What types of landslides occur in Maine, and where do they happen?
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,
