Assateague Island Geology: A 5,000-Year-Old Barrier Island on the Move

Coast, dunes and salt marsh. Image from the National Park Service's page

Assateague Island National Seashore lies on the Delmarva Peninsula, in Worcester County, Maryland, and Accomack County, Virginia. Established on September 21, 1965, it covers about 16,732 hectares (41,347 acres) and protects 60 km (37 miles) of sandy beaches, salt marshes, dunes, maritime forest and the surrounding ocean and bay waters. Visitors come for crabbing, surf fishing, cycling, hiking, horseback riding, camping, swimming and wildlife watching.

A very young island

Assateague's geology is Holocene — less than 11,700 years old — made of loose or partly consolidated sand, silt and clay. The island itself formed about 5,000 years ago, which makes it a very young landscape.

Its sediments and landforms record how a barrier island responds to a changing climate: rising seas, shifts in storm frequency and strength, and changes in rainfall.

An island that migrates

As sea level has slowly risen, the island has moved toward the mainland:

1. storms and waves push beach and dune sand westward
2. that sand buries the salt marsh and bay deposits behind it
3. the result is a stack of alternating layers of sand, mud and peat

Waves, wind and tides rework the island continually, and together with its geologic foundation they decide the character and location of its many habitats.

Setting and fossils

Assateague belongs to the Embayed section of the Coastal Plain physiographic province, sharing its geologic history with a region far beyond the park. Any fossils in national parks are protected under the Paleontological Resources Preservation Act of 2009.

Maps and reports

The park's Geologic Resources Inventory report. Image from the National Park Service's page

The NPS Geologic Resources Inventory makes digital geologic maps, reports, posters and scoping summaries for more than 270 natural resource parks. Assateague's are in the NPS DataStore, along with a completed Soil Resources Inventory.

Geomorphic map of Assateague Island. Image from the National Park Service's page

More: the park's pages on natural features and ecosystems and coasts and shorelines.

Sources

Based on "NPS Geodiversity Atlas—Assateague Island National Seashore, Maryland and Virginia," National Park Service, citing Anderson (2017) and Schupp (2013); a work of the United States government in the public domain.