The New Meadows River, along with the other embayments of Casco Bay, originated from volcanic activity.  The resulting volcanic formations subsequently underwent substantial deformation approximately 350 million years ago that caused linear folding of the bedrock which resulted in a northeast-southwest alignment of the bedrock, evident today in the northeast-southwest orientation of the peninsulas, islands, and ledges of the area.  Erosion and scouring during glaciation slowly removed softer portions of the bedrock leaving only the harder layers which ultimately formed the foundation for the bays, necks, and other geological features of today's coastline. 

(Duffy, 1989.  Geological Assessment of Maquoit and Middle Bays, Brunswick, ME, Dept. of Geological Science, Univ. of Maine pp24 ).

The geology of the present-day New Meadows River includes a wide variety of formations from exposed and thinly covered bedrock on the terrestrial side to thick layers of glaciomarine mud deposits below the water.  The interface between the land and the sea, or intertidal area, varies similarly from highly exposed, vertical bedrock walls to very soft mud in sheltered bays and coves.  This varied geology provides a wide range of habitats and ecological niches, all of which combine to make the New Meadows River highly diverse and productive. 

(from "State of the New Meadows River Report," prepared by Christopher Heinig for the New Meadows River watershed Project Steering Committee, April 16, 2002).