Maine's coastline exhibits a striking morphological dichotomy: west of Portland, beaches are predominantly sandy and smooth, whereas northeast of the city, the shoreline becomes rugged and rocky. This difference, noticeable even from space, is attributed to fundamental variations in the underlying bedrock geology and the dynamics of riverine sediment deposition, according to recent observations.

The key lies in the composition and resistance of the parent rock. West of Portland, the geology favors the formation of sandy beaches, a process shaped by erosion and sediment transport. Conversely, to the northeast, the bedrock is more resistant to erosion, resulting in a coastal landscape dominated by cliffs and rocky formations. Furthermore, the distribution of fluvial sediments plays a crucial role; rivers emptying into the western coast deliver a greater quantity of sand, contributing to the formation and maintenance of its extensive beaches.