As a result of tectonic formation, Vermont east of the Green Mountains tends to be formed from rocks produced in the Silurian and Devonian periods, and western Vermont mainly from the older Pre-Cambrian and Cambrian material. Several large deposits within the state contain granite. The remains of the Chazy Formation can be observed in Isle La Motte. It was one of the first tropical reefs. It is the site of the limestone Fisk Quarry, which contains a collection of ancient marine fossils, such as stromatoporoids, that date to years ago. At one point, Vermont is believed to have been connected to Africa (Pangaea); the fossils found and the rock formations found on the coasts in both Africa and America are evidence affirming the Pangaea theory.