Shaping Cape Cod •

Last update: October 15th, 2019 at 1:07 pm

An astronaut aboard the International Space Station used a very long lens to shoot this detailed photograph of the northern tip of Cape Cod, showing 14 kilometers (8.5 miles) of its 105-kilometer (65-mile) length. Cape Cod is one of the biggest barrier islands in the world, and it protects towns like Provincetown and its harbor from storm waves coming in from the Atlantic Ocean. It thus also protects the southeastern Massachusetts coastline.

In the photo, the cream-colored features are symmetrically shaped dunes built mainly by northeasterly winter winds. On the water, the white streaks are boats and their wakes.

In terms of geological age, Cape Cod is young, having been laid down about 20,000 years ago by Ice Age glaciers when they were at their fullest extent. The cape consists of moraine and associated river deposits that were laid down by two large lobes of ice that extended into the area from Canada. Though Cape Cod today has water on both sides, during the Ice Age one great ice lobe lay to the seaward side and another was on the Cape Cod Bay side. Many ponds remain on Cape Cod as a result of the receding glaciers.

Why is Cape Cod shaped the way it is? When the glaciers were at their fullest extent, sea level was more than one hundred meters (325 feet) lower than it is today because so much water was bound up in huge continental ice sheets. As the ice melted, sea level kept rising and areas that had been covered by ice were submerged under the rising Atlantic waters. At this point waves and sea currents started to erode the moraines and river deposits of the cape.

Because it juts into the Atlantic Ocean, the cape shoreline is heavily eroded by waves. Sometimes the ocean breaks through the barrier island, as happened during Hurricane Bob in 1991. But even without major storms, strong wave erosion smoothes and disrupts the beachline facing the Atlantic Ocean.

In this photo, the sands eroded by the waves are swept northward to feed the growth around Provincetown. When a sandy coastal spit like Cape Cod advances into deeper water, it grows more slowly and wave action starts to bend the line of spit growth inshore. In the case of Cape Cod, this wave refraction around the tip has turned the growth direction progressively to the west and then to the south, forming the hook shape we see today.


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