This dataset contains line data describing the shoreline type and relative stability of bluffs along a section of the Maine coast mapped at a scale of 1:24,000. 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. In 2026, the Maine Geological Survey updated the Casco Bay portion of this dataset to include new factors associated with bluff stability and sediment supply from coastal bluffs to the coastal wetland and a summary of mapped adjacent habitats. Further updates covering the rest of the Maine coast will be completed by 2029. The Maine Geological Survey uses this dataset to produce Coastal Bluffs Maps.

• Shoreline segments are classified in the BLUFF field as either being bluff (Y), non-bluff (N), or unmapped or undetermined (X).

• The relative stability of a bluff face is classified in the STABILITY field as being either not a bluff (N), stable (S), unstable (U), highly unstable (H), or unmapped or undetermined (X). This classification is based on observed factors that reflect recent activity on the bluff face. Ratings for these individual observed factors are also provided where data have been updated (areas without an update will show blank values). Refer below for a table describing these factors.

• The shoreline type is classified in the TYPE field as either ledge (exposed bedrock outcrops) (L), armored (seawall, riprap, gabion, bulkhead, etc.) (A), salt marsh (V), beach, mud flat, or other loose sediment (N), or unmapped or undetermined (X). Multiple shoreline types may co-exist on any given section of coastline (example, TYPE L-V indicates, landward to seaward, presence of ledge and vegetation).

• Where data have been updated since 2026:

  • Maine Geological Survey has added several new variables indicating visual and terrain characteristics of stability. See the field table below for more information.

  • Sediment supply is classified in the Sediment_Value field with ratings of None, Low, Medium, or High. Areas without an update will have a blank value. The classification is based on observed evidence of erosion and sediment transport from coastal bluffs to the coastal wetland.

  • The map now documents the adjacency of coastal bluffs to special mapped habitats and a total count of these habitats. See the field table below for more information.

Bluff stability data collection and scoring

MGS ratings for bluff stability are based on visual observations of coastal oblique imagery and terrain metrics derived from topographic and bathymetric lidar collected by NOAA from 2022-2023.

• Visible factors: Observations of coastal bluff stability are based largely on interpretation of oblique aerial imagery. Field visits and images or notes taken in the field are also used where available and are a preferred data source. In locations where field or oblique aerial imagery is not available, vertical aerial imagery is used but provides limited interpretation because shoreline erosion details are often not visible from a vertical angle.

  • Visible stability factors consist of toe erosion, exposed sediment on slopes, landslides or slumps, fallen trees, evidence of groundwater or runoff, failed armor, and armor type. Refer to the “All Coastal Bluffs Fields” table for more details.

• Terrain factors: Terrain interpretation is important for estimating bluff stability. Metrics such as slope and relief are important for understanding whether a bluff may be approaching an unstable slope condition. Aspect is an important metric for interpreting how freeze-thaw cycles may impact stability during seasonal changes. Combining terrain information with visual characteristics provides an overall more comprehensive measure of bluff stability

  • Terrain factors consist of bluff slope, height, and aspect.

• Calculating bluff stability: Stability is the sum of the following criteria ratings:

Sediment source data collection and scoring

MGS ratings for sediment source represent the importance of each mapped coastal bluff as a source of sediment for coastal environments. As a result, the sediment source rating depends on a combination of factors that denote active erosion of the bluff and the presence of coastal habitat that benefits from bluff sediments.

• Visible factors of erosion/sediment supply: As with bluff stability, these factors are based on interpretation of oblique aerial imagery and/or field studies.

  • Visible sediment source factors consist of toe erosion, exposed sediment on slopes, landslides or slumps, failed armor, and noted evidence of sediment transport.

• Terrain factors of sediment supply: Bluff height is included in sediment supply scoring as an estimate of reserve sediments that may be supplied to coastal environments in the future. Much like bluff stability, this terrain factor is included conditionally based on visible evidence of bluff erosion.

• Habitat factors denoting sediment supply: mapped habitat areas are included in this assessment as recipients of bluff sediments. Identification of these areas consists of simple habitat type classifications on the seaward edge of coastal bluffs as well as adjacent special habitat areas.

  • Habitat type factors are completed by MGS review and include vegetation (V) or beaches, sandflats, or mudflats (N). Types that are recorded but are not used in sediment scoring are ledge (L), armor (A), and unmapped (X). Besides X, all types may coincide with one another at any one shoreline segment.

  • Special habitat types used for sediment source scoring include Sand dune habitat, Shellfish habitat, and Marine worm habitat.

• Calculating the Sediment Score: The sediment score is equal to the sum of the following criteria ratings: