More than 21,000 years ago the Laurentide ice sheet advanced south from today’s northern New England and Canada and transported eroded rock debris until it reached its southernmost limit. Several glacial ice lobes deposited the glacial drift that now comprises most of Cape Cod.
The forward progression of the ice sheet was often balanced by melting at the leading edge, so that the ice front maintained its position even as it began to retreat.
While the ice front was stationary, frequent warm periods caused large amounts of water to melt from the glaciers. It is assumed that the ice had vanished from the Cape Cod area in less than 3,000 years and that most of its glacial landforms were created within about 1,000 years.
The most common glacial landforms on the Cape and the Islands are end moraines, outwash plains and kettle terrain.
Glacial moraines were formed when rocks of various sizes were deposited near the edges of the melting glacier. When the glacier was vigorously advancing over older deposits, it piled up thrust sheets and formed a large ridge beyond the ice front.
Huge meltwater streams emerged from the glacier ice and passed through moraines before inconceivable quantities of sediment loads were dumped and gently sloping outwash plains of stratified drift were formed.
Outwash plains are the most dominant glacial landforms on the Cape and the Islands. But they are incomplete because their downstream ends eroded away by the sea.
Depressions indicate another glacial feature at many places of the outwash plain. Isolated blocks of ice buried in the outwash deposits melted slowly, long after the glacial lobes had retreated far to the north. As sediments collapsed around the melting ice blocks, kettle holes formed within the outwash plains.
About 6000 years before the present, sea level continued to rise and Cape Cod’s landform began to take shape (green color).
The present pattern of erosion is shown on the map on the right.Red lines show areas of undergoing wave erosion,whereas green lines show areas of deposition. Eventually, the familiar shape of Cape Cod is formed by wind and waves. But these forces are still in action.
The overlook you are standing on is part of a steep cliff; called bluff. From here you can view the vast Atlantic and the drama of sand and surf.
As waves break against the base of the bluff, sediment is carried away to be deposited elsewhere. Wind-blown sand, waves and rain erode the deposits again. So beaches gain (accretion) and lose (erosion) width through the seasons. Sand spits and dunes get longer and wider.
Measurement values of shoreline erosion show a variation over time. Looking at average rates of erosion short term may be misleading, since the location also changes over time as well as the intensity of the erosion. Over about the last hundred years, the average rate of erosion along the cliffed section of the ocean side of Cape Cod is about three feet per year. But this also includes parts of the Lower Cape that are undergoing rapid erosion while other parts have been stable for decades.
Historic shorelines at Marconi Beach show the glacial bluff in this area retreats at fewer than 3 feet per year.
The more important factor of coastal change may be the recent sea-level rise indicated by tide gauge records. Over the last 2,000 years, the sea level rose only about 6 1/2 feet, but about 1 foot over just the past 100 years. This much faster rate of sea-level rise may cause increased rates of erosion.
All in all, coastal change is a dynamic process with geological and physical influences of different kinds and intensity.
Geological Influences:
- Geomorphology, i.e. shore of rocky or low cliffs, cobble beaches or sand beaches
- Shoreline erosion/accretion
- Beach profile slope
Physical Influences:
- Wind speed and direction
- Relative sea-level change
- Mean wave height
- Tidal range
In order to determine and also to predict coastal changes, a vulnerability ranking of endangered sections of the shoreline has been developed.
Bear in mind that all changes of the landscape are not the result of single storms or big waves.
Also under normal conditions, longshore drift moves sand down the beach to shape and extend the barrier spits. Tidal currents erode and carry debris into the embayments and build marsh surfaces upward and shoreward. Prevailing onshore winds erode and deposit sand and change the shape and position of sand dunes. During both heavy and fair weather, marine erosion, marine deposition, and the movement of sand by the winds have all modified the glacial Cape and Islands and still alter the landscape of today.
Cape Cod's exposure to erosion is a process that is essential to the Cape's existence. It is a natural process that allows the Cape to adjust to rising sea level.
Task:
Go to the listed coordinates and get some information about the glacial origin of the island. Then answer the following questions:
1a) What are the three main types of glacial landforms you find on Cape Cod still today?
1b) Identify and characterize the appropriate type of this location.
2) Observing the difference in elevation of the area, determine the cardinal direction of the flow of the glacial outwash; for instance, with the help of a map or a GPS receiver.
3) Looking down at the beach, do you think features like wind and waves contribute to the process of coastal erosion or deposition at this point today? Explain your observation using terms of the description above.
Optional) A picture at the platform showing your "Bared and Bended Arm" with GPS receiver in your hand would be nice.
Once you have sent your answers to the e-mail address of our profile, you can log your visit online. We will only contact you if something is missing or incorrect.
Have fun outdoors.
Description is based on the following sources:
[1] Cape Cod and the Islands: The Geologic Story, Robert Oldale, Parnassus Imprints, 2001
[2] Coastal Vulnerability Assessment of Cape Cod National Seashore to Sea-Level Rise, U.S. Geological Survey
