Middletown 20,000 Years Ago and Its Enduring Legacy: Part 2
Contributed by Dana Royer
In Part 1, I wrote about how the events in the Middletown region 200 million years ago left an enduring legacy, persisting to the present-day. Landscapes holds an accumulated memory, bearing numerous witness marks from its past. In this second and final part, I turn to another key time in Middletown’s geological past: the melting of ice sheets from the last Ice Age ~20,000 years ago.
The Enduring Legacy of the Ice Sheet’s Demise: Lesson 1 – Connecticut Landscapes
The last Ice Age reached its maximum extent about 20,000 years ago. In our area the ice reached what is today Long Island. Here in Middletown the ice sheet was around a mile thick, so if you were walking around town then the air would have been thin like in Denver today. As the ice advanced, it scraped away soil and pulverized rock, resetting the landscape. The action of ice dragging rock across bedrock makes literal scratches and grooves in the bedrock, and the orientation of the marks tells you the direction of ice flow. There are many places to see these grooves, including on top of Mt. Higby, where they have a north-south orientation (as you would expect).
As the ice began to recede, a large amount of material was dumped along the southern edge, called a terminal moraine. If you have friends in Long Island, you can tell them that they live on a (geological) garbage dump. It took about 10,000 years for the ice to recede to its (roughly) present-day position. As it melted, it released its content: lots of freshwater and rock of all sizes, from clay particles to car-sized boulders. Because these rocks were moved, they don’t always match the underlying bedrock. Geologists call boulders like these “glacial erratics”. They are common throughout New England. There is a nice example on Home Avenue south of Wesleyan University, near the turn to Beach Street (see photo).
On top of Mt. Higby, I have found in the soil pieces of not only basalt (rock fragment on left in photo below), which you would expect because the bedrock on Mt. Higby is basalt, but also brownstone (middle fragment) and fragments of rock from outside the central basin (right). It’s amazing to think that ice—thousands of years ago—brought this material up the mountain, sometimes from many miles away.
The melting of the ice sheet was chaotic, not smooth and uniform. Not only was rock left behind, but also chunks of ice, which melted more slowly. This rock and ice sometimes formed dams, creating temporary lakes behind them. The most massive glacial lake in New England was Lake Hitchcock, whose southern shoreline was protected by a massive sediment dam in Rocky Hill (see map).
Over the thousands of years that melting was happening, multiple lakes formed and drained, including some in Middletown. One place to see the “wreckage” of one of these drained lakes is in the Dividend Pond park in Rocky Hill (accessed from Old Forge Road). Head south from the parking lot and continue south past Dividend Pond into an open, dune-like space sometimes called the Mustard Bowl (see satellite image). Here you will see what seems like an infinite amount of sand, which was deposited after the catastrophic failure of Lake Hitchcock. It is not a coincidence that adjoining to the south is the River Highlands golf course (plenty of sand for the sand traps).
The Enduring Legacy of the Ice Sheet’s Demise: Lesson 2 – Connecticut Soils
I mentioned in my first post that Connecticut’s central valley has some of the richest soils in New England. This is partly because the basalt and brownstone contain more nutrients like calcium and magnesium. But our glacial history also shaped our soils. The soils in most of New England are chock full of glacial cobbles: fist-sized rocks that melted out of the receding ice sheet. If you’ve ever dug into a New England soil you’ll know what I mean. Farmers curse these cobbles, which due to freeze-thaw action are brought to (or close to) the surface every spring: the curse of the spring potatoes. This is one reason for the ubiquity of stone walls in New England: they are linear garbage piles (which we now see in a more romantic light).
Soils sitting on top of the remnants of glacial lakes are different. When these lakes existed, sediment would have been accumulating along their bottoms. Lake sediment tends to be well sorted (that is, the individual grains are all about the same size) because the sediment must first pass through a column of water. After the lakes drained, what remained was a flat expanse of (mostly) silt and sand. For farming, flat = good and no cobbles = good. You see very few (if any) stone walls in the areas that were once occupied by glacial lakes. Separate from lakes, glacial cobbles—and stone walls—are less common in the central valley than elsewhere in Connecticut. This is because the bedrock in our valley—basalt and brownstone—weathers more quickly than the most common rock types elsewhere in the state (mostly schist and gneiss). So, the cobbles that were deposited in the soil thousands of years ago had a greater chance of getting broken down into sand, silt, and clay.
Further resources for the general public:
Michael Bell. 1982. The Face of Connecticut: People, Geology, and the Land. Bulletin 110 of the State Geological and Natural History Survey of Connecticut. Wonderful book that explores the intertwining of human and natural history in Connecticut; available online for free.
Jelle de Boer. 2009. Stories in Stone: How Geology Influenced Connecticut History and Culture. Wesleyan University Press. Another outstanding book on Connecticut’s human and natural histories.
Robert Thorson. 2002. Stone by Stone: The Magnificent History in New England’s Stone Walls. Walker. A deep dive into one of the most distinctive features of our current landscape.
Contributed by Dana Royer
Dana Royer is a Professor of Earth and Environmental Sciences at Wesleyan University. He is interested in the paleoclimate and paleoecology of terrestrial ecosystems that are millions of years old, and has published over 60 papers on these topics. He teaches courses on environmental studies, climate change, paleontology, and soils.