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Cape Cod Hikes: The Parabolic Dunefields

The Parabolic Dunefields are easily one of the most awesome geologic features on Cape Cod. The highest of the dunes, such as Mount Ararat, stand more than 100 feet in elevation, and the entire range of dunes extends more than four miles from Pilgrim Springs to Race Point. Parabolic dunes are named for their characteristic half-moon shape. They are highest in the middle and they taper down to the edges. The Provinceland dunes are migrating to the southwest, driven by winds that blow from the northeast. On GoogleEarth you'll find them at 42deg03'39.05"N 70deg07'42.05" W.

The migration of these dunes is a hazard to man, animal and plant life. At the parking lot for Pilgrim Lake, you can plainly see the dunes overtopping a forest at the lot's edge, while a short walk away, the dunes are slowly filling in a portion of Pilgrim Lake. A little further up Route 6, bulldozers constantly remove sand from the highway in an unceasing battle against the dune's advance.

The dunes are migrating, not only because of the strong northeast winds and steady supply of sand from the coast, but also because of man's impact on the environment. Long ago, Cape Cod was covered by a scrub pine forest. This provided a root system that acted as an anchor for the earth of the Cape, and kept the dunes in check. But early settlers cleared all of these forests for agriculture and firewood.

At the beginning of the nineteenth century, beach grass and pitch pine was replanted on the dunes which again checked their movement, but recent increased dune buggy and foot traffic has uprooted the delicate grass. This problem was solved by establishing well-marked trails for vehicular traffic, but in places the dunes continue to move.

Sand dunes migrate grain by grain. When the wind blows the individual sand grain forward, they fall to the earth and bump other sand grains forward. This is called saltation. The sand is moved by wind and saltation up over the dune crest, where it slides down the lee side, or slip face, and this is how the dune migrates. The slip face of a dune usually rests at a 30 to 34 degree angle- any steeper and the sand slides down by force of gravity.

The dunes can be seen from behind at the observation towers at Race Point. From here, you can see low flat blow-outs between the dunes created by the wind. As in the dunes at Sandy Neck, these blow-outs usually reach close to the waterline and eventually fill with vegetation, which helps stabilize the dune.

For More Cape Cod Hikes see: Nature Examiner

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