LANDSCAPE EVOLUTION OF THE SOUTH MOUNTAIN-GREAT VALLEY REGION BETWEEN THE POTOMAC AND SUSQUEHANNA RIVERS

The history of landscape evolution along the South Mountain-Great Valley margin between the Susquehanna and Potomac rivers is geomorphologically complex. Transported alluvial deposits of many different ages interfinger with one another, and with residual deposits, in many ways. Surficial geologic mapping along the South Mountain-Great Valley fringe revealed at least four distinct ages of surficial deposits exist, each with unique geochemical weathering signatures. The youngest pre-Holocene surficial deposits are concentrated near the mountain front as alluvial-colluvial fans across the footslope and on some low level terraces throughout the Great Valley. Intermediate and old surficial deposits spread out further into the valley as dissected remnants of old alluvial fans and higher stream terraces. These weathered deposits underlie younger surficial deposits in some excavations closer to South Mountain. The age and distribution of these alluvial deposits varies between drainage basins (Susquehanna vs. Potomac) and within sub-basins (e.g. Yellow Breeches Creek vs. Conodoguinet Creek); thus, providing information about regional drainage development.

Competition between Susquehanna River and Potomac River for drainage control also plays a role in preservation of multi-aged surficial deposits through controls on accommodation space, drainage divide location and stream channel migration across the valley floor. High level stream terraces with highly rubified soils occur near the drainage divide between Conodoguinet Creek, a tributary to Susquehanna River, and Conococheague Creek, a tributary to Potomac River, possibly recording a significant Late Cenozoic stream piracy event. Highly rubified soils developed within stream terraces are not present elsewhere within the Conococheague Creek Basin, implying southward drainage to Potomac River is relatively recent. Rubified terrace soils are more common throughout both Conodoguinet and Yellow Breeches drainage basins, suggesting flow to Susquehanna River has been a long-term process.