An impact of colossal scale
About 35.5 million years ago, during the late Eocene epoch, a bolide estimated to be 3 to 5 kilometers in diameter struck the shallow sea covering the continental shelf. Traveling at approximately 17.8 kilometers per second, the impactor vaporized on contact, blasting a hole through the water, layers of sediment, and deep into the underlying granite basement rock. The impact fractured the bedrock to a depth of 8 kilometers.
The initial explosion created a transient crater that quickly collapsed. The surrounding sedimentary walls slumped inward, widening the structure to its current diameter of about 85 kilometers and creating a central peak ring. Within minutes to hours, the entire structure was filled with a chaotic mixture of shattered rock, melted material, and tsunami deposits, forming a layer of rubble 1.3 kilometers thick known as the Exmore breccia. The impact also generated a massive tsunami, with waves possibly reaching hundreds of meters high, that washed across the ancient coastline and may have traveled as far inland as the Blue Ridge Mountains. The event produced the North American tektite field, which scattered glassy debris from the impact across a wide area.
A buried structure's modern influence
For millions of years, the crater was buried by new layers of sediment, hiding it completely from view. Its existence was unknown until 1983, when a drilling core taken off the coast of Atlantic City, New Jersey, revealed an 8-inch-thick layer of ejecta containing shocked quartz and tektites. The crater's full extent was finally mapped in 1993 using oil exploration data.
Today, the crater lies 300 to 500 meters below the surface of the southern Chesapeake Bay and the Delmarva Peninsula. It remains one of the best-preserved "wet-target" impact craters in the world. Its structure is studied through seismic reflection profiling and deep scientific drilling projects. A joint U.S. Geological Survey and International Continental Scientific Drilling Program project in 2005-2006 drilled a corehole 1,766 meters deep near the crater's center.
The buried structure affects the modern landscape. The gradual compaction of the breccia inside the crater causes the land above it to subside. This subsidence influences the course of local rivers, causing the James and York Rivers to make sharp, unnatural turns where they cross the crater's outer rim. The impact also shattered the region's aquifers. Below the freshwater aquifers lies a reservoir of ancient, highly saline water, which is roughly 1.5 to 2 times saltier than normal seawater and dates back to the Early Cretaceous period.