Dean Bennett

Sand beaches are most prevalent on Maine’s southern coast from the New Hampshire border to Portland. Here the early post-glacial Androscoggin River, which probably ran south at that time through Sebago Lake, deposited abundant quantities of sand along the present coastline. Southern beaches tend to be the state’s longest, due in part to a relatively straight coastline. While most of the glacially derived sand for these beaches comes from nearby sources, two of the largest beach systems near Casco Bay are formed from sand carried down from upland deposits: the Popham Beach system (near the mouth of the Kennebec) and the Old Orchard Beach system (supplied by the Saco River).

North of Casco Bay, where sand is limited, beaches are fewer. The highly irregular coast has deep embayments, and the beaches are primarily formed by wave action at the heads of bays. This activity, in conjunction with the erosion of rocky headlands, tends to straighten the coastline. Erosion of bedrock usually provides only minor amounts of sediment to Maine’s beaches, but Sand Beach on Mount Desert Island is an exception; 60 to 70 percent (by weight) of its sand comes from nearby bedrock. Sand Beach illustrates another source of beach sediment: mollusk shells. It is one of few Maine beaches with a high percentage (by volume) of carbonate shell fragments from marine organisms – a composition more typical of tropical regions.

Each beach results from a precise correlation between the size of particles of glacial sediment – sands, silts, and clays – and the speed of moving water. The finer silts and clays are carried away, while the coarser grains of sand are left behind to form the beach. Waves are the most important agent in the process of building beaches as well as eroding them. Steep, high-energy waves, such as those produced by storms, can remove large amounts of sand from beaches very quickly. In general, beaches are eroded in winter and built up during the summer months.

In profile, a beach is divided into offshore, foreshore, backshore and dunes. The offshore is always submerged and often contains bars formed of sand removed from the foreshore and backshore. The foreshore is the zone between high and low tides. The berm, a built up terrace, is the major feature of the backshore. Dunes occur between the backshore and the lagoon, marsh, or land area behind the beach. On most undeveloped beaches, the frontal ridge is the most outstanding feature of the dunes. It is also the most important, for this ridge buffers the backdunes and marshes during storms and stores sand for rebuilding beaches depleted by wind and wave.

Dunes form due to a reciprocal interaction between sand, blown landward from the berm, and vegetation—primarily American beach grass. Once beach grass gets a foothold, it begins to trap windblown sand. More sand is trapped as the plant grows until a balance between sand, plant, and wind is reached. The vegetation stabilizes the sand and establishes conditions for the formation of dune fields.

Gravel and cobble beaches occur where material of coarse size (eroded outcroppings of bedrock and glacial deposits) can be worked by the waves. Along the eastern U.S. seaboard, they occur only along the New England coast, becoming more prevalent as one proceeds northeastward. Maine has more gravel and cobble beaches than any other New England state, (with more than 450 east of the Penobscot River alone). They’re composed of materials ranging in size from pebbles (from .125 inches to 2.5 inches) to cobbles (2.5 to 10 inches).

The profile of gravel and cobble beaches differs from that of the typical sand beach. Unlike the wind-whipped dune ridge that rises twenty feet or more above the sand beach berm, the highest point of gravel and cobble beaches is a ridge, four to twelve feet in height, created by storm waves. The face of gravel and cobble beaches is also steeper than that of sand beaches and may contain three or four berms.

Adapted from Maine’s Natural Heritage, by Dean Bennett. Published by Down East Books and the Maine Critical Areas Program.