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Geological > Geologic History > Connecticut Geology


The geologic events that shaped the north shore of Long Island Sound, and much of landscape to the north, played out over the past 500 million years of earth history. The crushing and folding of warm pliable bedrock, as mountain ranges formed and New England was assembled against eastern New York, resulted in an initial north-south alignment of ridges and rock units.
For definitions of italicized terms see a Glossary of Geologic Terms provided by the National Park Service.

As time passed, and the rocks cooled and became more brittle, tension associated with the opening of the Atlantic Ocean caused fracturing, faulting and rifting that roughly paralleled the overall north-south alignment of the aggregated bedrock units This set the stage for the formation of a network of south-draining valleys as stream erosion and two fairly recent glaciations worked to wear away the Appalachian Mountains. Valleys formed where less resistant rock units, and fragmented rock in fracture/fault zones, were preferentially eroded (Figure 1).

Shaded Relief Map of LIS basin

Figure 1: A shaded relief map showing the N-S grain of Connecticut's landscape. The effects of preferential erosion of less resistant bedrock are particularly evident in the Newark terrane, where resistant basalts form prominent north-south trending ridges and less resistant sandstone and shales underlie the rest of Connecticut's "Central Valley". After Hatcher, 1993.

An appreciation of the extent to which the resulting bedrock surface has imparted a regional grain to the land can be gained by recalling one's driving experiences. Driving east-west across Westchester County, New York, and throughout much of Connecticut, often entails circuitous routes around and over hills, and across stream valleys (somewhat akin to traveling across the grain of a corrugated roof), but driving north-south usually involves more direct and flatter routes (with the grain) up valleys (e.g. Driving from Peekskill, New York to Shelton, Connecticut vs. driving from Shelton, Connecticut to Waterbury, Connecticut or driving from Chesterfield, Connecticut to North Stonington, Connecticut vs. driving from Groton, Connecticut to Norwich, Connecticut).


Plate Tectonics (Building Mountains and Creating Oceans): The crust of the earth is composed of bedrock segments called plates. Plates can be composed of relatively "light and thick" continental crust or relatively "dense and thin" oceanic crust. The theory of plate tectonics holds that these plates move over time (at about the rate that your fingernail grows). See the National Park Service Plate Tectonics web site "Putting the pieces together" and "Moving through time" for additional information.

When plates move they interact with neighboring plates in three basic ways. They move apart (diverge) they come together (converge/collide), or they "sideswipe" each other (a transform relationship) as is happening along the San Andreas Fault in California. This is shown on the National Park Service Plate Tectonics web site "The action is at the edges" and the illustrated cross section on the U.S.G.S. web site. It can also be seen on the Public Broadcasting System's "A Science Odyssey" web site go to "Plate Tectonic Activity".

These interactions do not happen independently. They are part of the Earth's tectonic system which recycles oceanic crust over time. Since the size of the Earth is fixed, the creation of new crust in one place must be accommodated elsewhere. The processes associated with rifting and sea-floor spreading supply new oceanic crust to the sea floor in expanding ocean basins. This helps to drive subduction and melting of oceanic crust where ocean to continent or ocean to ocean plate convergence results in the closing of ocean basins. See the National Park Service Plate Tectonics web site page "A Slice through the Earth" for an illustration. Expansion of the Atlantic Ocean, for example, is presently being accommodated by an attendant "closing" of the Pacific Ocean as shown on the U.S.G.S. web page "Developing the Theory, Seafloor Spreading Recycling the Ocean Floor".

Convergence/Collisions (460- 270 Million Years Ago): Owing to differences in their thickness and/or density, collisions between oceanic and continental plates, or two oceanic plates, have a different result (subduction/volcanism) than collisions between two plates of continental crust (mountain building). This is illustrated on the National Park Service web page "Convergent plate boundaries". When plates composed of oceanic crust and plates composed of continental crust meet, the lighter, thicker continental plate rides over the thinner, more dense oceanic plate. The overridden oceanic crust sinks toward the interior of the earth (subduction) and melts as shown on "The Continental Slide" on the PBS A Science Odyssey page. As oceanic plates age, they cool and get denser. When two oceanic plates meet the older, denser plate is subducted.

Continent to continent collisions occur when ocean basins completely close and all of the oceanic crust separating surrounding continental plates has been subducted. In this type of convergence, the last plates to be involved are composed of thick, light continental crust. Neither plate is subducted, so the edges of the colliding plates get heated, metamorphosed and rumpled up as mountains form under compression (e.g. Himalayan Mountains result from India colliding with Asia). An example is shown on the PBS A Science Odyssey page "The Continental Crush". Since there is no longer any subduction involved, the colliding plates survive. Continents can grow larger (accrete) over time, as ocean basins close, and masses of continental crust collide and stay joined (sutured) together.

During the assembly of the supercontinent of Pangaea, the metamorphic bedrock of Connecticut (and the rest of New England) was sutured to eastern New York (Laurentia), which sat on the northwestern coastline of the Iapetos Ocean (Figures 2 and 3a) at the time. These rock units were sequentially accreted from west to east through a succession of continent-to-continent collisions involving the Taconic and Nashoba Island Arcs, Avalonia and the African portion of Gondwana (Figure 2).

Cross section of Cambrian-Ordovician

Figure 2: A depiction of the area between the eastern margin of Laurentia (today's eastern New York) and the African portion of Gondwana prior to the succession of collisions that emplaced, heated and deformed the bedrock units of Connecticut. After Coleman, 2005. Artwork by Janet Zeh.

The order and direction of these collisions was dictated by the position of the northwestern and southeastern portions of the Iapetos Ocean, and the Rheic Ocean relative to Laurentia. The northwestern Iapetos Ocean closed first (Figures 3 a and b), followed by the southeastern Iapetos and Rheic Oceans (Figures 4a and b).

Illustration of Ordovician 480 mya

Figure 3a: A depiction of the earth during the early stages of the closing of the Iapetos Ocean. By this time Avalonia had rifted from the "African" sector of Gondwana as the Rheic Ocean was forming along a sea floor spreading ridge. The red star shows the approximate position of eastern New York (and the yet to be formed Connecticut) on the continental margin of Laurentia (see also Figures 2 and 3b). After Coleman, 2005. Artwork by Janet Zeh.

Cross section of Northwestern Iapetos Ocean

Figure 3b: A depiction of the closing of the Northwestern Iapetos Ocean as would be seen along section A-B of Figure 3a (above). The carbonate shelf along the eastern margin of Laurentia depicts conditions in eastern New York before the Taconic Island Arc was accreted. After Coleman, 2005. Artwork by Janet Zeh.

Along the eastern margin of Laurentia, the bedrock units of each accreting land mass were heated by the east-west compression of these collisions and their depth in the earth. The rocks became pliable as they warmed, and tended to fold along north-south axes. The heat also modified their character and they were changed (metamorphosed) from what they were to some type of metamorphic rock (typically schists and gneisses in Connecticut). The character of rock that they become was, in part, determined by the type of rock they were before they were heated (metasedimentary, metavolcanic, etc.). The resistance of these rocks to weathering and erosion varies depending on their origin and history. The chronology of the three mountain building events that assembled, crumpled and metamorphosed the bedrock of the region is as follows:

Taconic Orogeny (460-440 Million Years Ago): The Taconic Island Arc (Figure 4a) collided with, and was sutured to, the eastern margin of the Laurentia as the Northwestern Iapetos Ocean closed. The western half of Connecticut was thereby assembled against eastern New York, and the Taconic Mountains formed.

Illustration of Silurian 440 mya

Figure 4a: A depiction of the earth as the Taconic Island Arc collided with the eastern margin of Laurentia and the rocks of western Connecticut were accreted against the rocks of eastern New York during the Taconic Orogeny. After Coleman, 2005. Artwork by Janet Zeh.

As this occurred, a portion of the ocean bottom sediment from the Northwestern Iapetos Ocean was scraped off (Accretionary Wedge), (Figures 3b and 4b) the surface of the subducting oceanic crust, incorporated in the collision, turned to rock, and also made part of western Connecticut (Figure 4c).

Cross section of Late Ordovician

Figure 4b: A depiction of the accreted Taconic Island Arc as would be seen along section A-B of Figure 4a (above). The metamorphosed accretionary wedge sediments and volcanic rocks of the Taconic Island Arc that were involved in this "collision" (Taconic Orogeny) now form much of western Connecticut. After Coleman, 2005. Artwork by Janet Zeh.

Map of CT bedrock from Late Ordovician

Figure 4c: A sketch map of Connecticut showing the rocks that were added during the Taconic Orogeny. After Coleman, 2005.

Acadian Orogeny (440-350 Million Years Ago): The Taconic Mountains were eroded as the Southeastern Iapetos Ocean closed. The Nashoba Island Arc and the small continent of Avalonia, which had rifted from Gondwana as the Rheic Ocean formed (Figure 3a), were sutured to (and further expanded) the eastern margin of Laurentia during the Acadian Orogeny (Figures 5a and 5b).

Illustration of Acadian Orogeny

Figure 5a: A depiction of the earth as the Nashoba Island Arc and Avalonia collided with Laurentia during the Acadian Orogeny. After Coleman, 2005. Artwork by Janet Zeh.

This orogeny emplaced the rocks of the eastern half of Connecticut (including the metamorphosed accretionary wedge sediments scraped from the floor of the Southeastern Iapetos Ocean) as the Acadian Mountains formed (Figure 5b and c).

Cross section of Acadian Orogeny

Figure 5b: A depiction of the accreted Taconic and Nashoba Island Arcs and Avalonia at the end of the Acadian Orogeny. This mountain building event added the bedrock of eastern Connecticut to the eastern margin of Laurentia. After Coleman, 2005. Artwork by Janet Zeh.

Map CT bedrock from Acadian Orogeny

Figure 5c: A sketch map of Connecticut showing the rocks that were added during the Acadian Orogeny. After Coleman, 2005.

Allegenian Orogeny (350-270 Million Years Ago): The Acadian Mountains were eroded as the Rheic Ocean closed (Figure 6a), and the "African" portion of Gondwana collided with the accreted eastern margin of Laurentia. This collision played a major role in the assembly of the supercontinent of Pangaea, and completed the "collision phase" of the geologic history of eastern North America with the creation of the Appalachian Mountains (Figure 6a).

Illustration of  Alleghenian Orgeny

Figure 6a: A depiction of the earth as the supercontinent of Pangaea formed and the Alleghenian Orogeny created the Appalachian mountains along the eastern margin of Laurentia, which we now know as the North American continent. After Coleman, 2005. Artwork by Janet Zeh.

Cross section of Alleghenian Orogeny

Figure 6b: A depiction of the soon to be crumpled eastern margin of North America (Laurentian, Taconic, Nashoba and Avalonian bedrock) joined to the remains of the subducted Rheic Ocean and the western margin of Gondwana. Compare to the "before" depiction in Figure 2. After Coleman, 2005. Artwork by Janet Zeh.

The heating and tremendous east-west compression of this collision crumpled and further metamorphosed most of the previously emplaced bedrock of Connecticut as the Appalachian Mountains were created (Figure 6c).

Map CT bedrock from Alleghenian Orogeny

Figure 6c: A sketch map of Connecticut showing the rocks that were heated and metamorphosed during the Alleghenian Orogeny. After Coleman, 2005.

The general alignment of the bedrock units (exotic terranes) that were accreted, heated and deformed during the Taconic, Acadian and Alleghenian Orogenies set the stage for the pattern of rifting and fracturing that would accompany the breakup of Pangaea and the formation of the Atlantic Ocean.

Rifting/Divergence (225 million years ago to the Present): By about 225 million years ago, the super continent of Pangaea was completely assembled and covered much of the earth. The internal heat of the earth built up under this continental "heat blanket", and localized upwelling of hot magma (like thick pea soup or oatmeal boiling on the stove) began to push parts of the Super Continent in different directions (Figure 7a). (For additional illustrations go to the U.S.G.S. page and the NPS page.)

Illustration of Early Jurassic

Figure 7a: A depiction of the earth as the supercontinent of Pangaea began to rift apart, and the Atlantic Ocean started to separate the African and North American continents. After Coleman, 2005. Artwork by Janet Zeh.

The east-west compression associated with the assembly of Connecticut, and the rest of New England, was replaced by tension in the opposite direction (Figure 7a). These conditions allowed most of the bedrock of New England to cool. As the rocks cooled, they became more brittle and tended to break rather than bend and fold when stressed. Zones where rock units had been joined together (sutured) under compression, belts of weaker, easily broken rock and faults separating terranes, became susceptible to fracturing, faulting, fault reactivation and rifting when stretched. These zones of susceptibility were part of the overall north-south bedrock fabric of the region.

In Connecticut, this manifested itself in the formation of the Newark (Rift Basin) terrane and numerous faults and fractures of Mesozoic age (Figure 7b).

Map of Newark Terrane in Connecticut

Figure 7b: A map showing the Newark (Rift Basin) terrane (shaded), and some of the Mesozoic faulting associated with the pulling apart of Pangaea and the formation of the Atlantic Ocean. After Coleman, 2005.

The Newark terrane is a rift valley that failed to produce an ocean. To the east, a more successful rift (Figure 7c) split the Avalonian terrane apart and created the Atlantic Ocean.

Cross section of Atlantic Ocean opening

Figure 7c: A depiction of an early stage in the opening of the Atlantic Ocean. The failed Hartford Basin rift produced the Newark terrane of Connecticut's Central Valley, while the more successful Atlantic rift zone, to the east, split Avalonia which is now shared by New England and northwestern Africa (Gondwana in this figure). After Coleman, 2005. Artwork by Janet Zeh.

Figure 8 shows the distribution of the assembled Laurentian (Proto-North American), Iapetos and Avalonian terrane rocks that remained in New England as the Atlantic Ocean opened. It has taken 200 million years for the Atlantic to grow as wide as it is now. To see animation of Atlantic Opening go to Ocean Drilling Stratigraphic Network (ODSN) and or the National Park Service.

Illustration of New England Metamorphic bedrock

Figure 8: Map of New England showing the distribution of the metamorphic bedrock associated with the Iapetos and Avalonian terranes, and the sedimentary and igneous rock of the Newark terrane. The Proto-North American terrane is made up of rocks that once formed the eastern margin of an ancient continent (that pre-dated North America) called Laurentia. Courtesy of the State Geological and Natural History Survey of Connecticut, Connecticut Department of Environmental Protection.

WEATHERING, EROSION AND DEPOSITION (The last 350 million years) Back to top

Weathering/Erosion: The bedrock of the Appalachian Mountains has been subjected to weathering, mass wasting and erosion since the inception of the Alleghenian Orogeny. (For an explanation of the different processes see the NPS page.) Some geologists believe that up to 30 km (18.63 miles) of the bedrock cover has been removed from Connecticut during this period. The configuration of the landscape, that developed as a result, was influenced by the north-south trend of bedrock fabric of the region. This fabric developed during the three orogenies that assembled and crumpled the exotic terranes of New England and during the subsequent rifting, faulting and fracturing that accompanied the opening of the Atlantic Ocean (Figures 7b and c and 9a).

The existence of folds, faults and fractures within and separating aligned bedrock units having various susceptibilities to weathering and erosion created conditions favorable for differential removal of weathered material. South-draining valleys and lowlands tended to develop along fault and fracture zones and in areas underlain by less resistant bedrock, whereas intervening ridges were generally preserved over more resistant rock types.

As the Atlantic Ocean began to form, streams draining the eastern flanks of the Appalachian Mountains transported large quantities of sediment to the shoreline. During Cretaceous and Tertiary time, these sediments accumulated to form the continental shelf and Atlantic Coastal Plain (Figure 9a).

map of New England Terranes map of Connecticut Terranes

Figure 9a and b: Maps showing the general north-south alignment of the accreted exotic terranes of New England (A) and Connecticut (B). These terranes lie east of the blue and red band of rocks that represent the ancient eastern margin of Laurentia. The gray areas show the distribution of Cretaceous Coastal Plain strata in the region (A) and underlying Long Island (B). (N.Y Geological Survey Web Site)

Locally, the core of Long Island is composed of Cretaceous Coastal Plain sediments (Figure 9b) that were delivered to the ocean by the very well developed south-flowing, regional drainage system that existed prior to the arrival of the two ice sheets that are known to have spread southward across New York and New England (Stone and others, 2005).

Glaciations (Illinoian? ~150,000 to ~130,000 years ago and Wisconsinan ~26,000 to ~ 15,500 years ago): (Note: all ages less than 50,000 years are given in radiocarbon years.) A glacier inferred to be of Illinoian age is thought to have stripped away most of the soil and "rotten rock" that had weathered from exposed bedrock since the Alleghenian Orogeny. Striations (grooves on bedrock) and local outcrops of "older till" provide evidence for this glaciation (Stone and others, 2005), but not much is known about it. The second (Wisconsinan) glaciation to affect Connecticut was thick enough to completely cover Mt. Washington (6,028 feet or 1,837 meters high) as it flowed southward and advanced into the state about 26,000 years ago. It eventually extended as far south as the middle of Long Island.

By about 19,000 years ago, the ice sheet had melted back to a position along the north shore of Long Island, through Fishers Island and along the Rhode Island coast. The moraine that marks this position was the dam for Glacial Lake Connecticut (Figure 10). The Wisconsinan ice sheet had nearly melted out of Connecticut by about 15,500 years ago (Figure 10).

Map of Connecticut glacial lakes

Figure 10: Map showing major glacial lakes (darker blue and green), modern drainage in Connecticut (light blue), and radiocarbon dates for recessional Wisconsinan ice positions on Long Island (black) and in Connecticut (red lines). The darker blue shading, in the area now occupied by Long Island Sound, delineates the approximate extent of Glacial Lake Connecticut. After Stone and others, 2005.

Glaciers "flow" down hill under the influence of gravity. At the base of the glacier, ice flow directions can be influenced by the topographic features that are being overridden. In Connecticut, both known glaciers flowed over and around hills, and along existing valleys. Hills were rounded and valleys were widened and deepened as the glaciers flowed from north to south across the state. Any Cretaceous Coastal Plain deposits that may have covered southern Connecticut were removed, and the overall effect on the bedrock surface was a slight streamlining and modification of what already existed. The north-south "grain" of the bedrock surface was preserved and in many cases enhanced. The nature and distribution of glacial deposition in the region was strongly influenced by the shape of the underlying bedrock surface, and the modern topography and character of the Connecticut coast is a reflection of this influence.

Image of Connecticut and Long Island relief

Figure 11: This digital image shows the north-south grain that the folded, fractured, rifted and glacially smoothed bedrock has imparted to the lands that abut Long Island Sound to the north. The east-west trend of Long Island is also apparent. After Thelin and Pike, 1991.

Summary: It took about half a billion years for the sequence of geologic events that shaped the bedrock surface of Connecticut to unfold. Three mountain building events associated with the closing of the Iapetos and Rheic Oceans successively heated, metamorphosed, crumpled and accreted the exotic bedrock terranes of the region. These assembled bedrock units have a north-south "grain", the weaker components of which, preferentially yielded to stream erosion as the Appalachian Mountains were worn down, and a well developed south-flowing bedrock drainage system evolved. The action of the two glaciers that are known to have overridden Connecticut worked to enhance most aspects of the pre-glacial drainage system. The north-south grain of the land, which started to develop 470 million years ago, was preserved as the last glacier retreated, and is now manifested in the numerous promontories and inlets that characterize the coastline of Connecticut.

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