Great Falls consists of three distinct rock formations. The oldest rocks were laid down from 420 to 435 million years ago as continents collided, then slowly metamorphosed into the Sangerville Formation, whose light green to dark grey rocks appear along the southern and northern areas of the falls. Granite of the Sebago Formation lies along the southern and eastern patches, and these dense white rocks make the falls act as a natural dam. The most recent change came with the opening of the Atlantic Ocean more than 180 million years ago, when rising magma created a basalt dike near the trestle bridge.