Holbrook Island Glacial Bluff EarthCache
Holbrook Island Glacial Bluff
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Welcome to Holbrook Island Sanctuary preserving over 1230 acres of spectacular Maine coast. Bordering Penobscot Bay in Brooksville, Holbrook Island Sanctuary protects many different ecosystems, which visitors can explore and enjoy. Most of the property is on a point of land on Cape Rosier but it includes a 115-acre island just off shore from which it takes its name. From the beaches, mudflats, and rocky coast to the tops of steep hills that are actually old volcanoes, the sanctuary hosts a great diversity of plants and wildlife.
The south shore of Holbrook Island has a coastal bluffs formed of ice contact drift from the most recent glacial event. A bluff is defined as a steep shoreline slope formed in sediment (loose material such as clay, sand, and gravel) that has three feet or more of vertical elevation just above the high tide line. Cliffs or slopes in bedrock (ledge) surfaces are not bluffs and are not subject to significant erosion in a century or more. Beaches and dunes do not form bluffs, except along the seaward dune edge as a result of erosion.
The story of this coastal bluff begins with the last glacial retreat. The last glacial episode in Maine began about 25,000 years ago, when the Laurentide ice sheet moved south overspreading New England and Eastern Canada. During its peak development, this ice sheet flowed east to southeast across the area into the sea. The ice itself was thousands of feet thick, with its massive weight shaping the land as it slowly traveled. Climatic warming forced the Laurentide ice sheet to stop and begin to recede as early as 21,000 years ago. This melting of the ice sheet released more water and gravel then is possible to imagine. Even as the ice margin withdrew, internal flow within the glacier continued to transport its sediment load southward toward the edge of the vast ice sheet. Large quantities of sediment were dumped into the ocean at the edge of the melting ice sheet, and these deposits are now exposed to view because the land rose above sea level. Remnants of the glacial sand and gravel are to be found here under the high banks located along the south shore.
This coastal environment is dynamic and subject to continuous change. Gravitational processes of Creep (slowest movement that operates every day, everywhere, no matter how gentle the slope), slumping (most commonly after heavy rains saturate the ground) and occasional land-sliding (fastest movement) modify the shape of coastal bluffs. Rising sea level along Maine's coast allows storms and coastal flooding to reach further inland and erode sediments at the base of bluffs. Steepening of bluffs by erosion at their base may lead to increased slumping and deposition of clay, sand, or gravel in the intertidal zone which then acts to stabilize the bluff for a period of several years to decades as coastal processes rework and remove the slumped material. Once the material at the base of the bluff is removed entirely, the bluff may then be undercut again and the cycle of slumping followed by protection of the bluff base will be repeated. Most bluffs erode erratically; perhaps losing ground one year and not the next. Over a period of many years a bluff may permanently retreat landward. Historical analysis can help determine the average rate of bluff retreat.
This bluff is a highly unstable bluff with an unvegetated bluff face and a beach/boulder beach shoreline. The bluff face is too unstable to support vegetation and the few tree on the face have come down from above. These bluffs are constantly being eroded by waves to create this mixed sand and gravel beach in front the bluff and are subjected to movement by the sea.
At the posted coordinates you will find yourself viewing the bluff. You should be able to see evidence of trees that have been under cut by the water and evidence of man’s effort to protect the bluffs. The park has trails leading to this area and the bluff can be view from above or below without damaging the bluffs. The bluffs that you are looking at are the remains of the Ice Contact Drift that is slowly falling into the ocean. Remember this is an earthcache so there is no container just an earth science lesson at an amazing natural feature that needs to be protected. To log this cache, you must post a photo of yourself or your hand with your GPS showing a view of the bluff’s instability in the background. You should be able to see the layering of the sands and gravels making up the bluffs and the boulders on the beach that have not washed away. Email me through my profile the height of the bluff above the beach. Please include the name of the earthcache and the number of people in your group in your email. In your log please take the time to describe what you find special about this beach and its bluffs.
Please remember that this park is maintained for recreational purposes. So please make sure to follow all regulations carefully and register before you begin any hike. Remember that forethought and a little preparation are the key elements to a successful and enjoyable outdoor experience. Do not leave the trail and make sure to practice “Carry-in-carry out" to keep this park special for the future visitors.
If you enjoy this earthcache you may want to check the Maine Geological Survey located at (visit link)
They have developed a number of information sheets or field localities giving a great deal of information about geologic features. They also have a number of books and maps about Maine’s natural history/ geology that you might find interesting.
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