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Sandy Point Beach, Cousins Island, Yarmouth
Sandy Point Beach in Yarmouth is an excellent place for a school group or an individual to see some diverse geology and evidence of active geological processes. The beach parking area is located on the Cousins Island side of the Ellis C. Snodgrass Memorial Bridge, commonly called the Cousins Island Bridge (directions to Cousins Island). An access road leads from the west side of the lot down to the beach near the bridge. Most of the beach is covered at high tide, so it is best to arrive close to low tide and you can begin the walk here at the sandy beach adjacent to the bridge (Figure 1).
The beach adjacent to the bridge includes a sand spit that one can walk along at low tide (Figure 2). It is formed by currents moving and depositing sand in the channel beneath and beyond the bridge. If you look at the sand closer to the high tide mark near where you enter from the access road, you will notice in places that the sand has areas where it has a purple color (Figure 3). If you look closely at the purple sand, especially with a hand lens, you will see that it contains mostly small crystals or fragments of the purple mineral garnet, known as almandine (Figure 4). The garnet is heavier than the surrounding lighter-colored sand. It is often found in segregated patches because wind and water currents can't move it as easily as the lighter minerals that make up the lighter-colored sand (mostly quartz and feldspar mineral fragments).
The part of beach that is below the north side of the parking lot has large stones in the nearshore tidal area and the beach is generally of coarser sand and pebbles than the sandy spit beach (Figure 5). This part of Sandy Point Beach was formed by wave activity eroding the bluff that the parking lot sits atop. Numerous small and medium size slump scars are found along the bluff face, and many trees have been toppled or rotated by the slumping (Figure 6). A large slump scar is found on the other side of the bridge, best accessed from below the bridge (visit on return if there is time). When fresh, the slump scars expose the deposits left during the last ice age (Figure 7). These include bouldery sediment called till by geologists, but more commonly called hardpan. Also, there are sandy and silty deposits above the till that were laid down in the ocean as the ice front retreated from the area. Because of the weight of the massive ice sheet, the earth's surface was depressed. When the glacier began to melt and retreat, the sea flooded the depressed region and the glacier front, so that it was in contact with the ocean as it retreated (Retelle, 1997).
Clearly the erosion poses a potential problem for the parking lot and for the power line towers that are set there. The larger boulders in the nearshore area give an indication of how much erosion has gone on since the ocean has risen near to its present position in the last few thousand years (Figure 8). The large boulders were eroded from the bluff when it stood seaward of where it is today.
Continue walking eastward along the shore away from the bridge and parking lot. You will approach a home that has a large wooden retaining wall engineered to prevent erosion (Figure 9) like the slumping seen below the parking lot. Here you will cross a small stream where the bluff angles to the north (left). Continue walking along the shore until you come to an exposure of bedrock. The outcrop and the rock you will see farther along the shore is mapped as the Eliot Formation (Berry and Hussey, 1998). The Eliot Formation is part of a sequence of rocks known as the Merrimack Group, and can be traced from southeastern coastal New Hampshire to as far north as Freeport, Maine. The rock has been deformed by folding and shearing of the rock layers (Figure 10) during episodes of dynamic earth activity associated with a now inactive fault zone known as the Norumbega fault system (Ludman and West, 1999; Swanson, 1995). During these episodes the original mineral composition of the rock was altered by pressure and temperature over time to become a metamorphic rock known as a phyllite (an altered shale or mudstone).
Many millions of years after the deformation and metamorphism of the bedrock, a much younger geological event took place; the last ice age. As the ice advanced over the land surface, boulders and rock fragments at the base of the ice scraped and gouged the underlying solid rock leaving features on it called striations and grooves. The orientation of these features parallels the direction of the flowing ice (Figure 11). Compass measurements of these ice-flow indicators show that during the last ice age, the flow direction of the ice in this region of Maine changed from a southeasterly flow (160-degrees) to a southerly flow (175-degrees) as the ice began to melt and retreat.
Berry, H. N. IV, and Hussey, A. M. II, 1998, Bedrock geology of the Portland 1:100,000 quadrangle, Maine: Maine Geological Survey, Open-File Map 98-1 (online version - 4.3Mb PDF file).
Ludman, A., and West, D. P., Jr., 1999, Norumbega fault system of the northern Appalachians: Geological Society of America, Special Paper 331, 202 p.
Retelle, M. J., 1999, Surficial geology of the Yarmouth quadrangle, Maine: Maine Geological Survey, Open-File Report 99-105 (1:24,000 scale map with 8-page report).
Swanson, M. T., 1995, Distributed ductile dextral shear strain throughout the Casco Bay area, in Hussey, A. M. II, and Johnston, R. J. (editors), Guidebook to field trips in southern Maine and adjacent New Hampshire: New England Intercollegiate Geological Conference, 87th Annual Meeting, Brunswick, Maine, p. 1-13.
Text and photos by Tom Weddle.
Originally published on the web as the April 2008 Site of the Month.
Last updated on January 20, 2011
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