Plate 4: Thickness and Geology of the Potomac Formation—Expanded Explanation City of Alexandria, VA and Vicinity

Introduction The Potomac Formation is a heterogeneous assemblage of ancient river deposits consisting chiefly of unconsolidated and poorly consolidated sands, silts, clays, and small amounts of gravel, along with localized zones of diamictons of uncertain origin. It is the dominant geologic unit in the City of Alexandria. It crops out or is close to the surface everywhere west of Old Town and Del Ray except for a small strip along Holmes Run in the extreme western section of the city, where it has been stripped off by erosion to expose the underlying bedrock. The Potomac Formation also forms the large majority of the Coastal Plain section beneath Old Town and Del Ray, but is covered in that area by thick, younger alluvial deposits of the Potomac River, referred to herein as the “Old Town terrace”.