Devereau Beach and Marblehead Harbor - Tombolo
When most people spend a day on the beach, they're admiring the beautiful ocean without a thought to how this beautiful landscape formed. In fact, all beaches are constantly undergoing changes! Strong waves erode shorelines every day, changing the shape of the coast. Some beaches even had past glacial activity that sculpted them even more. An example of one of these beaches is in North Shore itself - Devereau Beach.
Devereau Beach is a submerged beach, which means that the sea level rose, covering part of the beach. When the Laurentide Ice Sheet was covering North Shore, a lot of the ocean water was inside the glacier, which meant that the global sea level was lower. When the ice sheet melted, there was a eustatic rise. A eustatic change in sea level refers to sea level change on a global scale; it's a change in the physical volume of liquid water in the ocean. Isostatic change, on the other hand, is on a local scale. During an ice age, the ice adds a lot of weight to the land, which actually causes the land to sink. When the land is sinking, the sea level appears to be rising, even though the volume of water isn't actually changing. When the ice melts, there is no longer a huge source of compression on the land. This causes a process known as glacial rebound - when the land springs back up and rises. When the land rises, the sea level appears to fall.
The images above illustrate what the land does during glacial and interglacial periods. Note that isostatic, not eustatic sea level is depicted in these images.
Since the ice sheet melted, Devereau Beach (and the rest of the area) has been going through a eustatic rise (the frozen water in the glaciers melted, adding more liquid water to the ocean) and an isostatic drop (the land is rebounding up, making the sea level appear to decrease).
One of the features associated with a submerged coast is a tombolo. A tombolo is a sand bar that connects an island to the mainland. In the case of Marblehead harbor, the island is Marblehead Neck and the mainland is Marblehead. Tombolos form through the process of longshore drift. Longshore drift is when material is transferred along the coastline by the longshore current. A spit then forms in the shallow water when there is a change in the direction of the coastline. Deposition occurs, starting with the largest particles, due to the loss of energy (Ismail). The material deposited gets finer as it builds up the spit. If and when the spit builds up long enough to connect to an island, then it becomes a tombolo.
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The partially submerged tombolo in Marblehead Harbor is perfect evidence to show that the sea level has risen over the past 10,000 years due to a eustatic sea level rise.
Click Here for more pictures of Devereau Beach and Marblehead Harbor.