The Geology of Mount Desert Island A Visitor's Guide to the Geology of Acadia National Park Maine Geological Su rvey DEPARTMENT OF CONSERVATION The Geology of Mount Desert Island A Visitor's Guide to the Geology of Acadia National Park by Richard A. Gilman Department of Geosciences State University of New York, College at Fredonia Carleton A. Chapman Department of Geology, Emeritus University of Illinois Thomas V. Lowell Department of Geology University of Cincinnati Harold W. Borns, Jr. Department of Geological Sciences University of Maine at Orono Maine Geological Survey DEPARTMENT OF CONSERVATION Walter A. Anderson, State Geologist Copyright © 1988 by the Maine Geological Survey, Department of Con­ servation. All rights reserved. Printed in the United States of America. No part of this publication may be reproduced, stored in a retrieval sys­ tem, or transmitted, in any form or by any means, electronic, mechani­ cal, photocopying, recording, or otherwise, without the prior written permission of the publisher. Cover photograph: Otter Cliffs at sunrise, by Robert M. Neff, copyright @ 1986. The laws and policies of Maine and the United States prohibit dis­ crimination in Department of Conservation programs and/or employ­ ment because of race, religion, national origin, sex, age, or handicap. Any person who believes discrimination has occurred should con­ tact the Commissioner, Maine Department of Conservation, Station #22, Augusta, ME 04333; Telephone (207) 289-221 J. CONTENTS Page Preface . i Acknowledgments ................................................................... ii Introduction . 1 Bedrock Geology of Mount Desert Island . 3 The Rocks of Mount Desert Island . 3 Stratified Rocks . 4 Ellsworth Schist . 4 Bar Harbor Formation . 6 Cranberry Island Series . 8 Intrusive Igneous Rocks .................... ................ ............. ...... 10 Gabbro-Diorite ....................... ........................................ 10 Granite of Southwest Harbor ............. ..... ............ ... ............... 11 Granite of Cadillac Mountain ................ ................ .................. 11 Granite of Somesville .......................................................... 13 Other Granites . 13 Diabase Dikes ................................................................ 14 Joints and Faults ................................................................ 16 Schoodic Peninsula ............................................... ................ 19 Isle au Haut ....................................... ................ ............. 21 Summary of the Bedrock Geologic History of Mount Desert Island ... ... .. ..... .. ... ....... ... ........ .............. 22 Glacial Geology of Mount Desert Island ................................................ 27 Introduction ................................................ ..................... 27 Shaping of the Landscape by Glacial Erosion ..... ................ ................... 29 Glacial Transportation and Deposition of Rock Debris . ................................. 33 Disappearance of the Glacier .................................. ...................... 34 Glacial Downdraw into the Sea .................................................... 35 Melting of Glacial Ice on Land .................................................... 36 Rebound ................................................. .. .. ................. 36 Summary of Glacial History ...................... ................. ............... 40 Recent Geologic Processes on Mount Desert Island ....................................... 43 Conclusion ........................................................................ 45 Supplementary Reading ........... ....................... .................... ..... 46 Glossary of Geologic Terms . .. .................. ....... ..... ......... ............ 47 Note: The Surficial Geology map is superseded by: Braun, Duane D., Lowell, Thomas V. and Weddle, Thomas K., 2016, Surficial geology of Mount Desert Island: Maine Geological Survey, Open-File Map 16-1, map, scale 1:30,000. 0 10 I--· I kilonle ters 0 10 I -- I I mltes .. O'a . Swan$ l$land t} p Figure I. Map of mid-coastal Maine and Mount Desert Island. Acadia National Park is shown by ruled areas. Maine has long been famous for the beauty of its coast. Mount Desert Island in particular has at­ tracted numerous tourists and summer residents for well over a century. Visitors and residents alike en­ joy the scenic vistas at Bar Harbor, Cadillac Mountain, Jordan Pond, Sand Beach, Thunder Hole, and many other places within Acadia National Park and elsewhere on the island. The purpose ofthis visitor's guide is simple: we hope that an understanding of the geologic processes which formed the island's spec­ tacular scenery will leave the reader with a greater appreciation for both Mount Desert Island and the national park. Mount Desert Island is the largest of the many islands located along the coast of Maine (Figure 1). Measuring almost 108 square miles in area, the island is located 150 miles northeast of the Maine-New Hampshire border and 70 miles southwest of the U.S.-Canadian boundary. Access to the island is via Maine Route 3 south from Ellsworth. Named in 1604 by the French explorer Samuel de Champlain, the island and much of the adjacent coastal area were the site of continual skirmishing between the British and French during the French and Indian Wars. English colonists settled the island after the British victory over the French and the signing of the Treaty of 1763. Farming, lumbering, shipbuilding, and fishing were major industries for the new settlers, and fishing remains important today. Although one of the nation's smallest national parks, Acadia is one of the most scenic and most visit­ ed. Much can be viewed from the state routes and park roads that wind around the island, or from the various hiking and carriage trails within the park. Unlike most other national parks, Acadia was not purchased with public funds or set aside from public lands. During the middle and late 1800's, the island became a summer colony for wealthy patrons from southern New England, Philadelphia, and New York. Concerned about the dangers of overdevelopment of the island, a number of prominent citizens, among them George B. Dorr, Charles W. Eliot (a former president of Harvard University), and John D. Rocke­ feller, Jr., together with many other Maine residents and summer visitors, worked to preserve the is­ land's beauty. Out of their efforts came the donations of land that formed the nucleus of Acadia National Park. Legislation enacted by Congress in 1986 established permanent park boundaries, addressing con­ cerns of adjacent towns that continued expansion of the park would hurt their economic growth. This book summarizes the geologic history of Mount Desert Island and serves as a guide to geologic features located within and adjacent to the park. This history begins more than 500 million years ago with the formation of the oldest rocks on the island and continues today with the geologic processes that are presently shaping the face of the landscape. The accompanying geologic maps, used in conjunc­ tion with the text, allow readers to identify various geologic features on Mount Desert Island and under­ stand how they came into being. l. ACKNOWLEDGMENTS Writing an interesting, understandable, yet scientifically accurate geologic summary for the non­ geologist is a monumental task, much more difficult in many respects than writing a technical paper. We are grateful for the many helpful comments and suggestions and careful editing by Carolyn Lepage, Marc Loiselle, Sheila MacDonald, Bill Metzger, Ernest Muller, Woody Thompson, and Bob Tucker. Cathy Stultz's assistance with manuscript preparation is also appreciated, as are the illustrations drawn by John Poisson, and the photographs taken by John Poisson, Woody Thompson, and Joe Kelley. The writers are also indebted to the staff of Acadia National Park for their assistance, especially Lois Winter, Park Naturalist, who read early drafts of the manuscript and offered many suggestions for making it more readable for park visitors. Not only is Mount Desert Island one of the most scenic places on the entire Atlantic Coast, it is also one of the most interesting from a geologic point of view. This short book will introduce you to the rock formations and glacial landscapes that record the more than 500 million years of earth history that went into the making of Mount Desert Island. Maybe your imagination will be sparked by the fact that the location of the Acadia National Park Visitor Center was once part of a gigantic body of molten rock that cooled several miles below the sur­ face of the earth, or that at another point in its history the same location was covered by hundreds of feet of glacial ice. And perhaps you'd find it intriguing to take a boat trip to the Cranberry Isles to see rocks that were spewed from volcanoes about 400 million years ago. If these things interest you, read on; we will help you interpret the story that the landscape tells us. Part of this story is recorded in the bedrock exposed along the shore and on the mountain tops; other parts are recorded in the deposits of sand and gravel left by the glacier that once covered Maine's coast; and still other evidence is found in special features of the landscape that were carved by erosion over the past thousands of years. The geologic history of the island can be conveniently divided into three parts: the period when the island's bedrock was