This book describes the geologic origin of the exceptional scenery of Mount Desert Island, Maine, the home of Acadia National Park, in relatively non-technical terms for those visiting the area. Starting with a short review of basic geologic concepts, the book follows the geologic history of the island from 550 million years ago to the present. A slice of South America with MDI on its southern side was plastered on the edge of North America to form Maine. The collision of another slice of South America caused MDI to become a violently erupting ten mile wide volcanic caldera. The Island then underwent a long period of stream erosion culminating in a period of glacial erosion to form the present landscape. Self-guided trips with thirty-three stops help the visitor better understand the origins of this unique place.