Most people have heard of the "Ice Age" in which Connecticut (and most of northern North America) was covered by great sheets of ice, or glaciers. To learn more about glaciers in general, check out this Glacier Web Site from Rice University.
The Ice Age in New England probably began 2 or 3 million years ago,
but there were several -- at least two and probably more -- times when
the ice receded back into Canada, to be followed by new advances.
Evidence concerning older ice advances is scanty, because most of their
land features and glacial sediments have been destroyed by the later advances.
However, there are a few locations where two or three layers of glacial
gravel each record different ice movement directions, as shown by different
orientations of elongated cobbles and pebbles in the layers. Each
of these layers might record different episodes or advances of ice.
The most recent advance came through about 24,000 years ago (Lowell
and Dorion, 2001), and that ice sheet spread rapidly as far as the
southern edge of Long Island, and also far out into the present Gulf of
Maine. Because it has been only 13,000 years or so since the ice
left in the last recession, some people think the Ice Age might not be
over, and that the great glaciers could advance once again within a few
thousand years (see this news
release). Such blocks of time are only the blink of a geologist's
eye in the great calendar of earth events.
|The end of the ice sheet is where the "conveyor belt" of moving ice
melts and deposits all the detritus that it has scraped up along the way.
Whenever the ice sheet is stable for some period of time (perhaps a few
decades to centuries), the materials dumped at the edges build up into
long ridges, called moraines. If these moraines contain some
woody material mixed in, they can be carbon-dated. Work on end moraines
in New England has resulted in maps that show where the edge of the continental
glacier stood at different times in the past. The version presented
here was published in a New
England Intercollegiate Geological Conference field trip guide.
Thanks to Byron Stone for permission to use his map.
Note that the farthest extent of the continental glacier left moraines that created our present-day Long Island, almost 22,000 years ago.
The thickness of this "Laurentide ice sheet" (named for the Laurentians of Quebec) must have exceeded the highest mountains at the peak of glaciation, perhaps over 6,000 feet. As the ice sheet melted later in the last cycle, scattered local glaciers were left along some mountain sides, with ice that did not overtop the mountains but only filled and flowed down the valleys in higher elevations.
|In the Connecticut River Valley, late in the Ice Age the view might
have been something like this (photo courtesy of the National Science Foundation).
Some of the time, smaller valley glaciers in the eastern and western "highlands"
may have flowed into larger glaciers. At other times, the entire
land was hidden beneath one great ice sheet.
Where valley glaciers move along exposed mountain sides, as in this photo from Antarctica, the mountain side tends to be eroded into a fairly steep slope, while the valley beneath the ice is cut deeper. Where the ice flows over everything, the hills beneath the ice tend to be rounded off, and the low areas are not significantly deepened but instead may be dumping areas for glacial sediment. Indications are that the last glacier flowed right over most of Connecticut and then left, while local valley glaciers continued for a few thousand years in the higher mountain areas of northern New England.
|We can tell which way the last glaciers moved from grooves and scratches in bedrock surfaces such as this one, in East Haven. The grooves were made by stones caught in the base of the ice, which pressed hard against the bedrock as the ice moved. Here the ice moved in a south-south-east direction. There are also places where there are more than one direction of grooves, indicating a change in the ice flow before it receded. Glacial grooves are very common all over New England, as is the smooth "polish" that the ice imparted to many bedrock surfaces.|
|The great moraines north of present-day Long Island were originally more continuous than today, so that when the continental ice sheet started to recede, fresh water filled what is now Long Island Sound behind a dam made of the long moraines. This is called "Lake Connecticut." At that time, so much water was still locked up in continental glaciers that sea level was much lower, so that the Atlantic Ocean was many miles out from the present shoreline. Eventually the ocean rose and replaced Lake Connecticut with sea water some thousands of years ago.|
|As the ice melted farther to the north, Glacial Lake Hitchcock was formed behind a dam made by a large delta of sand and gravel in the present town of Rocky Hill. It has been quarried for many years, but you can still see the level top of the sandy delta around the sides of the quarry in this photo. Varves, or thin layers of mud deposited in Lake Hitchcock, have recently been used to determine very precise ages for events that affected the lake (see the recent NEGSA abstract by J. Brigham-Grette and others, for a talk given in March 2001. Varves are a little like tree rings, and they are especially good at recording the climate -- see this New England Varve site.|
|Lake Hitchcock was very long, stretching all the way from Rocky Hill to Lyme, New Hampshire and probably part way up some side valleys as well. There were other glacially-dammed lakes, such as Lake Albany and many smaller lakes in upland regions. An arm of the ocean ran up the present-day St. Lawrence River of Quebec and into what is now the Lake Champlain Valley, forming the "Champlain Sea." Marine animals such as oysters and whales have left their fossil remains in that valley. Lake Hitchcock did not drain down today's Connecticut River south of Rocky Hill, but instead detoured to the west down the present central valley toward New Haven.|
|Immediately after the ice left, plants and animals returned, even while it was still very cold. It is amazing to find nearly complete skeletons of large animals still preserved in lake muds from 10,000 years or more in the past. The most recent elephant excavation took place just across the border in New York, and it has a great web site -- the Hyde Park Mastodon excavation.|