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Types of Bluffs Along Maine's Coast
The Maine Geological Survey Coastal Bluff Maps show the shoreline type and relative stability of bluffs along the Maine coast. The slope, shape, and amount of vegetation covering a coastal bluff and the adjacent shoreline are directly related to the susceptibility of the bluff face to ongoing erosion. As might be expected, less vegetated bluffs are more likely to be eroding than completely vegetated bluffs. Another important factor related to bluff stability is the material which makes up the bluff. Materials such as clay, gravel, and sand react differently to erosion and, when combined with variations in vegetation and slope, affect the rate of erosion for an individual bluff. When planning to reduce the risks of coastal erosion, it is important to take vegetation, slope, erosion rate, and sediment types into consideration.
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.
Coastal environments are dynamic and subject to continuous change. Gravitational processes of creep, slumping, and occasional landsliding modify the shape of coastal bluffs. Rising sea level along Maine's coast (at a rate of about 2 mm/year, slightly less than a foot per century) 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.
Over time, the appearance of bluffs will change. Coastal flooding, erosion, slumping, creep, deposition in the intertidal zone, ice action, ground-water drainage, and human alteration will change bluff appearance. For example, unvegetated beaches are more easily eroded than salt marshes, armored shorelines, or ledges. Also, bluffs covered with vegetation and having a gentle slope have been more stable than those with steep slopes and little vegetation. Understanding the processes that erode bluffs can help determine what solutions are appropriate to reduce future rates of property loss.
The Coastal Bluffs Maps can help identify shorelines with increased risk of coastal erosion. Review the legend below in order to understand what the maps show and do not show. The colored categories correspond to field conditions when the map was made. Conditions may have changed in the last few years. Comparison of the information on the map with current conditions may indicate that a stability category is different now. Since bluff erosion may be cyclical, the change may only be temporary. If coastal development is near any bluff, we recommend a professional evaluation be made to determine the risk to structures.
Last updated on January 20, 2011
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