A proto-continent ripped open during the Late Proterozoic era 700 million years ago, giving birth to an ocean named Iapetus. Then Iapetus's edges came together again essentially squeezing the sea out of existence.
Huge tectonic plates drifted and collided in a series of stages, eventually merging into the land mass of Northeastern U.S. as we know it today. The Roxbury Land Trust has an excellent illustrated guide (PDF) about the geological history of New England replete with details on the tectonic plates movements:
|Source: Roxbury Land Trust (Geology)|
Cameron’s Line in west Connecticut is a major thrust fault that separates (1) rocks that were originally part of the ancient Iapetus Ocean (deep ocean metasedimentary rocks) from (2) metamorphic rocks derived from the previous continental shelf and slope deposits to the west.
This line marks an abrupt change in the earth’s crust that resulted from a collision between North America and a European-African land mass 400 million years ago.
Weir Farm Geologic Resources Inventory Report