The island is comprised of three major geomorphic components: 1) Pleistocene island core, 2) Holocene accretional terrains, and 3) Holocene marshes.
A series of linear depressions collectively referred to as the Central Depression can be traced along the middle of the island core trending in north-northeast to south-southwest directions
The hydrogeology of St. Catherines Island may be considered within the geologic framework of both a shallow and a deep groundwater system.
The dune field adjacent to McQueen Inlet has displayed tremendous dynamics where this portion of the shoreline was originally accretional.
Erosion also exists on the marsh side of the Island. An assessment of the total marsh side shoreline of St. Catherines Island, found erosion at a rate of nearly 3.3 feet (1.0 meter) a year from 1933 to 2004.