Connecticut Geology

This is essentially a mirror of several pages at Wesleyan University. It is a brief, well-illustrated geological history of Connecticut.  


RalphsSlide12.jpg (40235 bytes) 500 million years ago, the plates were arranged something like this.  Instead of the modern Atlantic Ocean, we had the Iapetus Ocean to the east of what is now Connecticut.  In Greek mythology, Iapetus was the father of Atlantis, thus the name. The bar connects similar points in this and the following images.
RalphsSlide13.jpg (39873 bytes) The plates slowly collided one by one, finally forming a super-continent called Pangaea (meaning "all one Earth") not long before the beginning of Mesozoic time.  The collision between proto-North America and Proto-Africa (possibly with a couple of smaller plates as well) wrinkled the earth to form the Appalachian Mountains.  Most of New England is within this mountain chain.
RalphsSlide14.jpg (40902 bytes) Again, here is the arrangement of the continents today.  Connecticut was involved in the events that rifted Pangaea apart to open the Atlantic Ocean and create this modern pattern, starting 200 million years ago.
RalphsSlide17.jpg (59473 bytes) New England today shows regions that had different geological origins, which are pieces of the ancient plates that collided and fused together.  Several un-labeled pieces, called terranes, actually constitute the Iapetus group, much of which was in the ancient ocean. Avalonia was left behind when Africa separated from eastern North America, which opened the present Atlantic Ocean.  The "Newark" terrane was also created by that event; it and others like it are named for a similar piece in New Jersey
RalphsSlide19.jpg (61541 bytes) Let's look at the rifting first.  This cartoon image shows the location of Connecticut in the rift zone of Pangaea.
RalphsSlide20.jpg (95888 bytes) As Africa separated from North America, a series of rift valleys formed (including the Hartford basin labeled by H), shown here in red. 
RalphsSlide21.jpg (54769 bytes) Earlier folding caused a N-S weakness, and the crust failed in Connecticut along that direction as rifting proceeded.
RalphsSlide22.jpg (63535 bytes) Mesozoic sediments and lava flows in this block diagram of the Connecticut Valley were originally horizontal.  As faulting tilted the rocks downward to the east, the asymmetric ridges came into existence.
RalphsSlide23.jpg (54689 bytes) Here is a view of the lava flow (basalt) ridges from the south.
RalphsSlide26.jpg (101412 bytes) The central Connecticut Valley, Proto-North America, Avalonia, etc. are reflected in the shapes of the land surface.
RalphsSlide24.jpg (78127 bytes) The landscape components of Connecticut include the N-S basalt ridges, Mesozoic sedimentary rocks, and the eastern and western terrane uplands flanking the central valley.
RalphsSlide25.jpg (103578 bytes) The geological components of Connecticut are arranged in terranes, which are sections of the earth's crust that have their own geological history.
RalphsSlide28.jpg (65085 bytes) The geological bedrock map of Connecticut arranges the different rocks that make up Connecticut according to the terranes.  The rock units show the common N-S trends and faults, which are also directions of "weakness" that were exploited by glaciers that covered Connecticut during the Ice Age.
RalphsSlide29.jpg (52625 bytes) Glaciers had an important influence on our land surface.  As great sheets of ice moved over our hills, they exploited the N-S valleys and carved away rocks that were weak and fractured.
RalphsSlide30.jpg (91298 bytes) This map shows the extent of the glaciers about 20,000 years ago.  Most of Canada and Greenland were covered, as well as New England.
RalphsSlide31.jpg (57148 bytes) Here is a map of the ice in our region.  The continental glacier covered Mt. Washington in New Hampshire (6018 feet).  This ice sheet entered Connecticut about 26,000 years ago, reached its maximum about 21,000 years ago, and was melted out of the state by 15,500 years ago.
RalphsSlide32.jpg (37111 bytes) The glaciers also covered the state earlier during the Ice Age, about 150,000 years ago, and many people think that they completely changed the topography.  In fact, the ice modified features already present.
RalphsSlide33.jpg (52431 bytes) As the ice moved southward, it deepened valleys and rounded hills.  The pre-existing N-S grain of the land was preserved and accentuated.
RalphsSlide34.jpg (51151 bytes) This mechanism is evident in modern glaciers, such as this one in Alaska where the rock surface (right side) is being rounded.
RalphsSlide35.jpg (34924 bytes) As the ice rounds off the hills, it picks up the broken rock fragments and other sediments.
RalphsSlide36.jpg (76342 bytes) Here is some recently-eroded rock material in Alaska.
RalphsSlide37.jpg (75880 bytes) In Connecticut, this sedimentary rock in the central lowland shows how harder sections were rounded while less-resistant rock was gouged out.  This happened at small scales like this as well as in major valleys.
RalphsSlide38.jpg (35315 bytes) Glacial plucking on the south sides of hills also influenced the shaped of the land, and provided rock shelters for paleo-North Americans.
RalphsSlide39.jpg (146595 bytes) This topographic map shows a glacially-plucked hill in Deep River.  The contour lines are closer together on the south side of the hill, showing a steeper slope.
RalphsSlide40.jpg (79900 bytes) The ice moved toward the viewer over this hill.  You can see how the plucked hillside could become an overhang and shelter for a weary traveler.
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.

(Perhaps early humans followed game into these newly-exposed lands - Ed.)