A ROCKY HISTORY
CONNECTICUT’S OLDEST STONES ARE HIDDEN IN THE WOODED WESTERN HILLS
In a few forested pockets of western Connecticut, there are a group of rocks older than the oldest humans. Far older. In fact, they date from a time when Earth’s continents were vastly different in shape, size and position than today. U.S. Geological Survey scientists date the rocks to around 1.3 billion years old, says Jay Ague, an earth and planetary sciences professor at Yale University. These granite gneiss rocks are found in remote sections in New Fairfield, including on Wanzer Mountain east of Pine Ledge, at Hubble Hill in Sherman, and on the eastern slopes of Mount Tom in Litchfield.
Earth was formed more than 4.5 billion years ago. Connecticut’s gneiss rocks — characterized by layers of lighter and darker minerals — were created during the formation of a supercontinent called Rodinia that predates the betterknown supercontinent Pangea.
“Rodinia was formed by the amalgamation of the continental masses, right around a billion years ago,” Ague says. “So, the rocks of western Connecticut, the Adirondacks, and a number of other places along the Eastern Seaboard record that amalgamation. It was a big collision of the continental masses that produced mountains and is recorded in rocks all over the world, from Norway down through Africa, the United States, and South America. It’s a very interesting time frame.”
Around 700 million years ago, Rodinia rifted apart, helping to form the rocks of the state of Connecticut. “You may have heard of the marbles of Western Connecticut; there are quarries there and all the way up through the Berkshires,” Ague says. “These were sediments that were deposited on the rifted margin of eastern North America at that time. So once Rodinia split apart, at least for part of that time, it was facing an ocean, and sediments could accumulate on the margin of that.”
The granite gneisses “were probably buried under a large amount of oceanic sediments. And that sediment is preserved,” Ague says. “Then Connecticut has had a series of geological collisions, if you will, with terrains impinging on it for hundreds of millions of years. The rocky core of the North American plate is called Laurentia, and the eastern margin of Laurentia is where we’re at. Around 450 million to 480 million years ago, there was a collision between Laurentia and what’s typically interpreted to be an island arc terrain, something like Japan, for example, a volcanic terrain.”
It was the beginning of millions of years of collisions between what we now call Connecticut and other landmasses. About 250 million to 300 million years ago, these plate shifts resulted in the formation of Pangea.
Connecticut is a particularly good state to track the history of the world in rocks because the southern coast of the state was adjacent to western Africa at the time. “Pangea stayed together for about 100 million to 150 million years, but began to split apart around 200 million years ago,” Ague says. “The trap rock ridges — igneous rocks and brownstones — that Connecticut is famous for were formed around this time of breakup. The main zone of separation between New England and Africa lies along the Atlantic coast, but some faults that began to form during the separation extend up into the central part of the state. The I-91 corridor lies in this region. Eastern North America began to split up there as part of the larger breakup of Pangea, but it stopped before a fully developed rift between the two halves of the state could form.”
More recent prehistory also played a role in the state’s formation, Ague says. “The glacial ages are much more recent. And, of course, they helped shape the topography and the landscape. We’re talking tens of thousands of years, not hundreds of millions of years.”
North America and Africa continue to move apart today. “The average rate of movement is roughly the same as the rate that your fingernails
grow,” Ague says.
Even if you can’t track down the state’s oldest rocks, keep an eye on the striking geological formations and remember you’re looking at an era of a past so distant it can be hard to fathom. “You’ve got a lot of history preserved in our state, and it’s really quite spectacular,” Ague says.
Granite vs. granite gneiss
The rocks are essentially made of the same stuff, like quartz and mica, and are very hard and nonporous (which makes them great for countertops). The differences are in how they formed and what they look like today. Granite was formed when liquid magma cooled and crystallized, resulting in a speckled appearance. Gneiss — shown here at Harkness State Park in Waterford, a more recent creation than deposits in the western part of the state — came about when rocks were compressed and heated under extreme pressures and temperatures. That’s where its layered look comes from.
IT WAS THE BEGINNING OF MILLIONS OF YEARS OF COLLISIONS BETWEEN WHAT WE NOW CALL CONNECTICUT AND OTHER LANDMASSES.