Geologic History of Connecticut
Adapted from: http://geology.teacherfriendlyguide.org
The BIG picture
Geologic history is the story recorded in the rocks of an area. By understanding the history of the rocks
observed in the Northeast, we can make sense of geology. The rocks in your backyard fit into a much
larger story of shifting plates and colliding continents. By knowing more about the geologic history of our
region, you can better understand the type of rocks that are in your backyard and why they are there.
Rather than focus on specific periods in geologic time, we will look at the history of our region as it
unfolds: as a series of major events over the past one billion years and how those events created and
shaped the Northeast and Connecticut. Learning about these events will allow you to understand why
our region looks the way it does!
Geologic Time
How did geologists come up with a timeline for the history of the Earth?
Over the course of many years and through the combined work of
geologists around the world, the Geologic Time Scale was developed.
Geologists put together information by studying the ages of rocks all over
the world. There is not a rock record in any one place that has the
complete sequence of rocks from 4 billion years ago to the present.
Geology as a science grew as geologists studied individual sections of
rock. Gradually, fossils were discovered that helped us understand the
ages of groups of rocks. Once the ages of rocks were identified, they were
given names. The names you see for the different periods on the Geologic
Time Scale have diverse origins. Time periods were named after mountain
ranges and even ancient tribes around the world. Look at the time scale to
see the different time periods.
Mountain Building Part I: the Grenville Mountains
The dynamic plates of the Earth are constantly
in motion, made of rigid continental and oceanic crust overlying the churning,
flowing asthenosphere (Figure 1.2). Plates are pulling apart, colliding into
one another, or sliding past each other with great force, creating strings of
volcanic islands, new ocean floor, earthquakes, and mountains, melting rock
and injecting magma into the overlying crust. As these plates move, the
continents resting atop them are continuously shifting position. This not only
shapes the land, but also affects the type of rocks and minerals, natural
resources, climate and life present.
North America was not always the shape we see today. The continent was
formed over billions of years, and geologic processes continue to shape it
today. The Earth is estimated to be 4.5 billion years old. The oldest rocks that we know of are nearly 4
billion years old. Although these ancient rocks are found on almost every continent, none are found at
the Earth's surface in the Northeast. In North America, these most ancient rocks are found exposed at
the surface in many parts of Canada. These rocks make up the stable continental landmass that is the
core of North America.
The best evidence that we have shows that the North American continent formed about 1 billion years
ago from a series of collisions with other plates and stretching and pulling apart of the plates. North
America first formed as narrow strips of land smashed together to form the beginnings of a continent.
This small continent was called Proto-North America. Over time, this new continent’s rocks wore away
Geologic History of Connecticut
Adapted from: http://geology.teacherfriendlyguide.org
into small pieces of sediment which were carried off the continent and into the oceans. The sediments
deposited on the eastern margin of proto-North America are called the Grenville belt.
Over 1 billion years ago, Proto-North America collided with another
continent. The Grenville belt of sediments was caught in between the
colliding continents and was thrust up onto the side of proto-North
America. The collision crumpled the crust, creating a tall mountain
range that stretched from Canada to Mexico: the Grenville Mountains.
These mountains are the earliest
evidence of mountain building in our
region, and the rocks remaining from
that ancient mountain chain are the
oldest rocks that we see
exposed at the surface in the
Northeast today. Over time, the
Grenville Mountains eroded,
just as the Appalachians,
Rockies and Himalayan Mountains are constantly being
eroded today. By 600 million years ago, weathering and
erosion had worn away the mountains, leaving exposed only
the rocks that were on the bottom.
During the erosion of the Grenville Mountains, the geography of the world looked nothing like today.
North America was positioned on its side across the Equator, with today's east coast facing south.
Sediments were eroding from the Grenville Mountains on either side. The ocean breaking on the shores
of the east coast was known as the Proto-Atlantic Ocean (called Iapetos). Because continent was near
the equator, the Northeast was experiencing a warm climate. At this point in geologic time, eastern New
England had not formed.
Sometime about 470 million years ago, the ancient Iapetus Ocean began to close as the plate carrying
Baltica (proto-Europe) approached the North American plate. Baltica did not collide with North America
until several million years later, but the convergence of the two plates created a whole new look for
eastern North America. As the continents approached one another, the oceanic crust in the middle was
forced under the Baltica plate. The friction and melting of the crust from the intense pressure of the
colliding plates created a string of volcanic islands along the area where the plates converged (known as
the subduction zone). Over millions of years, more and more land was added on during collisions.
Mountains were built up on the east coast of North America. More land continued to add on to North
America over time
Geologic History of Connecticut
Adapted from: http://geology.teacherfriendlyguide.org
Pangaea Forms
Hundreds of millions of years of continental
collisions caused the creation of a giant
"supercontinent" known as Pangaea, beginning
about 460 million years ago. Africa collided with
North America between 360 - 245 million years
ago, and the supercontinent called Pangaea
formed!!! As this happened, mountain chains
formed on the east coast. Pangaea existed as a supercontinent for over 200 million years!
The Theory of Continental Drift tells us that all present continents were once together and collectively
known as a 'supercontinent' called a Pangaea. The word 'Pangaea' means 'all lands' in Greek, and
accurately defines the way the continents were 200 millions years ago before it split up. These split-up
pieces drifted slowly apart and became the way they are today. Even until now, the shape of the Earth
surface is still changing, and it will be forever, as long as the mantle underneath the Earth's crust gets
heated and convection currents in the magma keeps moving the tectonic plates.
Geologic History of Connecticut
Adapted from: http://geology.teacherfriendlyguide.org
Breaking Up is Hard to do…Pangaea breaks up!
By 225 million years ago, the vast landmass became unstable. Heat in the mantle below began to
stretch the crust apart along a zone where the Atlantic Ocean would eventually form, and separate Africa
from North America. Depressions in the crust, or rift basins, formed around the new ocean on both
continents. By around 200 million years ago, the continents rifted apart and set the stage for the
development of Africa, North America, and the Atlantic Ocean as we know them. As Africa pulled away
from North America, eastern North America was stretched.
Triassic: 273 million years ago
Cretaceous: 94 million years ago.
Great cracks in the crust appeared. Great valleys formed, with a HUGE valley in the area we call central
Connecticut. It is easy to imagine how the sedimentary rocks of the valley were formed as the large
valleys filled with sediment from the eroding mountains and lava. Volcanoes forced out great flows of
lava through long cracks in the floor of the Connecticut Valley.
Connecticut’s Rocks Today
Today, most of Connecticut is made of rock which changed under immense pressures during continental
collisions. This changed rock is called METAMORPHIC ROCK. In the middle of the state, rocks formed
from sand and mud which were stuck together (called SEDIMENTARY ROCK) can be found in the
middle of the state in the Connecticut Valley. Their layers kept a record the past that reveals details of
the climate, plants and animals of two hundred million years ago. Finally, IGNEOUS rocks, formed from
molten volcanic rock that later cooled and hardened, form the great ridges which run north-south in the
middle of the state and today. Remnants of ancient lava flows are seen today in the ridges that extend
nearly the length of the central Connecticut, from New Haven to Northfield, Massachusetts. Because
volcanic rocks are harder than surrounding sandstones, periods of erosion and eastward tilting have
worked to leave the ridges standing out above the rest of the valley.
Geologic History of Connecticut
Adapted from: http://geology.teacherfriendlyguide.org
The Ice Age in Connecticut
About 2 million years ago, an Ice Age began and covered most of the northern hemisphere. Since that
time, there have been several dozen periods of glaciation separated by warmer periods (like the one we
are in) The most recent glacial advance reached its maximum extent 25-20,000 years ago and lasted
until 10,000 years before the present. The most recent advance came through about 24,000 years ago.
It has been only 13,000 years or so since the ice left New.
Though the glaciers are long gone from the Northeast, they have left behind evidence of their advances
and retreats, smoothing over the mountains and
blanketing the surface with glacial deposits. The
Northeast owes a large share of its present shape to the
last glacial advance. The glaciers only extended as far
south as northern Pennsylvania and Long Island. The
end of the ice sheet is where the moving ice melted and
deposited all the rock and sediment that it scraped up
along the way. These piles of sediment are called
MORAINES and can be seen on the coast of
Connecticut and as the large pile called Long Island.
The farthest extent of the continental glacier left
moraines that created our present-day Long Island, almost 22,000 years ago.
When the continental ice sheet started to melt, fresh water filled what is now Long Island Sound behind a
dam made of long moraines. This former lake, formed while the Atlantic Ocean was many miles out from
today’s shoreline, is known as “Lake Connecticut.” As the ice melted and sea level gradually rose, the
freshwater in Lake Connecticut eventually came to be replaced by seawater.
Geologic History of Connecticut
Adapted from: http://geology.teacherfriendlyguide.org
Finally, great piles of rocks, called moraines, are the leftover piles of massive glaciers which moved
south through the continent and melted. Together, the great collisions, stretching, collisions, and
scraping have shaped the landscape of Connecticut as we know it.