Margaret M. Loos
Can we yet explain the numerous rocks of various sizes that do not match the rock masses there? They can be explained as sedimentary or other rocks deposited there by one means or another. If we expose a vertical cut several feet in depth in the area, we find the soil is typically dotted throughout by these rocks of all sizes and varying makeup. If we wash away the sand, silt or soil particles by running water we would create a pile of rocks similar to that on a rocky shore. This type of makeup overlying bedrock is classified as
till
. In the early 1800’s Scandinavian and European geologists observed that this till and other features that accompanied it could be explained by the same actions that were produced by existing glaciers. Dana in 1872 clearly classified this material in the United States as
glacial till
as well.
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In fact many of the geologic phenomena that exist in the New Haven area can be approached as evidence of glaciation. Glaciers did not occur very long ago in our geologic terms, just a mere 15 thousand years ago. That was in a very recent age called the Wisconsin age of the Pleistocene epoch of the Cenezoic era. (Ages make up epochs, epochs make up eras, and eras make up eons.) To give an idea of this time range, Dr. Gordon, of the geology department at Yale gives us a range of time for dealing with rocks, based on the radioactivity index for dating rocks.
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1. Oldest rock—4,300 million years old
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2. Oldest organized life’s fossil remains—500 million years
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3. Connecticut Rift valley formed—100 million years ago
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4. Beginning of ice-ages—1 million years ago
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5. End of glaciation—10 thousand years ago.
Glaciers
formed when layers of snow built up because it failed to melt for a period of years and more and more snow accumulated and pressed down on the under layers, forming a great sheet of ice. Since the ice melts sooner at contact points with the underlying material, melting took place and the water caused the glacier to move and advance as glaciation continued. Before the glacier moved down over the Connecticut shoreline a shelf of the continent made of the same material as the mainland extended outwards, above, then under the ocean. It was formed of hills and plains which gradually became lower until they reached the former shoreline which is calculated to have been at least 100 miles farther out from its present line. That area was called a coastal plain. That old coastal plain was “drowned” or submerged under sea water since the ice-ages as the sea level rose. The lowering of sea level that exposed the old coast was the result of the formation of the glacier which incorporated a great deal of the ocean’s waters. In order to understand that the effect could be so great students must know the great ice-sheets covered much of North America, Europe and Siberia, and geologists estimate that the ocean at that time was 300 to 500 feet lower.
18.
The glaciers acted as great conveyer belts bringing rock fragments obtained from the soil and bedrock over which the ice passed. It could grind and drag across the land. “Particles of clay, silt, sand and gravel and many large boulders are (were) in this way broken loose and mixed with the lower layers of the moving ice.”
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The glacier that affected the Connecticut shoreline, the Laurentide Glacier, was estimated to be at least ten thousand feet thick.
When the ice reached its farthest advance point it may have remained there for thousands of years. It began to melt and evaporate faster than it spread and for the long period when the melt and rate of advance balanced out the glacier deposited a great deal of its load of materials forming a chain of islands including the North shore of Long Island, Block Island and Fisher’s Island. Students should be shown pictures of presently active glaciers so they may visualize the tremendous amounts of materials they are capable of transporting, and the tremendous pressures they can exert. Evidence of the glacier’s power to weather and erode are everywhere around us.
20.
(figure available in print form)
21.
(figure available in print form)
Some landmarks of glaciation that we can observe in our area are:
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1. Glacial Plucking—When the glaciers deposited ice in the seams of igneous rock, rocks broke away in columns and pieces of those columns. Example: West and East Rock.
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2. Glacial Till—(Already cited) A variety of mud, sand, pebbles, rocks, etc., overlying bedrock. Example: Areas around Lighthouse Point deposited during the melt period about 12,000 years ago when the climate warmed.
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3. Kettles—At the time of the melt, the ice sheet wasted away. It lost thickness and blocks of ice separated. These isolated chunks would melt and the level of the land would drop as they did and form depressions in the terrain as hollows or lakes called kettles. Example: Beaver ponds (although this has been artificially deepened).
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4. Erratics—Large blocks of unconforming rock. Example: The largest of these is located at the top of Fountain Street, but many are indicated in the Lighthouse area and we will examine some. (See surficial maps)
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5. Striations and grooves—These are produced when material is dragged across preexisting rocks by the glacier. Many are indicated in Lighthouse area, but the longest (30 feet) is on the West Rock Ridge.
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6. Terminal Moraine—A deposit of material at the site of the most advancement of a glacier.
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Example: The North shore of Long Island, visible on a clear day from Lighthouse.
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Some other features of a glacier’s action are indicated on the drawing, but they are not present in our harbor study.
An Active Mountain Glacier of Canada
(figure available in print form)
(figure available in print form)
(figure available in print form)
Area of New Haven Harbor: Tip of the Rift Valley