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.

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.

Many of the park's loveliest features were carved out by the brute force of the continental glaciers that blanketed New England 2 to 3 million years ago: Jordan and Long ponds, Echo and Eagle lakes and stunning Somes Sound, a narrow, but deep, inlet surrounded by steep cliffs.

The last glacier, the one whose imprint remains most visible today, advanced out of Canada around 100,000 years ago, crept slowly across New England, and eventually spread 150 miles out to sea. It dug out deep valleys and lake basins and engulfed and reshaped mountain peaks, rounding and polishing the northern slopes and fracturing the southern faces into a series of sheer granite steps (including the Precipice Trail section of Champlain Mountain). As the ice sheet traveled, it gathered up large rocks-erractics-and carried them considerable distances (these boulders can be seen at the summit of Cadillac and South Bubble mountains).

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. It is now the third-largest island off the east coast of the continental United States.

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 depositing fine rock particles at Newport Cove, the only sand beach on Acadia's coastline.

Ancient native peoples made their home on Mount Desert Island long before European explorers ever ventured across the Atlantic. Few records of their presence remain: slate tools, pottery, red ochre burials and middens (large refuse piles of shells, which archaeologists have dated at between 3,000 and 5,000 years old).

The people who became known as the Wabanaki inhabited the island at the time the first Europeans made contact in the 1500s. Originally, it was believed the Wabanakis traveled to Pemetic or "Sloping Land," as they called the island, by birch-bark canoe from their winter homes near the Penobscot River's headwaters. During the summer months, they would hunt, fish and gather berries near Somes Sound. More recently, archaeologists have concluded that the Wabanakis may have had permanent settlements both inland and on the coast.

The history of these early island residents is told at Acadia's Abbe Museum, located just off the Park Loop Road near Sieur de Monts Spring, and the expanded Abbe Museum is located on Mt. Desert Street in Bar Harbor.

In 1524, the Florentine explorer Giovanni da Verrazano is credited with christening the area that is now Maine and the Canadian Maritimes with the name L'Acadie or Acadia. Some historians believe it to be a Wabanaki word; others say it is a corruption of Arcadia, an equally scenic and inspiring region of ancient Greece. Eighty years later, in 1604, the French explorer Samuel Champlain gave Pemetic the name: l'Isles des Monts-déserts which is now known as Mount Desert Island. Champlain, who crossed the Atlantic 29 times and later founded Quebec, is believed to have run aground at Otter Point, where he met members of the Wabanaki tribe.

A party of French Jesuits, who settled at the mouth of Somes Sound in 1613, were also warmly greeted by the Wabanaki. The priests intended to found a mission there but were soon pushed out by English explorers determined to expand northward from their settlements in Massachusetts. For the next century, the French and British would struggle for control of Acadia. In 1759, the British finally prevailed when they defeated the French in Quebec, but not before a young French noble-man laid claim to a large section of the Maine coast. Sieur de Antoine de la Mothe Cadillac stopped long enough on Mount Desert to lend his name to the island's highest mountain before moving on to found the city of Detroit (Michigan).

Many of Mount Desert Island's towns bear the names of the first settlers, including Abraham Somes, a Massachusetts sailor who, with his family, settled on the island in 1762. Because of its proximity to sailing routes, the western side of the island was settled first. Later arrivals gravitated to the island's eastern half, where the soil proved better for farming. Then known as Eden, Bar Harbor was incorporated as a town in 1796. By 1820, most island inhabitants were engaged in fishing, shipbuilding, lumbering or farming. This period of island life is well documented at the Islesford Historical Museum, located on Little Cranberry Island and accessible by cruise and mail boats.

By mid-century, a new industry emerged: tourism. First artists, including the distinguished landscape painters Thomas Cole and Frederic Church, traveled to Mount Desert. Then came journalists and sportsmen, drawn by the promise of the vast, unspoiled wilderness Cole and Church had depicted. Early visitors, known as "rusticators" or "summercators," bunked with local families. By 1880, Bar Harbor boasted 30 hotels and a national reputation as a summer resort.

That reputation was sealed soon after, when America's most socially prominent families - the Rockefellers, Morgans, Fords, Astors, Vanderbilts, and Pulitzers - began summering in Bar Harbor and nearby Northeast and Seal Harbors. They built magnificent summer "cottages" of palatial dimensions, entertained lavishly and forever altered the rustic character of the island. Ironically, these same summer colonists also helped preserve the natural beauty of Mount Desert Island, for it was they who created Acadia, the first national park whose land was donated entirely by private citizens.

A Maine politician once remarked that "the portable sawmill created Acadia National Park." Concerned that this tool of progress would cut a swath through their island paradise, a group of summer residents, led by the president of Harvard University, Charles W. Eliot, formed a public land trust in 1901 to protect the island from uncontrolled development. The group had the foresight to appoint George Bucknam Dorr as its director. A member of a highly regarded Boston family that had made its fortune in textiles, Dorr would spend the next 43 years (and much of his own wealth) tirelessly working to protect and preserve Acadia for public use.

The land trust's first notable acquisition was the chiseled headland known as The Beehive, in 1908, followed soon by the summit of 1,530-foot Cadillac Mountain. By 1916, Dorr secured national monument status for the trust, and in 1919, it became the first national park to be established east of the Mississippi. As a nod to its French heritage, it was named Lafayette National Park. Dorr was appointed the first superintendent, a position he held until his death in 1944.

Over the next 10 years, the park doubled in size, thanks in part to the acquisition of the breathtaking Schoodic Peninsula, which faces Mount Desert Island across Frenchman Bay. The family who donated the 2,000-acre peninsula had but one small stipulation: Being residents of England, they objected to the park's Francophile name. Dorr arranged to change the name to Acadia National Park, a move that required an act of Congress. The park's last major acquisition came in 1943, with the donation of 3,000 acres on unspoiled Isle au Haut, an island that is about 15 miles southwest of Mount Desert Island, in Penobscot Bay.

Next to George Dorr, Acadia has had no better friend than industrialist and philanthropist John D. Rockefeller, Jr. He not only donated more than 10,000 acres of parkland (including the stretch of coast between Thunder Hole and Otter Cliffs), but he was also responsible for one of Acadia's most picturesque features, the 45 miles of broken-stone carriage roads that wind through its sylvan interior. In 1913, alarmed by the prospect of a park overrun by automobiles, Rockefeller began building the single lane roads connected by a series of 17 bridges crafted from local granite and cobblestones. Today, the roads are enjoyed by equestrians, hikers, bicyclists, and, during winter, cross-country skiers. Carriage rides are available through Wildwood Stables.

In 1947, a great fire broke out on Mount Desert Island, consuming some 17,000 acres and burning for 10 days before it was brought under control. The blaze swept down Bar Harbor's "Millionaire's Row," destroying more than 60 grand summer cottages and bringing the uppercrust resort era to a close. Some of the surviving cottages have been converted into inns and bed-and-breakfasts. Others remain private residences, summer homes for the Fords, Rockefellers and Astors, as well as a new generation of summercators such as Katharine Hepburn, Julia Child and Martha Stewart.