ACADIA NATIONAL PARK A Geological Summary Lashing waves--sudden in their violence, relentless in their persistence--are only the most obvious of the forces that have created and shaped this land. Other seas built up then tore it down. Forces within the earth's crust raised and com- pressed the land. Molten rock from deep below the surface penetrated it and cooled and hardened and became a part of it. Ice--thousands of feet thick and laden with rock debris--moved over Acadia, pressing it downward and sculpturing it to the shapes you see today. Even now the land offers but temporary resistance to rain, wind, frost action, chemical reaction, and the pounding waves of a tireless sea. THE NEVERLASTING HILLS. Out of Acadia's dim past, traced back by geologists some 400 million years, comes a story of forces that alternately built up and tore down the landscape. Ancient streams and seas eroded highlands and deposited sediments, which later became layers of rock. Igneous (molten) rock was forced to the surface, to flow out as lava; pushed up the crust to form granite domes; or spread through fractures in the older rocks to form "dikes". Millions of years passed. Deposition, erosion, uplifts, depression, invasions of molten rock followed one another. Mountains sank beneath the sea only to be replaced by new land forms heaved upward by the unstable earth. A11 this was but prelogue to the great remodeling of Acadia's geologic architecture which occurred about 275 million years ago. FORMATION OF MOUNT DESERT RANGE. Miles beneath the earth's surface a great pool of molten rock began to undermine the crustal roof above it. In time the crustal roof collapsed, and the molten rock rushed upward to fill the space. This intrusive rock cooled slowly, to become the coarse-grained pink granite that forms the core of what is now Mount Desert Island. Eons passed. Erosion gnawed away much of the surviving sedimentary rock and laid bare the thick masses of intrusive pink granite. Thus did the Mount Desert Range emerge as a nearly continuous ridge running east and west. THE LAND IS SCULPTURED. About 1 million years ago, a thick layer of ice formed over eastern Canada. As the weight of ice increased at the center, the edges of the layer flowed outward, molasses like. When the ice front finally reached Mount Desert Island, the solid east-west barrier of Mount Desert Range lay directly across its course, As pressure from the north mounted, the ice sheet heaved itself up and finally over the crest of the range. In doing so it sought out stream courses, widened them, and carved the U-shaped valleys seen today. Rock debris in the grip of the ice sheet worked like a giant sheet of sandpaper and rounded off the north slopes. Once the ice overtopped the ridge, it moved onward, tugging at the south face of the mountains. This enormous quarrying action, aided by cracks and faults in the rock, created the steep cliffs and giant steps on the southerly faces of the mountains. The Mount Desert Range was buried to a depth of perhaps 5,000 feet. Under this gigantic glacial load the land yielded. It is no wonder then, that when the ice finally retreated, the once unbroken ridge emerged as a line of individual peaks separated by the deep valleys now occupied by lakes and a fiord. It is understandable that Maine's ancient coastal plain was depressed beneath the sea; that what were once its heights and ridges are the islands and peninsulas of today. CURRENT GEOLOGIC ACTIVITY. Even today, Acadia is being shaped by geologic forces. Most easily observed of these forces is the sea. Each wave sets in motion a new sequence of events. Nature's grindstones--the sand and rocks of the shoreline--swirl and rub against the land. Caves and chasms along the shore are cut a little deeper. Cliffs are undermined; and because the rocks in these cliffs are driven by joints, the undermined sections collapse into the water. What is eroded from one shore, the sea may deposit on another. Sand Beach is actually a sandbar built by the waves across a once longer cove. The section of the cove closed off by the sandbar is now a lagoon. One day piled high and steep by the waves, the next washed broad and flat, Sand Beach is an excellent place to observe short-term geological change. Back from the sea other kinds of erosion take place. Streams cut into the rocks. Frost cracks them. Chemicals dissolve them. Pounded by the sea, eroded from within, Acadia is slowly wearing away. Ironically the very forces that would destroy this beautiful land and bring it down to the level of the sea are the ones that have created its beauty.