Geology and Ecology of National Parks

Cape Cod National Seashore

Geology of Cape Cod National Seashore

Cape Cod looking to the north

Cape Cod looking to the north,

(Credit: John Mullaney, USGS. Public domain.)

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.

 

Glacial outwash plain

This image shows a modern-day example of a glacial outwash plain (Brady Glacier, Alaska) and represents what an active outwash plain on Cape Cod may have looked like in the past. Outwash plains begin at the foot of a glacier and can extend outward for miles. 

(USGS. Public domain.)

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.

Doane Rock

Doane Rock is the largest glacial boulder on Cape Cod.

(Public domain.)

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. A geologic map of Cape Cod can be found here.

 

Ecology

A variety of ecosystems can be found on Cape Cod including marine, estuarine, freshwater, and terrestrial. These ecosystems provide habitat for an abundance of plant and animal life. Pitch pine and scrub oak forests, heath lands, sand plain grasslands, and dunes make up the terrestrial ecosystems. Heathlands and sandplain grasslands are a direct result of Cape Cod’s glacial past. They lie on top of the sandy sediments left behind as the glaciers retreated. The grasslands are also a result of past disturbances by either settlers or fires. Without these disturbances, these ecosystems would likely be replaced with forests. Heathlands and grasslands are a unique ecosystem that provide habitat for similarly unique animals such as the vesper and grassland sparrow, which are types of open-land birds.

Dunes on Cape Cod are a result of a variety of interconnected forces including glacial activity, erosion, deposition, and destabilization of the ground surface by early settlers. Dunes are a prominent feature on Cape Cod National Seashore covering about one-third of the land surface. Plants that grow on the dunes are uniquely adapted to endure the harsh conditions of their sandy, windy environment. Vegetation helps to stabilize the dunes and combat erosion.

Freshwater ecosystems on Cape Cod, like most aspects of the peninsula, are a direct result of a glacial past. Kettle ponds are formed when the kettle holes discussed earlier are deep enough to intersect with the water table, thereby flooding the depression and creating a pond or lake. These special habitats provide refuge for a number of species including 12 different types of amphibians. Among those species is the remarkable eastern spadefoot toad. Spadefoot toads spend most of the time hiding underground, emerging only occasionally on warm, rainy nights to breed. A variety of reptiles are also associated with these freshwater ecosystems. The eastern hognose snake consumes almost exclusively on the toads that can be found near the kettle ponds. When threatened, the eastern hognose will rear up and hiss like a cobra and then roll over and play dead. A notable number of terrestrial eastern box turtles can be found on Cape Cod as well.

Salt marsh in Hatches Harbor

Salt marsh in Hatches Harbor on Cape Cod. Salt marshes are extremely productive ecosystems.  

(Public domain.)

Salt marshes and estuaries are prominent features of the Cape Cod landscape. Estuaries are places where saltwater and fresh water meet. These are extremely biologically diverse and productive ecosystems. Salt marshes filter out excess nutrients and provide habitat for fish, shellfish, and birds. Beyond the estuaries and saltmarshes, the Atlantic Ocean also teems with life just off the coast. Up-welling of cold, nutrient rich water support food sources for seals, sharks, dolphins, whales, and seabirds (Figure 8). In the summer, endangered North Atlantic right whales and humpback whales use the waters off the coast of Cape Cod for feeding and as a nursery ground. Colonies of seals can also be found in the summer, both in the ocean and on the shore.

All of these ecosystems from the oceans to the freshwater ponds and the grasslands provide habitat for over 370 different species of birds. Migratory birds that will fly as far north as the Artic and as far south as Antarctica rest and feed on Cape Cod. Among these many bird species that make Cape Cod home is the piping plover, a once endangered species. Although these tiny birds have rebounded from their smallest numbers, they are still a threatened species.

Surrounded by salty ocean water on three-sides, freshwater is a major concern on Cape Cod. All of the drinking water, and most of the freshwater for agricultural, industrial, and domestic purposes on the peninsula comes from groundwater. Groundwater is vulnerable to contamination and depletion. The USGS has been researching ground and surface-water on Cape Cod for over 50 years to help inform decision-making regarding this vital resource.

Recent studies have investigated the effect of climate change on groundwater and surface-water, sources of water to ponds, streams, coastal areas, and public-supply wells, as well as the transport of discharge of nitrogen from wastewater. USGS has also been involved with monitoring groundwater contamination of the underlying glacial aquifer at the Joint Base Cape Cod. This work helps to protect the communities that rely on that aquifer as their only source of freshwater.

USGS is also involved with research off the coast of Cape Cod in the Atlantic Ocean. In 2013, USGS participated in a joint expedition with NOAA and other organizations to explore deep sea canyons offshore of the Northeast United States. These canyons, which cut into the edge of the continent margin, can be thousands of meters of deep. Thus, scientists on the expedition used a special remotely operated vehicle (ROV) for exploration. The ROV captured fascinating images of the creatures that make their home in these dark and deep environments.

Image of Deep Sea Canyons

NOAA

(Public domain.)

Surprisingly, these canyons are areas of high productivity in the deep sea. Organic matter and sediment collect and are concentrated in the canyons allowing them to support a diversity of life, including cold-water corals and commercially important fisheries. These canyons are also associated with submarine landslides and hydrocarbon seeps. The research expedition will help scientists to better understand the unique biological, ecological, and geological processes occurring in the deep-sea canyons. More information and amazing photos of deep sea flora and fauna can be found here.

USGS also conducts important research at the Woods Hole Coastal and Marine Science Center which is located on Cape Cod. Their 2018 annual report about the science can be downloaded here. Woods Hole is one of three centers associated with the USGS Coastal/Marine Hazards and Resources Program (CMHRP). Work at the Woods Hole Coastal and Marine Science Center focuses on six key topic areas: Coastal and Shelf Geology, Sediment Transport, Energy and Geohazards, Environmental Geoscience, Sea-Floor Mapping, Information Science. Their work is not just limited to Massachusetts or New England but spans the nation. In 2018, Woods Hole scientists worked on a vast diversity of projects from providing support during the Mount Kilauea Volcanic Eruption in Hawaii to restoring salt marshes in the Herring River in Wellfleet, Massachusetts. and mapping the seafloor off the coasts of New Jersey, Delaware, Maryland, and Virginia.