Arundel Formation, Geological formation in Maryland and Washington, D.C., United States
The Arundel Formation is a clay-rich rock layer found beneath Maryland and Washington, D.C., containing iron-bearing minerals and organic material. It dates to the early Cretaceous and is known for yielding thousands of bone fragments from dinosaurs and other animals that lived in this region.
William Bullock Clark named this formation in 1897, drawing from Anne Arundel County in Maryland where it was first formally described. Later excavations, especially at the Muirkirk site, confirmed it as one of the most productive sources of early Cretaceous fossils on the East Coast.
Fossils from this formation are displayed in several regional museums, including the Smithsonian in Washington, D.C., where visitors can see actual specimens recovered from the area. The finds have shaped how scientists and the public understand what North America looked like during the early Cretaceous.
The most accessible site is in Muirkirk, Maryland, where iron mining once exposed the formation. Those who want to see specimens without visiting the site itself can find fossils on display at the Smithsonian National Museum of Natural History in Washington, D.C.
The formation contains petrified tree trunks found still standing upright, with root systems intact in place. This tells us the trees were buried so quickly by sediment that they had no time to fall or decay before being sealed in rock.
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