Evidence of sand movement, wind and wave action can also be seen in cross-bedding. Assateague has been marching westward toward the mainland for about 2,000 years. Eventually, the combined effects of years of storms and consequent sand movement have pushed the barrier islands westward. Evidence of this movement can be seen in overwash fans and sand deposits in the marshy bays on the western side of the island. Hurricanes and Nor’easters move sand from the eastward, ocean side of the island across to the western, land-facing side. Storms can have huge impacts on the form and location of Assateague Island. This east to west movement is mostly a result of storm processes. Assateague Island is also moving westward, slowly converging with Chincoteague Island. As you can see, the island has extended southward significantly. The first image shows Assateague Island in 1985 the second is Assateague Island in 2019. This movement is extremely evident in these aerial images taken by NASA. This movement of sediment has caused Assateague to migrate southward. Wave action is controlled by sea floor and shoreline features as well as the depth of the water. Longshore currents are the result of the energy released parallel to the shore when a wave hits land. 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. Barrier islands like Assateague are strongly affected by seasonal and storm processes. Today, they are only perceptible by a tiny change in elevation, but they once marked where the land fell into the sea.Īssateague 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. Driving north to south or east to west across the Delmarva Peninsula, one can observe these scarps. During times of high sea levels, the longshore current carried sediment towards the south extending the tip of the peninsula. Evidence of these changes in sea level can still be seen today in the swash faces of former shorelines, called scarps. As glacial and interglacial periods occurred over the past two million years, sea levels rose and fell. For thousands of years, the Delmarva Peninsula has been slowly getting longer. 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. Meanwhile, starting about two million years ago, the Delmarva Peninsula was being formed during periods of higher sea levels. The geologic features of the impact crater as well as the layers of sediments that now overlie the crater. 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. 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. Over the past 35 million years the crater has been filled with thousands of feet of sediment, but the crater created a depression in the landscape toward which rivers converged, which helped determine the location of the present day Chesapeake Bay. The bolide created a crater as deep as the Grand Canyon and as wide as Rhode Island. It was into this continental shelf that a 2-3-mile-wide bolide (also known as a meteoroid) collided. Much like today, a broad continental shelf lay beneath the ocean, extending east toward the edge of the coastal plain. Thirty-five million years ago, sea levels were much higher, and the coastline of present-day North America was much further to the west. 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. 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. Assateague Island lies off the western coast of the Delmarva Peninsula, which separates the Chesapeake Bay from the Atlantic Ocean. To best understand the geology of Assateague National Seashore, we must first explore its larger surroundings. Formation of the Chesapeake Bay and Delmarva Peninsula
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