 Researchers Les MacPhie, and John Wonnacott concluded that the deep deposits at the east end of the Island make up the drumlin formations.There are two types of bedrock that lie under Oak Island; the southeastern portion consists of "Mississippian Windsor Group limestone" and gypsum, while the northwestern part is Cambro-Ordovician Halifax Formation slate.Oak Island and the area that is now Mahone Bay was once a lagoon 8,000 years BP, before the sea level rose with the melting glaciers.