The harbor is a drowned river valley flooded as the sea rose at the end of the last ice age. Bedrock under the harbor and in the Harborview and Calf Pasture Beach area is interlayered Ordovician trap falls gneisses and Harrison Gneiss with dark minerals hornblende, biotite and garnet. The bedrock has two folds in the harbor area. The earlier, more southerly one was formed at the same time as the Taconic Mountains. That fault runs from Bell Island through Manresa Island to Sprite Island. The second (created during the Acadian mountain-building event of 485-440 million years ago) runs along a line north from Noroton Point on Sheffield Island Harbor toward Exit 15 on Interstate 95.