The continental crust is made up mostly of granitic rocks. It is thought that about 60% of continental crust was formed by the end of the Archaean (2.5 billion years ago). Since then, the continental crust has grown primarily through the addition material at subduction zones, both by volcanoes and by the accretion of exotic terranes. There are also several processes that modify the continental crust after it is formed. For example, the granitic bedrock could be eroded and redeposited as a sedimentary rock (e.g. conglomerates, sandstone or shale). When these igneous (e.g. granite) or sedimentary rocks are subjected to high pressures and temperatures they may recrystallize to form metamorphic rocks. Some examples of metamorphic rocks include schist, gneiss and marble. You can also get basalts erupted within continental crust, especially when the crust is being extended and pulled apart (e.g. during rifting).

Between 450-250 million years ago, several continental plates collided to form the super-continent Pangaea. This collision pushed up the Appalachian Mountains, which would have been comparable to modern Himalaya. The deformation and metamorphism caused by the collision also created gneisses and schists. The Northwestern part of Connecticut was originally part of the proto-North American plate, whereas the eastern part of Connecticut was originally part of the pro-African plate and much of the central part of the state was originally part of the oceanic crust that separated proto-North America from proto-Africa. Starting around 200 million years ago, Pangaea broke apart and the Atlantic Ocean formed. The Connecticut valley formed as an aborted rift (a place where the crust was extended and thinned but did not break apart entirely) during this time; sedimentary rocks were deposited in this basin interbedded with basalt lava flows.

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