Drowned Drumlins, Battered Bluffs, and Salt Marsh Sediments - Boston Harbor Through Time
Delivered at the 2011 Boston Harbor Islands Science Symposium
Boston Harbor Island shorelines evolved from the flooding and gradual reworking of a drumlin field originally deposited from the Laurentide Ice Sheet. Erosion of the drumlins by storm waves and the formation of steep bluffs is a common process seen in the harbor. Monitoring and sampling of these sedimentary deposits has helped determine that retreat of drumlin bluffs is controlled by toe erosion from storm waves, leading to oversteepening. It has been found that mass wasting processes correspond to bluff height: higher bluffs (>10m) are dominated by slope wash with rills and gulleys, while slumping dominates lower height bluffs. This is controlled by volume of sediment transported down the bluff. Lower bluffs deliver less material, so the base is more frequently oversteepened by storms, leading to slumping. Higher bluffs deliver sufficient quantities of material to the base to preclude slumping for decades, so gulleying becomes the dominant slope erosion process. As bluffs retreat, boulder-sized material remains on the retreating shore forming a boulder terrace. Terraces form a pavement protecting the underlying till and can extend offshore to the limits of the original outline of the drumlin landform.
Accretionary landforms commonly found in Boston Harbor include tombolos, salients, and saltmarshes. Tombolos are sedimentary deposits that link adjacent drumlins. This linkage appears to be controlled by the underlying geology rather than the wave regime: tombolos tend to form where there is an existing bathymetric high which serves as a platform for the accretionary landform. Most often, the bathymetric high consists of a broad boulder terrace and underlying bedrock. Salients are cuspate sedimentary features created by bidirectional longshore processes. They form on the leeward facing sides of the islands and are composed of prograding sand and gravel ridges. Embayments along the shoreline form both when glacial depressions are drowned, and when barrier beaches create lagoons. Fine-grained sediment can collect in these protected areas and eventually saltmarsh plants then take root. There are several small, protected saltmarshes that thrive in Boston Harbor, as well as fringing saltmarshes that survive along wave-protected shorelines.
A recent study on the geomorphic evolution of these islands yields interesting results. Extensive mapping and sampling of the sedimentary deposits has been accomplished through high-resolution GPS, a series of auger and vibracores, and a ground-penetrating radar (GPR) survey of the upland regions. Special attention has also been paid to development and evolution of Boston Harbor's fragile saltmarsh systems and wave-cut bluffs, and their sustainability with modern-day sea-level rise. Only by understanding Boston Harbor's elaborate geomorphic history can we begin to shed light on its future.