, over time it was compressed into limestone, which is primarily made of calcium carbonate. At the time of the ancient sea floor, Laurentia collided with the southern continent Gondwana, closing the ancient Iapetus Ocean. The limestone was changed, through heat and pressure, into the metamorphic rock called marble.
Because Stockbridge Marble is limestone changed through metamorphism, it might be hard to distinguish what organisms helped form it, but clues might be nearby. To the west, in Saratoga, New York, stromatolites have been found in Hoyt Limestone that dates to about the same time, so they might have been in the marble too. Stromatolites are layers of microorganisms that use photosynthesis, like cyanobacteria. Stromatolite mounds are among the oldest fossils on the planet, over 2 billion years old. These mounds were a primary contributor of oxygen (a by-product of photosynthesis) to the planet’s early atmosphere!
Recently, I was privileged to be able to collect samples of Stockbridge Marble, for the Peabody Museum’s geology classes, from one of northwestern Connecticut’s quarries, situated in the landscape known as the “Marble Valley.” It’s rather amazing to think that the piece of marble you are holding was once part of an ancient tropical ocean.
Some of these quarries have been in operation since the 1700s, when they produced stone used in ironmaking to remove impurities. Local maps of the area have place names like Lime Kiln Road from that period. Today the quarries extract marble mostly as chips for aggregate and powder that farmers add to their fields to “sweeten” the soil (make acidic soil more alkaline).
Immediately to the west of the Marble Valley are the hills of the Taconic Range. During that time of colliding continental plates, when the marble was formed, the mountain building process thrust peaks more than 20,000 feet (6 kilometers) into the air. These ancient rocks, now eroded, are composed mainly of schist and gneiss. The gneiss found here is Connecticut’s oldest rock, formed 1.2 billion years ago!
Because of this complex geology—high pH (alkaline) calcareous soils in the Marble Valley and acidic soils in the nearby hills—this area has the highest plant diversity in the state. The area also includes one of Connecticut’s most imperiled ecosystems—calcareous fens. These are places where springs trickle up through marble into a peat wetland. They contain threatened and endangered plants found nowhere else. This unique and fragile ecosystem is threatened by the encroachment of invasive plants such as Phragmites (Phragmites australis), which can overrun and completely change habitats. It will be important to manage these habitats and the beautiful hills around them, so that generations to come can enjoy this remarkable geology and its ecosystem. Next time you get a hankering for the tropics, give thought to that bit of ancient tropical ocean you might have nearby.