The Lower Cambrian Antietam Sandstone crops out in southeastern Pennsylvania as many elongate, narrow ridges, commonly surrounded by lowlands thought to consist of shale and/or limestone. Most interpretations for this pattern have relied on complicated faulting to terminate the ridges. Mapping by the writer and his students suggests a simpler explanation: namely, this discontinuous series of sandstone ridges is an exhumed line of barrier islands that stood in front of the Early Cambrian continent. Barrier sands are surrounded by argillaceous units (the Harpers Phyllite) and calcareous marine units (the Vintage Dolomite), standing in front of sandy shoreline deposits (the Chickies Formation). Detailed stratigraphic studies, combined with petrologic analyses of related form tions, grain studies, and comparisons of sedimentary features show the Antietam Sandstone to be very similar to such beaches as Fire Island, New York; Long Beach Island, New Jersey; and Pea Island, North Carolina. Very close similarities were found between the modern setting and the Early Cambrian formations, lending credence to this model for the Antietam Sandstone ridges. This porous sandstone, surrounded by less permeable lithologies, may be a potential reservoir in regions where metamorphism has not precluded the existence of hydrocarbons.

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