ANTIETAM FORMATION

Site Location: 

Dam 3, Mileage: 62.27        Map

     Nearest Access: Lock 34, mileage 61.57

  The Antietam is composed of light gray quartzite layers interbedded with green-gray, sandy metasiltstones that are a gradational continuation of the underlying Harpers Formation.  The uppermost layers of the Antietam are gray sandstones which grade upwards from fine to coarse grained and become somewhat calcareous.                                                                  The Antietam contains numerous Skolithos worm burrows attesting to its marine origin and its biological similarity to the Harpers. The upper level sandstones are generally well sorted and crossbedded attesting to significant water action during deposition.  The calcium carbonate content of the uppermost sandstones preview the next episode in rock formation and a decided change in the depositional environment.  That change is one from detrital deposition to chemical precipitation, beach/shoreline to lagoon/offshore.  The Antietam is separated from an enormous thickness of limestone deposits by a fault called the Keedysville Detachment.  The fault provided a conduit along which fluids rich in iron and manganese came in contact with limestone.  Limonite deposition was one result, creating sufficient quantity to support a thriving iron industry during the middle 1700s and 1800s along the Potomac and Shenandoah Rivers. 

  Antietam time marked a point in global geologic history when life began to explode numerically and generically.  Among the diverse life forms spawned at that time were the trilobites, crayfish-like animals belonging to the Phylum Arthropoda.  They were sea bottom dwelling invertebrates that became extremely important to geologists.  Their abundance, ubiquity, and diversity made them prime fossils for global zoning and time correlation.  Trilobites may be discovered throughout the rock sequence of the Potomac River valley upstream of this site.

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