Changing Coastline

Formation of Cape Cod

In a recently published brochure by Gordon Peabody of Safe Harbor, an environmental education and protection group working on Cape Cod, he posed the question:

How can Cape Cod be the newest, glacially formed land on the planet and at the same time be the fastest disappearing?[i]

In order to answer this question, one has to understand what types of coastal changes are occurring on Cape Cod and how these changes are the result of being situated within the larger coastal system of the Gulf of Maine. Roughly 18,000 years ago marked the peak of the Wisconsin Glaciation, the last major glaciation in North America. During this ice age, the Gulf of Main formed, making a semi-enclosed sea partitioned by large underwater elevated areas like George’s Bank. The Gulf of Maine, spans from Cape Cod in Massachusetts to Cape Sable in Nova Scotia. As a result of being a semi-enclosed system it retains much of the circulating cold water, buffering it from entering the gulf stream directly, making it some of the most productive water in North America and a good habitat for maritime creatures to flourish like lobster, cod, herring and whales, which have become synonymous with the Cape’s identity.

Morphology of the Cape

This glaciation, in conjunction with currents influenced by the Gulf of Maine were the forces that created the signature shape of the Cape. During this period, Cape Cod was covered under ice more than a mile thick, but as the glacier began to melt and recede, the sea level began to rise and the Cape began to take shape as it is more presently known today. Graham Giese, an Oceanographer and Senior Scientist for the Provincetown Center for Coastal Studies, documents the timeline along which the morphology of the Cape has gradually changed:

Much of this morphology was constructed during the late Wisconsin regression 13,500 years ago and the Holocene transgression that began less than 13,500 years ago and reached a depth of 8 meters 4500 years ago, 6 meters 4000 years ago, 2 meters 1500 years ago, and near its present level less than 1000 years ago.[ii]

The morphology of the Cape transformed concurrent to changes in sea level depths, which were the result of the retreating ice and disappearance of glaciated area. The last image in the above figure depicts the Cape as we currently know it today. In comparison to much more significant morphological changes, this crude map will probably be accurate for several hundred years but more detailed maps of the Cape have to be redrawn with some frequency as the Cape is in a constant state of transformation. This has implications for the jurisdiction of the National Park Service (NPS), which is allowed to have a dynamic boundary that is drawn ¼ mile off mean low-tide, meaning that as various land features change as in the case of the Nauset-Monomoy Barrier in the Mid-Cape, the boundary of the National Seashore has to change accordingly.

The geological profile of much of the Lower Cape is comprised of glacial cliffs with foundational bedrock, but the more newly formed areas are composed primarily of shifted sediment due to the longshore sediment transport system affecting the Cape’s coastline. This transport of sediment has resulted in the formation of two newer areas, which are depicted in yellow: Provincetown and the Nauset-Monomoy Barrier.

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[1] Resolution of digital Bathymetric grid constructed using 15 arc second (~1/2 km grid cell size) and 30 arc second (~1 km grid cell size) (Longitude = 71.5 – 63 W, Latitude = 39.5 – 46 N). Constructed by E. Roworth and R. P. Signell

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[i] Peabody, Gordon. Understanding the Coastal Process. Cape Cod: Safe Harbor, 2009. Print.

[ii] Elazar Uchupi, Graham Giese, Neal Driscoll and D. G. Aubrey. Postglacial Geomorphic Evolution of a Segment of Cape Cod Bay and Adjacent Cape Cod, Massachusetts, U.S.A. Journal of Coastal Research, Vol. 21, No. 6 (Nov., 2005), pp. 1085-1106+1192