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Soto and Huoppi - Glacial History of Connecticut The Traprock, Vol. 1, December 2002, pp 29 - 32

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The Glacial History of Connecticut and Its Effect on the Present Topography of Connecticut

Rachael Marie Soto, David Huoppi

It is difficult to imagine the entire Connecticut landscape covered with a massive ice sheet. The tall trees and the lush grass, our driveways and yards- all buried beneath an immense glacial sheet. When we think of the term 'Ice Age,' we envision a world of snow and ice, a world that is forever cold and deserted. However, this Hollywood image of an ice age is disingenuous- it suggests an environment without movement and without change. Quite the opposite, this very land we stand upon was buried under ice. Over two million years after the Ice Age began, it is far time New Englanders consider the surrounding environment in light of the glacial formations of the past. Today, glaciologists have located and examined plenty glacial evidence found all over the state of Connecticut. Found all throughout the Connecticut landscape, many rocks themselves tell the glacial history of Connecticut. They are testaments that the glacial formation, movement, thickness, and depth that existed long ago had lasting effects on the topography of the state. Namely, the glacial markings on rocks enable us to determine the orientation of the ice sheet. Examining the overall topography of Connecticut, we find glacial marking on both the highest and the lowest topography. Such a finding with rocks suggests that the entire state of Connecticut was once entirely glaciated. Examining the evidence left behind in glacial deposits further corroborates the Ice Age theory. Currently, glaciologists today have applied the evidence found in Connecticut's topography to determine the behavior of the last ice sheet. Even today, Connecticut remains in a state of recovery from the effects of glaciation. We find ourselves in what geologists call an inter-glaciated period. Meaning that scientists anticipate the formation of yet another ice age in the next 500-7000 years. Moreover, the glacial evidence found in the topology points in the direction of an Ice Age with similar occurrences as the last.

When one thinks of Connecticut, along with most of New England, beautiful landscapes come to mind. Landscapes filled with beautiful foliage and vegetation- a land filled with life. It is difficult to imagine the entire landscape of New England completely covered by a huge ice sheet, concealing what we have come to know as a verdant environment. While the Ice Age in New England began about two to three million years ago, yet its effects remain long after. The Connecticut ice age was a very cold and wet period. Although the ice sheets melted away years ago, ample evidence surrounds us. The glacial evidence is prevalent nearly everywhere we look, so much so that glaciologists are able to reconstruct the glacial formation, movement, thickness, and effects simply by observing the geological evidence the ice sheet has left behind. Each bit of glacial evidence serves as a piece of the puzzle,

uncovering the astonishing mystery and greatness of the Connecticut Ice Age. In the 1800's, curiosity among scientists over the formation, movement and effects of glaciers began to grow. Scientists were extremely intrigued and eager to study and uncover the history of the earth, focusing on any visible patterns. Louis Agassiz came to the United States from Switzerland in 1846 to teach at Harvard University. He brought with him innovative ideas in American glaciology. These ideas prompted T.C. Chamberlin and his colleagues to further the study of glaciations by identifying four separate drift sheets in America. This discovery pointed out that the glacial period incorporated at least four successive ice invasions. The third ice invasion being studied refers to the behavior of the last ice sheet. The ice sheet in New England spread as far south as Long Island, where it eventually became stagnant and began to melt (Flint, 1930)

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Soto and Huoppi - Glacial History of Connecticut The Traprock, Vol. 1, December 2002, pp 29 - 32

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Because nature does not follow state boundaries, the glacial epoch in Connecticut cannot be viewed separately from the rest of the glacial effects on the surrounding land. Most of Canada and northern United States were completely covered by ice sheets. It is likely that a large region was glaciated at once. The ice sheets are believed to have moved outwards from at least three distinctive central areas in the northern region. In fact, the debris deposited by the glacier is believed to have covered most of the pre-glacial valleys. For example, hills were modified by glacial erosion that changed the topography of the region. Clearly, we are able to study the behavior of these glacial ice sheets by observing and analyzing the topographic glacial evidence today (Flint, 1930). Rocks are the key in unraveling the spectacular history of the earth. There is a vast amount of glacial evidence that can be found in the rocks of Connecticut. Fresh and weathered pebbles and boulders of the same type of rock are found next to one another in drifts, indicating that a glacier moved them. In fact, we are surrounded by glacial evidence on our very Trinity College campus. The rocks that have been here for millions of years contain markings (scratches) attributed to glacier movement. Almost every outcrop we examine in Connecticut contains glacial markings. These markings allow geologists to determine the orientation of the glacier. Evidence of glacial abrasion is present on the surface of weathered rocks causing them to crumble. This, in turn, indicates that the rock sediments must have been carried away from their primary location by ice sheets. The average depth of till (sediment that remains as the glacier recedes) in Connecticut is measured to be roughly less than 10 feet (Flint, 1930). By examining the present overall topography of Connecticut, we can determine the magnitude of the last ice sheet. Glacial abraded bedrock is present in both the highest and lowest topography of Connecticut, proving that all of Connecticut was once fully glaciated. What remains uncertain is if all glacial evidence in Connecticut is due to the last ice sheet. However, we do know that the last ice sheet

directly caused some of the glacial evidence. Since the glacial evidence is present in both the lowlands as well as high hills, we can assume that the last ice sheet did bury the valleys and hills. Furthermore, we can conclude that the ice sheet must have been at least as thick as the highest topographic point in Connecticut, which is 1650 feet. The highest point in New England was 5500 feet; therefore we can assume that the ice must have been even slightly thicker (Flint, 1930). The valleys of Connecticut do not exhibit common evidence of glacial erosion as observed in more intensely glaciated regions. The valleys appear not to have been greatly altered, as would be expected in glaciated areas. Certain parts of Connecticut along the coast do in fact demonstrate bedrock floors to be below sea level, indicating glacial erosion. The lack of drastic erosion in the New England area suggests that the glacier could have been very slow-moving. The mantle of glacial deposits over the bedrock is recorded to be quite thin, suggesting once again that the slow glacial movement did not cause vigorous erosion. The most glacial erosion that is evident in Connecticut is the smoothing of the contour of the hills and mountains (Flint, 1930). By examining the rock outcrops we can observe various evidence of glacial movement. Where a glacier once covered the land we can now see distinct horizontal layers, as in most sedimentary rocks. However, the layers tend to slope downward in one direction, indicating the direction of the flow of the glacier. Within the general scheme of horizontal layers, we see distinct groupings. Typically, the most finely sorted sandy rocks will make up the bottom layers. On top of the sandy layers is usually a mix of sand and gravel layers. On the very top are layers of very coarse gravel. Overall, we can see a difference between well-sorted and poorly-sorted particles from lower elevations to higher elevations. The sorting occurs because the fine-grained sediments travel through the ice as it advances, while the coarse grained sediments are stagnant in the ice. As the ice retreats, it drops all the

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coarse grained sediments on the top of the fine grained ones. Since the ice of a glacier is solid, it can carry sediment of all sizes, which makes glacial till poorly sorted. Many areas in Connecticut are made up of the same type of rock yet with different mineralogy. One such area is called Glacial Lake Quinnebaug in Eastern Connecticut. This ancient glacial lake exhibits a distinct north to south division between two groupings of gneiss, but it is made up of different minerals. The gneiss on the eastern side of the division is very dark because it has a lot of amphibole, pyroxene, garnet and epidote. The gneiss on the western side of the division is much lighter in color due to large amounts of quartz and feldspar. The difference in color can possibly be related to the movement of the glacier. The glacier moving in a very slight southeast direction could have carried the darker, coarser minerals farther than the lighter, finer grained minerals. And this could have created the north to south division. The direction of ice movement can be assumed by analyzing the channels and scratches on the surface of bedrock. Geologists have found the direction of the ice movement to be several degrees east of south. The marking on the rocks suggest that the movement was horizontal, yet some markings do cut vertically into the rock (Force and Stone, ?). The glacial evidence of Connecticut suggests that as soon as the glacier reached its southernmost point, the glacier slowed down losing its forward momentum and eventually became stagnant. This explains why the glacier did not extended further south. Once it became stagnant. it then slowly melted away. Evidence of the stagnation is directly observed in the land. For example, valleys are flanked at different levels by terraces, gravel, sand and other debris particles. Thus, nearly all material in the bedrock of Connecticut can prove the formation and movement of glaciers. Whenever the ice sheet is stagnant for a period of time, the material is accumulated along the edges and forms into long ridges, known as moraines. Recessional moraines are responsible for the

formation of many islands along the cost. Moraines are a clear indication of a standstill in glacial retreat (Flint, 1930). As mentioned before, the glaciers extended as far south as Long Island. The magnitude of these glaciers was so great that our present-day Long Island is in fact a moraine left from the southernmost glacier. The glacial moraines on Long Island were so large that when the glaciers receded, they formed a massive fresh water lake between Connecticut and Long Island in what we know as Long Island sound. The moraines acted like a dam protecting the lake, until the water level of the Atlantic Ocean rose up and flooded the lake, converting the freshwater into saltwater (Anonymous, 2002). Glaciers flattened the Connecticut Valley by eroding the brownstone and sedimentary rocks in the area, making the Valley ideal for farming. The Connecticut valley is good for farming because a lake covered it. The glacier moved all boulders and other material from the valley and deposited them elsewhere. Glaciers creating lakes dug into brownstone rock. Lake Saltonstall in Branford is a clear example of a lake formed by glacial erosion (Force and Stone, ?) Connecticut's topography is greatly affected by the glacial epoch. Glacial ponds are filled in and turn into swamps, which are filling into wet forests. Rivers and streams continue to transport with them glacial sediments while depositing them in various locations. The costal recreational areas in New England are all largely due to glacial deposits (Anonymous 2002) Further glacial evidence can be found in examining glacial deposits themselves. Glacial deposits in Connecticut could form in two separate scenarios: the deposits made in marginal lakes whose spillways held for a prolonged period of time filled the lakes completely with sediment. The valley, on the other hand, served as a drained kettle lake between terraces by the melting of the ice sheets. Most of the deposits in Connecticut are made up of irregular stratified sand, gravel, and poorly sorted sediments. The presence of eskers in Connecticut

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serves as further evidence suggesting that the ice sheets became stagnant (Flint, 1930). The glacial sculpting of Connecticut's topography is likely to have been caused by the erosive effects of pro-glacial streams rather than the ice sheets themselves. The pro-glacial stream erosion is also apparent in the presence of large potholes, and plunge pools excavated in bedrock (Flint, 1930). The massive ice sheets that once covered Connecticut gradually vanished; the higher altitude was the first to melt, leaving the ice in the valleys to remain the longest. The ice in the valleys was well protected from melting by the sun's rays. The ice in the valleys became sheeted over a protective layer of debris over time. Because the valleys remained the longest, the drainage had no other choice but to flow along the sides of the marginal lakes. In comparison to the rest of New England, the ice sheets in Connecticut moved relatively fast. Keep in mind that movement is not meant to be defined as the whole glacier moving entirely back and forth. Rather, movement is meant in terms of advancing when more ice accumulates, and receding when the ice melts. Throughout most of Connecticut the glaciers advanced at a rate of up to 243 feet per year, whereas farther north, they advanced as slow as 100 feet per year. One factor affecting the rate of movement was the thickness of the ice. The largest parts of the glaciers were up to seven thousand feet thick, but in Connecticut, they were as thin as eighteen hundred feet in New Haven, and twenty-five hundred feet in Hartford (Anonymous, 1995) Another factor was temperature. Areas with lower temperatures experienced slower movement whereas areas with higher temperatures experienced faster movement. The topography of land was also an important factor in the movement of the glaciers. Ice moved faster in valleys, while on mountains and other high elevations, the ice moved much more slowly. The last main factor was the amount of precipitation in the area. Regions that experience greater amounts of rain and snow saw slower moving masses of ice. The drier regions saw faster moving glaciers (Anonymous, 1995).

Connecticut is still in a state of recovery from glaciation. "Glacial ponds are filling into swamps. Swamps are filling into wet forests. Erratic boulders are tumbling down from ridges. Rivers and streams still carry glacial sediments to the ocean and sea level is still rising." (Bell, 1985). In present day we find ourselves in what geologists call an inter-glaciated period. Scientists anticipate the formation of another age to occur in 500-7000 years. In that time, will we see the same patterns in the glacial movement that we have seen in past glacial periods? Climate and temperatures will most likely be similar at this future time as they were in the past, so the effects of topography, and temperature, and precipitation will be similar. Will the rate of movement of the glaciers be the same? Connecticut will most likely see the mildest form of ice coverage because it is the southernmost part of New England. The ice will probably move at the greatest speed because of Connecticut's location, whereas the northern New England states will be covered in ice for a much longer time. (Bell, 1985) Scientists continue to study patterns in the earth's history, and although many remain a mystery, we are constantly surrounded by clues. As seen in uncovering the mystery of the ice age it is clear that rocks, being the oldest available evidence, hold a vast amount of information from the past. References:

Anonymous, http://www.wesleyan.edu/ctgeology/Glacial/GlacialGeo logy.html, 2002 Anonymous, http://www.yale.edu/ynhti/curriculum/units/1995/5/95. 05.01.x.html#g , 1995 Flint, R.F., The Glacial Geology of Connecticut, State Geological and Natural History Survey, Bulletin No. 47, 1930 Force, E.R., Stone, B.D., .Heavy - mineral Dispersal and Deposition in sandy deltas of glacial Lake Quinebaug, Connecticut. U.S. Geological survey Bulletin 1874.

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