Asteroid-Impact Coasts

A highly unusual category of general coastal morphology is created by asteroid impacts at some time in the geologic past. Asteroid impact creates a circular or ovoid crater beneath which is a brecciated zone that extends many thousand meters below the former surface of the Earth’s crust. The eroded relics of these ancient craters have been called “astrob-lemes” (Dietz 1961).

Three coastal areas in North America are believed to owe their morphology, at least in part, to asteroid impact. They are:

• 1. Chesapeake Bay, and adjacent areas of Maryland and Virginia. The coast is characterized by an unusual pattern of a drowned dendritic drainage system, that is to say, organized like the branches of a well-shaped tree. It is fed by the valleys of the Susquehanna, Potomac, and Rappahanock rivers. The asteroid or “bolide” struck 35.2 (+/−0.3) million years ago, in Late Eocene times in soft coastal plain and shelf sediments which to the impacting object, about 3–5 km in diameter and travelling...