A massive downfaulted block (graben) of Horton sandstone (Unit 572) is bounded on the south by Goldenville Formation quartzite (Unit 413) and to the north by Avalon Zone rocks of the Pictou-Antigonish Highlands (Unit 312) and its Dissected Margins (District 320). The more-resistant rocks form prominent dissected scarp slopes. The St. Marys Fault has been excavated by the St. Marys River, while the Chedabucto Fault is followed by the East River of Pictou.