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The central Connecticut rift valley of the late Jurassic Period had no seaward outlet. It filled with sands and gravels pouring down the steepwalled escarpments. Sediments built up between these horsts on the downfaulted valley floor grabens.
During the Triassic and Jurrasic, 245-145 million years ago, violent earthquakes and stretching signaled Pangea's brake up, culminating in the rift that was to become the Atlantic ocean. It tore first at New Jersey, then at Connecticut, and finally succeeded at ripping the supercontintent 100 miles to the East of New Haven. Floods of lava gusts forth repeated and were eroded and buried over millions of years. These layers were subsequently tilted, faulted, and eroded to expose them at the surface as the basalt ridges we now see in Connecticut.
Explanations from Chet and Maureen E. Raymo (1989). Written in Stone. Globe Pequot Press, Old Saybrook ,CT.
