120,000 years ago, ice began to accumulate in massive amounts in northern Canada. This process initiated the formation of the Laurentide ice sheet, a huge amount of glacial ice that eventually covered all of Canada and the United States' New England region.

The formation of this ice sheet began due to a decrease in incoming solar radiation because of the elliptical shape of the Earth's orbit at the time.

                         / obliquity of the axis
             _.-""""-._ /
           .'          `.
          /              \
         |                |
         |     earth      |
         |                |
          \              /
           `._        _.'

tilt of the Earth's axis (obliquity)
Right now, the tilt of the Earth's axis is 23.50. Throughout the existence of the Earth, its obliquity has varied between 24.50 and 21.50 on a cycle of 40,000 years. Glacial conditions are most favorable when obliquity is the lowest.

Over the past 2.5 million years, variations in the obliquity of the earth's axis as well as of the shape of the planet's orbit have caused many glaciations. The various differences in the orbit of the Earth are called Milankovitch orbital cycles, named for the Serbian astronomer Milutin Milankovitch who worked with orbital properties. Another property of the Earth's orbit is the "wobble" of the axis itself due to the gravitational pull of the sun and the moon on the equatorial bulge of the Earth. This property causes the time of the equinox to change in a cycle of 23,000 years. 900,000 years ago and earlier these variatons caused many glaciations that were relatively short (23 - 40,000 years in duration).

After this period, glaciations were longer. As the Earth's orbit changes from elliptical to circular and incoming raditation increases and decreases, glaciations develop during periods of low raditaion and circular orbits.

21,000 years ago, the Laurentide ice sheet reached its maximum size when it stretched to Long Island in the northern United States. The Cordilleran ice sheet covered the western half of Canada beyond the Rocky Mountains. The two sheets met somewhere east of the mountains around Montana in the present day United States. During this last glacial maximum, sea levels all over the world were reduced dramatically because of the huge amounts of water trapped in the continental ice sheets globally. Ice in the New England area advanced out onto the continental shelf which was not covered by water because of the low sea levels. Terminal moraines were deposited during this time. The moraines formed the topography of Long Island, Cape Cod, Martha's Vineyard, and other surrounding areas. Once the Laurentide ice sheet retreated and sea levels rose, these areas became islands.

All bedrock mountains and hills in New England were shaped by the glacial ice sheet. The ice eroded rock leaving smooth surfaces covered with striations. Using these striations, a map can be made which outlines ice flow directions.

The advance and retreat of the glacier left till in its wake all across New England. Glacial till is formed around glaciers as rocks are carried and ground by the ice flows. Till is usually made of grain ranging from boulder size to clay deposits and is often found resting on the bedrock of the area. Deposits of till can range from a thin mantle layer to dozens of meters thick.

Also, glacial-fluvial, glacial-lacustrine or glacial-marine sediment was deposited during Laurentide deglaciation. Glacial-fluvial (outwash) sediment is composed of sand and gravel deposited in meltwater streams of the ice sheet. These deposits are now usually found in New England valleys. The other types of sediment are usually made of silt and clay which are found in lakes. Most of the eastern coast of New England is composed of glacial-marine clay. The Hudson River Valley was also once filled with glacial lakes and therefore has deposits of glacial-lacustrine sediment.

Today, not much of the former huge ice sheet remains. Some ice fields and ice caps in the Arctic islands is all that is left.


http://www.museum.state.il.us/exhibits/ice_ages/laurentide_deglaciation.html (has a neat video of Laurentide deglaciation)

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