A landscape shaped by fire, ice, and the steady pounding of the surf

fortress, Acadia appears impervious to change. But, in fact, the history of the park—and of the island where it is located—is one of imperceptible but ceaseless change, occasioned both by nature and by man. Of the two, nature has been the more tireless sculptor, shaping Acadia's contours with fire, with ice, and always with the steady pounding of the surf.

Some 500 million years ago, what we now know as Mount Desert Island began taking shape on the ocean floor. Erosion swept sediments from the North American continental plate - sand, silt, and mud, and later volcanic ash and seaweed—out to sea. There they slowly amassed and hardened into what would become some of the island bedrock. Mountainous in size, these sedimentary deposits were built and then leveled by the heat and pressure of continental drift three times over the next 100 million years.

Magma, or molten rock, further transformed this sedimentary rock. Churning and rising through the earth's crust, the magma eventually weakened and consumed the overlying bedrock, producing diorite and then the coarse-grained granite that defines much of the island today. Later intrusions of magma created the dark basalt dikes that course like thick veins through the Schoodic Peninsula section of the park. For millions of years, until the onset of the Ice Age, erosion gradually molded a single ridge of gently sloping mountains running from east to west.

Nothing else sculpted Acadia with the force and grace of the continental glaciers that blanketed New England 2 to 3 million years ago. Many of the park's loveliest features were carved out by the brute force of these immense sheets of ice: Jordan and Long ponds, Echo and Eagle lakes, and the stunningly beautiful Somes Sound, a narrow but deep inlet of seawater surrounded by steep cliffs. The glaciers were staggering in weight and size; geologists estimate their thickness at anywhere from 3,000 to 9,000 feet. Completely refashioning the lay of the land, glaciers hewed out a series of 17 individual mountains separated by U-shaped valleys running from north to south.

The last of the glaciers, the one whose imprint remains most visible on the island today, advanced out of Canada around 100,000 years ago, crept slowly across New England, and eventually spread 150 miles out to sea. This glacier not only dug out deep valleys and lake basins, but also engulfed and reshaped the mountain peaks, rounding and polishing the northern slopes and fracturing the southern faces into a series of sheer granite steps. The Precipice Trail section of Champlain Mountain, much beloved by serious hikers, was created during this period. As the ice sheet traveled, it gathered up large rocks and carried them considerable distances. Known as erratics, these boulders can be seen at the summit of Cadillac and South Bubble mountains, testimony to the strength of the glaciers.

Climatic changes eventually halted the glaciers' progress around 18,000 years ago. As the ice sheet receded, the ocean advanced, flooding the valleys and cutting the island off from the mainland. (Now the third-largest island off the coast of the continentalUnited States, it is connected to the mainland by two short bridges spanning the Mount Desert Narrows.)

Acadia's coastal headlands had sunk beneath the glaciers' crushing weight. But as the ice sheet receded, the mountains and hills gradually rebounded and regained some of their former stature. Nonetheless, due to melting of the polar ice caps, the ocean is slowly overtaking the depressed land at a rate of two inches every hundred years, creating a "drowned coast." The smaller islands that ring Acadia were once mountain summits, just as the bays that surround them were once river valleys. Everywhere Acadia reveals the imprint of the glaciers that covered it 100,000 years ago.

Today, the sea remains the key agent of change at Acadia. Daily, it buffets the steep face of Otter Cliffs, while polishing the pink and blue-gray cobblestones at Little Hunters Beach, and grinding rock particles finer still, mixing them with shell fragments, and depositing them at Newport Cove, the only sand beach on Acadia's coastline.