Geology of Cape Cod National Seashore
Learn about the geology of Cape Cod National Seashore in Massachusetts.
Cape Cod National Seashore sits on a peninsula that juts off the coast of Massachusetts into the Atlantic Ocean. Although formed relatively recently in geologic time, its history is one of fascinating glacial processes. The peninsula continues to be shaped and transformed today by changing sea levels, erosion and deposition of sediment.
Cape Cod is also home to the Woods Hole Coastal and Marine Science Center, which is a part of the USGS Coastal/Marine Hazards and Resources Program. Research conducted at Woods Hole provides information for decision-makers in other federal agencies, state and local entities, private organizations, industry, and the public. Projects range from studies of the water resources of Cape Cod to deep sea exploration and cover topics in coastal and shelf geology, sediment transport, energy and geohazards, environmental geoscience, sea-floor mapping, and information science.
Geologic History
Twenty-three thousand years ago a massive continental ice sheet called the Laurentide stretched across much of present-day New England. This enormous glacier was the result of the last ice age during which glaciers covered almost one-third of the land on earth, and the sea level was four-hundred feet lower than it is today. The location of the islands of Nantucket and Martha’s Vineyard mark the maximum extent of the Laurentide. The edge of the Laurentide ice sheet was made up of a series of lobes, which determined the location of Cape Cod as the glacier began to recede. As the last ice age came to a close, the glacier retreated quickly, and by 15,000 years ago it had receded from all of southern New England.
Glacial retreat leaves behind a number of unique geologic features which can be found today on Cape Cod. These features include drift, moraines, outwash plains, boulders, valleys, kettle holes, and distinctive soil features. Drift is the rock debris that glaciers deposit. Drift can either be stratified or unstratified. Stratified drift has been transported by water flowing off the glacier and consists of rock debris that has been sorted by size into strata, which are just layers of rock debris. Unstratified drift, or glacial till, consists of rock fragments of all different sizes and shapes. Glacial till is unsorted because, unlike flowing water, glaciers are not capable of separating rock debris by size.
Outwash plains and moraines consist of drift. Moraines are piles of rock that had accumulated at the front edge of the glacier and were left behind as the glacier retreated. In Cape Cod, one can find a very particular type of moraine called a push or thrust moraine. These moraines were formed as an advancing glacier pushed sediment previously deposited forward like a giant bulldozer. Outwash plains are created by meltwater from the glacier. The meltwater flowed out from the foot of the glacier carrying with it huge loads of sediment that were deposited as the velocity of the water slowed further from the glacial edge. Outwash plains left behind by the giant Laurentide dominate the landscape of Cape Cod. Outwash plains also lead to the formation of a uniquely glacial topography called a kame and kettle terrain. Kames are formed when sediment filled in a hole in the ice. Later, when the ice melted, the sediment collapsed and formed a hill. Kettles are depressions in the earth formed when sediment covers up a mound of ice, and then collapses when the ice melts.
Another uniquely glacial feature that can be found in Cape Cod are giant boulders, known as erratics. These boulders are too large to have been carried by anything other than a glacier. An example of such a boulder is Doane Rock in Eastham, Cape Cod. Doane Rock is the largest known boulder left behind on Cape Cod by the retreat of the Laurentide. Glacial processes are also responsible for the presence of U-shaped valleys in Cape Cod. These valleys have flat floors and steep sides, but do not contain rivers or streams. Research suggests these valleys were eroded by groundwater seeps associated with lakes that formed in front of the glaciers and were dammed by moraines. This process is called spring-sapping, and it was made possible by the higher than usual water-table associated with the glacial lakes. The groundwater seeps would erode away rock material in the outwash plain and form the valley.
Cape Cod’s glacial past has also left it with an interesting type of soil called a podzol. Podzols are formed in temperate climates as forests begin to grow over the sand, silt, and clay left behind by glaciers. Within the soil, particular stones called ventifacts can be found. These little rocks once lay on the surface of the of the drift and outwash plain and were carved, polished, and sandblasted as wind blew across the barren landscape.
Of course, glacial processes are not the only thing that shaped Cape Cod. The landscape has also been shaped by coastal processes, and continues to be changed today. When the Laurentide was at its maximum extent, sea levels were about 400 feet lower than they are today. During this time, much of the continental shelf, which is now underwater was exposed (Figure 1). As the glaciers began to melt, sea level rose rapidly. By 6,000 years ago, waves began to erode the glacial sediments that formed Cape Cod. Eroded material was transported and deposited down the coastline by longshore currents, the result of energy released parallel to the shore when a wave hits land. These erosional and depositional processes are responsible for the bay mouth bars, spits, and barrier islands that can be found on Cape Cod today. Spits allow for the creation of a calm-water lagoon and saltmarshes on their landward side. They also provide the base for coastal sand dunes. These features are in constant flux as wind and waves batter the shoreline. Particularly during storms, waves and wind will wash sand on spits and barrier islands into the lagoons and marshes behind them, in a process that moves the spit of island landward. Onshore winds transport sand and create sand dunes, which can be 40 to 100 feet tall.