Almost all of Maine’s beaches are "transgressing," or moving landward in response to coastal storms and gradual sea-level rise at a rate of about an inch per decade. This landward migration of the beach and dune system is like the motion of a tank tread; the beach basically migrates over itself in response to storms and sea-level rise.

Long-term erosion is considered permanent erosion that occurs over decades due to numerous factors:

• Structures like jetties prevent sand from being transported to the beach.
• Upstream dams on rivers prevent sand from reaching the beach;
• Tidal river dynamics force sand into deltas that serve as sediment sinks or traps.
• Sea-level rise or storm-related erosion occurs faster than the rate of sand deposited on the beach.

The photo to the left shows Scarborough Beach after the Patriots' Day Storm in 2007, when the beach "washed over" into the dunes.

A sure sign of beach and dune transgression is the continual presence of stumps or peat (marsh) deposits in the surf zone; these materials are from historic backbarrier marsh and forested uplands that became covered by sand and then exposed, as in this photo of Ferry Beach in Scarborough.