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The Eroding Sandy Point Esker, Stockton Springs, Maine
Eskers are sand and gravel deposits that form inside tunnels within glaciers. They are usually less than 300 m wide and 20 m high, but may extend (with occasional gaps) for more than 100 km. Eskers are usually steep-sided, but often contain multiple ridges. They are usually composed of sediment layers shaped by the ice-tunnel rivers; these layers were often disturbed by movement of the ice during deposition, however. Maine has many eskers, all of which formed during the last Ice Age, which ended between 14,000 and 11,000 years ago.
Where it nears the sea, the Sandy Point esker has several separate ridges that are about 150 m in width and 10 m in height (Figure 2). The most seaward tip of the esker faces directly into Penobscot Bay and experiences the force of winter storm waves. As a result, it is eroding relatively rapidly (estimated at 0.5-1.0 m/yr), and the eroded sand and gravel is forming beaches in front of and along the sides of the esker (Figures 2, 3, 4, 5) (Barnhardt and others, 1998). The beach directly in front of the esker (Figure 3) is very coarse-grained, with boulders scattered above and below the high-tide line. Despite erosion of the bluff sediment, wave and current action is too strong here to permit much of a protective beach to remain. Thus, the bluff face remains devoid of most vegetation.
On the eastern side, a 50 m wide beach has developed from eroded esker sand (Figure 4). This beach is finer grained than the one in front of the esker, and all the material in it has moved up the Penobscot Estuary by waves and currents. The beach has been leveled for a parking lot and lost any sand dunes it might have once had. This beach has prevented storm waves from reaching the side of the esker for a long time, and large trees cover the formerly eroding bluff face here.
The sides of the esker are draped with glacial-marine muddy sediment. Where exposed (Figures 2, 5), this material contains fossil seashells from around 12,000 years ago (Ives and others, 1967), when the sea covered much of coastal Maine. On the western side of the esker, less sand is available from the eroding esker, and only low, narrow beaches exist (Figure 5). Bluffs of fine-grained glacial-marine sediment are retreating rapidly here, producing mud for intertidal salt marshes and flats, not sand for beaches.
Sand and gravel eroded from the Sandy Point esker is driven by waves and currents up the Penobscot Estuary for several hundred meters. Where the sand beach ends, bluff erosion begins again (Figures 6, 7). Material from this eroded bluff of esker sediment is all driven upstream and forms an extensive beach up to 100 m wide and 400 m long. Though altered for a parking lot, some natural sand dunes remain in the area that was purchased by the Land-for-Maine's-Future Board (Figure 6).
Time-series photographs of the eroding bluff that supplies sand for this beach reveal the slow rate of sand production (Figure 7). When photographed in 1998 (Figure 7a), a block of esker sediment was observed on the esker side. The bent trunks of the trees on the block indicate that the block is moving downslope slowly by a process called "creep." The trunks bend in an attempt to remain vertical as the block slides down. A photograph of the same site two years later reveals that 1 tree has fallen, and about 0.6 m of sediment has eroded away (Figure 7b).
This small exposure of eroding glacial sediment demonstrates the close connection between bluff erosion and the creation of beaches in Maine (see also Kelley and others, 1989). If the Hersey Retreat (Figure 2) had been built close to the bluff edge a hundred years ago, seawalls to prevent erosion might have been constructed. If that had happened, sand for the adjacent beaches would have remain trapped in the bluff, and no beaches would now exist. Without beaches along the flank of the esker, erosion would have occurred here, and probably necessitated more seawalls along the length of the esker.
Barnhardt, W. A., Bryant, M., Dickson, S. M., and Kelley, J. T., 1998, Coastal Bluff Map, Castine quadrangle: Maine Geological Survey, Open-File Map 98-112, scale 1:24,000.
Ives, P. C., Levin, B., Oman, C. L., and Rubin, M., 1967, U. S. Geological Survey radiocarbon dates IX: Radiocarbon, v. 9, p. 506.
Kelley, J. T., Kelley, A. R., Pilkey, O., 1989, Living with the Maine Coast: Duke University Press, 174 p.
Thompson, W. B., and Borns, H. W., 1985, Surficial Geologic map of Maine: Maine Geological Survey, scale 1:500,000.
Selected Additional References on Eskers
Ashley, G. M., Boothroyd, J. C., and Borns, H. W., Jr., 1991, Sedimentology of late Pleistocene (Laurentide) deglacial-phase deposits, eastern Maine; An example of a temperate marine grounded ice-sheet margin, in Anderson, J. B., and Ashley, G. M. (editors), Glacial marine sedimentation; Paleoclimatic significance: Geological Society of America, Special Paper 261, p. 107-125.
Borns, H. W., Jr., 1979, Eskers in Maine: Maine State Planning
Office, Critical Areas Program, Planning Report 67, 44 p.
Thompson, W. B., 1979, Surficial geology handbook for coastal Maine: Maine Geological Survey, 68 p.
Trefethen, J. M., and Harris, J. N., 1940, A fossiliferous esker-like deposit [Maine]: American Journal of Science, v. 238, no. 6, p. 408-412.
Text and photos by Joseph T. Kelley.
Originally published on the web as the April 2001 Site of the Month.
Last updated on October 6, 2005
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