I. Glacial History of Long Island Sound
Approximately 18,000 years ago coastal Connecticut and Long Island Sound were covered by a thick sheet of ice. It is estimated to have been as much as 1,000 meters thick in the interior and 400-500 meters thick along its southern margin. This ice sheet was part of the Late Wisconsin Glacier, which covered much of northern North America at the time. It was the most recent of a series of glaciations to have spread from the polar regions across the continent in the past 10 million years.

Exactly why climatic changes allowed vast layers of ice to repeatedly cover North America is not well understood. Some think that sunspot activity or cosmic dust were responsible, while others feel that variation in the Earth’s motion, movements of the Earth’s crust, or changes in the atmosphere may have been the cause. No single hypothesis has yet achieved widespread acceptance, and it is possible that several different phenomena were involved.

Impact of the Glacier on Southern New England

Among the most dramatic changes brought about by glaciation were changes in sea level.  At the maximum size of the Late Wisconsin Glacier, sea level stood about 100 meters lower than it does today. As a result, large areas of land once above sea level are now submerged. The great weight of the advancing glacier also depressed the land masses in interior areas by as much as one-third the thickness of the ice.

The ice masses scoured vast quantities of rock and soil from the land as they moved south. It is believed that the Late Wisconsin Glacier eroded an average of 20 meters of surface materials from the preglacial New England landscape. The estimate is based on the quantity of all sediments deposited as a result of glacial activity that are found on and offshore in the New England region. Such glacial sediments are collectively known as drift.

 Moraines

18,000 years ago, the ice sheet was at its maximum size, extending across the present Long Island Sound basin to southern Long Island. The southern margin of the glacier remained there for a time in an “equilibrium” position, as the movement of the ice southward was offset by the melting of the ice in front due to a regional warming trend. At this time, a large quantity of drift was deposited, known as a terminal moraine. The southernmost deposit was called the Ronkonkoma Moraine. Some time later, a more northern equilibrium position resulted in the formation of the Harbor Hill Moraine.

Evidence indicates that no moraines were formed in the central basin of Long Island Sound, which suggests that the glacier receded across that area without establishing equilibrium positions. Apparently, there are no moraines between Harbor Hill on the south side of the basin and a group of moraines near the Connecticut coastline on the north.

The Connecticut moraines, which are located both on and offshore, are discontinuous and considerably smaller than the Long Island moraines. They occur in a zone which is less than 10- kilometers wide in most places. The smaller size and discontinuous nature of these moraines is due to minimal deposits, presumably resulting from the relatively short-term equilibrium position of the ice front in that area.

There are two principle areas at which lines of terminal moraines intersect the Connecticut shoreline: the Norwalk area and the Madison-Old Saybrook area. In these areas, relatively greater quantities of drift have led to the formation of sandy plains and beaches. East of these areas the drift thins and there are more numerous bedrock exposures. Where the drift cover is thinnest, rocky headlands backed by marshes predominate.

Within a ten-kilometer strip of the Connecticut coast, numerous islands and shoals are found. Some, like the Captain Islands near Greenwich, the Norwalk Islands, and Faulkner Island off Guilford, are segments of a terminal moraine. Other islands, like the Thimbles off of Branford, are principally exposed bedrock with a thin, discontinuous cover of drift. Still other shoals and islands fall somewhere between these two extremes and appear to be bedrock with a more substantial mantle of drift. Also offshore are several sandy deltas, including those in Bridgeport and New Haven. These deltas were formed by materials deposited by melting glaciers.

 The Long Island Sound Basin

The basin of Long Island Sound, which predates glaciation and is probably a stream-carved feature, received a relatively thick cover of sand and gravel as the ice front receded. Such materials, deposited by glacial water streams, are termed outwash.

The basin is bounded on the west by a ridge about 20 meters below present sea level and on the east by another ridge, the Mattituck Sill. The lowest point of this ridge is approximately 25 meters below sea level. These ridges, together with the lowered sea level during the glacial period, kept Long Island Sound isolated from the open ocean. As a result, glacial melt water pouring into the Sound formed a freshwater lake.  The Sound remained a freshwater lake until approximately 8,000 years ago, by which time sea level had risen to 25 meters below its present level. At this point, seawater entered the central basin of Long Island Sound, changing it from a non-tidal, freshwater lake to a tidal, saline arm of the sea.

Continued climate change and the shrinkage of glaciers led to further rises in sea level. Studies of marsh sediments on the Connecticut coast indicate that sea level rose at the rate of approximately 1.2 millimeters per year from 8,000 to 3,500 years ago; about 0.8 millimeters per year until 100 years ago; and two-four millimeters per year (or about 1 to 1.5 feet per century) since then.

 Some Interesting Long Island Sound Facts

• Approximately 1/8 of the United States population live within 50 miles of Long Island Sound.

• Over 110,000 pleasure boats are registered on Long Island Sound.

• New Haven Harbor is the Sound’s busiest port, handling about 1/2 the tonnage of Boston Harbor.

• The majority of cargo carried on LIS is oil.

• The loss of “clarity” in LIS is mostly due to phytoplankton (microscopic drifting plants) and sediments. Phytoplankton accounts for about 1/3 of the loss.

• Average depth of the Sound is 60 feet; its maximum depth is 320 feet.

• The Sound stretches from the Throgs Neck Bridge to Little Gull Island.

• The Sound is 21 miles across at its widest point and has over 500 miles of coastline.

• The Sound’s most important commercial fisheries are oysters and hard-shell clams. Historically lobsters were very important but they have largely disappeared in recent decades.

• Sports fishing accounts for most of the fish caught in LIS.

• LIS has over 100 species of fish, from sharks to sea horses.

• LlS has a large variety of marine animals. Among these are two species of coral: the Northern Coral, a star coral, and Dead Man’s Finger, a soft coral.

• Harbor seals enter LIS during the winter months, as do grey seals and occasionally harp seals.

• There are at least 1000 shipwrecks on the bottom of Long Island Sound. The most recent wreck occurred in November of 1984 when the tug Celtic sank off Norwalk, taking with it all six of its crewmen.