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Landslides in Maine - Frequently Asked Questions
Landslides are classified based on the type of earth material that is affected, and how it moves. Most Maine landslides are either slumps or debris avalanches. Slumps occur in clay-rich sediments that were laid down on the ocean floor when parts of southern Maine were drowned by the sea at the close of the Ice Age. These clay deposits are called the "Presumpscot Formation", after the Presumpscot River near Portland. The marine clays are most prone to slumping on slopes, such as stream banks or bluffs along the ocean shore. Factors that may promote slumping are steep slopes, saturation by ground water, shaking of the ground, and/or the addition of weight on top of the slope. When a slump occurs, large blocks of clay drop downward from the head of the landslide, while the bottom part (toe) of the landslide spreads outward toward lower elevations. The movement may be very slow over a period of years, or can occur in just a few minutes. Not all clay deposits are equally prone to slumping, but because slumps occur in populated coastal and valley locations, they may cause substantial property damage.
Debris avalanches occur on steep mountainsides of western and northern Maine. They involve the sudden failure of loose sedimentary material and/or blocks of local bedrock. Avalanches are rapid movements. They may result from heavy rains or snowmelt saturating the ground, and the slope failure commonly develops along the surface of the solid underlying bedrock. Avalanche scars are seen on many of the highest mountains in northern New England. Usually the affected areas are unpopulated, so there has been little or no property damage in Maine, and many landslides are not detected until sometime after they happen. More spectacular, and sometimes destructive, avalanches have occurred in the White Mountains of neighboring New Hampshire.
In addition to the above types of landslides, other types of slope failures may occur on a generally small or local scale. These include rockfalls (sometimes seen in cliffs along highways), and shallow soil creep on slopes excavated in glacial till.
Surface evidence of past and present earth movements should be taken as warning signs of future landslide potential. Previous slumps, and slumps that may be still active but very gradual, are sometimes indicated by depressions in the ground on the upper portions of slopes. The depressions form as a result of downward movement and backward tilting of blocks of clay at the heads of the slumps. Tilted trees on the affected ground are another good indication of slumps. However, it is usually difficult to predict the timing or rapidity of future slumps.
It's best to avoid adding weight or water content at the top of a potentially unstable slope, such as stream banks or ocean bluffs located in eroding clay. Examples of activities that might promote slumping include placing a building on the site, heaving watering of lawns next to the slope, or dumping heavy loads of fill. Along the ocean shore, areas of known landslides will generally continue to be problems due to rapid shoreline erosion coupled with the unstable ground. The Maine Geological Survey has a series of maps and reports detailing coastal bluff stability and landslide topics. If you have specific questions about landslide potential on your property, or remediating a slide that has already occurred, consulting geologists in the private sector can evaluate the site and offer advice for possible slope drainage and other protective measures.
Last updated on October 6, 2005
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