Assateague Island is a barrier island in the east coast of the US. The geology here is similar to other barrier islands along the coast.
Formation of the Chesapeake Bay and Delmarva Peninsula
To best understand the geology of Assateague National Seashore, we must first explore its larger surroundings. Assateague Island lies off the western coast of the Delmarva Peninsula, which separates the Chesapeake Bay from the Atlantic Ocean. The Delmarva Peninsula lies within the Atlantic Coastal Plain, which consists of thousands of years of sediment accumulation that has been weathered, eroded and deposited offshore of the continent. The existence of the Delmarva Peninsula in its present location can be attributed to two main factors: a bolide (asteroid) impact some 35 million years ago and thousands of years of changing sea-levels and consequent sedimentation. Thirty-five million years ago, sea levels were much higher, and the coastline of present-day North America was much further to the west (fig. 1). Much like today, a broad continental shelf lay beneath the ocean, extending east toward the edge of the coastal plain (fig 1). It was into this continental shelf that a 2-3-mile-wide bolide (also known as a meteoroid) collided. The bolide created a crater as deep as the Grand Canyon and as wide as Rhode Island. Over the past 35 million years the crater has been filled with thousands of feet of sediment (fig. 2), but the crater created a depression in the landscape toward which rivers converged, which helped determine the location of the present day Chesapeake Bay (https://pubs.usgs.gov/fs/fs49-98/) (https://pubs.er.usgs.gov/publication/7000063). The impact crater was identified by USGS scientists in the late 1990s, and has helped to further understanding of the Virginia Coastal Plain as well as studying other processes related to this ancient event such as land subsidence, river diversion, disruption of coastal aquifers, and ground instability (https://pubs.usgs.gov/fs/fs49-98/). Other processes key to the formation of the Chesapeake Bay, the Delmarva Peninsula and consequently Assateague Island National Seashore are sea-level fluctuations, erosion, and deposition.
For millions of years, rivers cut through the upper Atlantic Coastal Plain carrying loads of sediment from the quartz-rich Appalachian Mountains. As the velocity of the rivers decreased closer to the coast, the sediment settled out of the water column and was deposited. Deposition of sand and other sediments by the rivers flowing into the Chesapeake Bay has been a key force in the creation of the shallow, wide bay we know today. A As glaciers to the north of this area melted around 15,000 years ago, these river valleys were flooded as sea level rose and the Chesapeake Bay as we know it today began (Larson, 1998).
Meanwhile, starting about two million years ago, the Delmarva Peninsula was being formed during periods of higher sea levels. The peninsula, which started as a spit (beach material that projects seaward and is attached to the mainland at one end), directed the flow of the rivers and was key to formation of the bay. For thousands of years, the Delmarva Peninsula has been slowly getting longer. As glacial and interglacial periods occurred over the past two million years, sea levels rose and fell. During times of high sea levels, the longshore current carried sediment towards the south extending the tip of the peninsula (Mixon, 1985). Evidence of these changes in sea level can still be seen today in the swash faces of former shorelines, called scarps. Driving north to south or east to west across the Delmarva Peninsula, one can observe these scarps. Today they are only perceptible by a tiny change in elevation, but they once marked where the land fell into the sea.
Assateague Island, and the rest of the Delmarva Peninsula, continues to be shaped and changed by a number of processes today, particularly by storms and changing sea-levels. Barrier islands like Assateague are strongly affected by seasonal and storm processes. During the winter, strong longshore currents caused by increased wind and wave action move vast quantities of sand from the northern end of Assateague to the southern end. Longshore currents are the result of the energy released parallel to the shore when a wave hits land. Wave action is controlled by sea floor and shoreline features as well as the depth of the water. This movement of sediment has caused Assateague to migrate southward (fig. 3 & fig. 4). This movement is extremely evident in these aerial images taken by NASA. The first image shows Assateague Island in 1985; the second is Assateague Island in 2019 (fig. 3 & fig. 4). As you can see, the island has extended southward significantly. Assateague Island is also moving westward, slowly converging with Chincoteague Island. This east to west movement is mostly a result of storm processes. Storms can have huge impacts on the form and location of Assateague Island. Hurricanes and Nor’easters move sand from the eastward, ocean side of the island across to the western, land-facing side. Evidence of this movement can be seen in overwash fans (figure 5) and sand deposits in the marshy bays on the western side of the island. Eventually, the combined effects of years of storms and consequent sand movement have pushed the barrier islands westward. Assateague has been marching westward toward the mainland for about 2,000 years. Evidence of sand movement, wind and wave action can also be seen in cross-bedding (figures 6 &7). The layers of coarser material or darker, heavier mafic (iron-rich) material were likely deposited during times of increased wind and wave action when stronger winds were able to transport the heavier grains.
Assateague Island and Hurricane Sandy
Assateague was also impacted by Hurricane Sandy in 2012. The southern end of Assateague experienced erosion and loss of sand as the storm pushed sand across the island in overwash deposits and breached the beaches (figs. 8 and 9). These two images show from 2009 and 2012, show how the beach has been pushed westward toward the mainland significantly, and in figure 9 an inlet has formed where the storm breached the narrow coastline.
Following Sandy in 2014 and 2015, the U.S.G.S. collected a wealth of data on sediment sources, transport pathways, and sediment sinks related to the Delmarva Peninsula. These data were collected through geophysical mapping of the inner continental shelf off the coast of the Delmarva. This science will help to predict which areas of the coast are most vulnerable to storm-related forces and will be key to the management of coastal systems by other agencies. The U.S.G.S.'s Coastal Change Hazard Portal predicts how coastlines will be affected by major storm events like Hurricane Sandy. The model uses coastal elevations, wave forecasts, and storm surge projections to determine the probability and extent of shore and dune erosion. Figure 10 shows the archived probability of storm-induced overwash from Hurricane Sandy from the Coastal Change Hazard Portal.