A Tombolo of Our Own

Massachusetts Sea Level Curve from USGS. Click image to go to source.

Last night’s public lecture was a big success with a full house of students and visitors alike. Much of the talk by Dr. Rosen was focused on the dramatic changes in landscape since the Laurentide Ice Sheet receded from the area about 13-14 thousand years ago. The Boston Harbor Islands as well as many of the hills in the area are the result of drumlins, produced as the ice sheet formed and revealed only after the glacier had melted. Due to the weight of the ice, the relative sea level just as the glacier was melting was actually about 30 meters higher than it is today – but over about 2000 years, the land rebounded, and relative sea level dropped to about 40 meters below today’s sea level. As the land gradually settles back down, relative sea level has risen again. With global climate change and rising sea levels, it is interesting to consider the historical processes that have determined what our coastline looks like today, and how they continue to affect what it will look like in the future.

How the Nahant tombolo was formed by transport of sediments by currents on either side of the original island.

In the lecture, one of the curiosities of my daily commute was also answered – that is: how the Nahant causeway was formed. It is a naturally formed feature, called a tombolo. Although it has been stabilized so that we can generally get on and off the island without incident – it’s position was determined by currents carrying sediments around the island and depositing them on the downstream side of Nahant. Longshore drift/transport took care of the rest.

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