Acadia National Park is located in an area which has been subjected to hundreds of millions of years of geologic activity, including more recent glacial activity and coastal erosion.
Over 500 million years ago, pressure beneath ancient oceans and heat from tectonic activity turned a mixture of underwater sedimentary rocks (mudstone and sandstone) and volcanic ash into the metamorphic rocks of the Ellsworth Schist, the oldest rock unit in the Mount Desert region. The Ellsworth Schist, identifiable by its green chlorite bands and white-gray quartz and feldspar bands, was exposed approximately 450 million years ago when a micro-terrane (mini-continent) called Avalonia slowly collided with North America. The Ellsworth Schist was left in a position which allowed sand and silt to accumulate on top of it. Eventually this accumulation hardened to become the Bar Harbor Formation, which today is visible as layers of sandstone and siltstone ranging in color from brown to grey. Layers of ash were deposited in the Bar Harbor Formation from nearby volcanoes (Cranberry Island Volcanics). Molten rock below Earth’s surface (magma), later intruded into this area, forming the igneous rocks (granites) of the Mount Desert Island as it cooled. The Cadillac Mountain Granite is amongst the largest bodies of granite on the island, and it is also one of the oldest in the region (~420 million years old).
During the last two to three million years, thick glaciers flowed across the state of Maine, eroding mountains and carving valleys into the land across which they flowed. This activity peaked about 18,000 years ago during a period called the Wisconsin Glaciation. As large masses of ice (glaciers) receded, their bulldozer-like erosive power left scratches in the rocks called “striations” and gouges called “crescentic gouges.”
As the Earth warmed, the glaciers began to melt. As they melted, they deposited materials they had been scraping up and carrying along their journey. This left piles of rock, gravel, and sand in the wake of the melting glaciers, many of which are still present in Acadia National Park today.
Further changes to the land occurred around Acadia where heavy glacial ice created indentations in the land which allowed the Atlantic to flow inland once glaciers began melting. As the ocean flowed inland, mountains became islands and marine material covered what had been dry land. As the ice retreated further, the land around Acadia stabilized, rose up, and once again settled above sea level. Lakes and rivers remained in the valleys and the land became dry enough for animal habitation. Plants and animals began to recolonize what had previously been underwater.
Today, Mount Desert Island is home to 18 mountain summits and the highest cliffs on the Atlantic coast north of Rio de Janiero, as well as a population of over 10,000 people. Lakes and ponds are abundant within the park, including several large (>10 acres) ponds, known as Great Ponds.
Acadia is also home to several miles of beaches. Each beach is composed of unique material, shaped by geological history and the strong force of the waves. Tidal valleys, protected from the brute force of ocean waves, are home to salt marshes while rocky headlands are battered by powerful waves from the Atlantic Ocean. Beaches such as Sand Beach, which are not subject to forceful wave action, are made up of fine-grained sand. Sand Beach is the longest continuous sand beach in the park and is located at the end of a cove between two headlands (Great Head and Otter Cliffs). The rock formation commonly known as “Old Soaker” is located near the center of the mouth of the cove between the headland points, adding further protection from the Atlantic Ocean waves. Much of Acadia’s coast is tectonically young and has only been subject to a limited geologic period of wave action, making sandy beaches quite rare. Less-sheltered beaches, particularly those facing the open ocean, are subject to strong wave action which removes fine-grained sand and deposits pebbles, cobbles, and even boulders along the rocky shores.