SURFICIAL GEOLOGY, GEOMORPHOLOGY AND PALEOCLIMATE IMPLICATIONS OF COLLUVIAL DIAMICTONS AND FEATURES ALONG THE SOUTH MOUNTAIN-GREAT VALLEY MARGIN, MARYLAND

A variety of paleoclimatologic and geomorphic evidence suggests the presence of a paleoperiglacial environment with tundra/permafrost in the unglaciated Central Appalachians during Late Wisconsin glaciation. High-resolution light detection and ranging digital terrain models (LiDAR-DTMs) coupled with field-based surficial geologic mapping have characterized a variety of gravelly through finer-textured matrix- and clast-supported colluvial diamictons and other features interpreted to be of paleoperiglacial origin along the South Mountain-Great Valley margin of Maryland. LiDAR-DTMs clearly link lobate or curvilinear topography to field-mapped colluvial diamictons, which are interpreted to represent Late Wisconsin paleoperiglacial solifluction lobes that were activated during thaw and flowed down slope. Topographic signatures of solifluction lobes are best expressed downslope of the Weverton Formation, and to a lesser degree downslope of Antietam Quartzite in the northern portion of the study area. On lower hillslopes and footslopes, the surficial diamictons stratigraphically overlie older, highly weathered diamictons of colluvial and alluvial origin, which in many places are subsiding into subsurface karst accommodation space.

Although the solifluction lobes are not directly indicative of permafrost, the presence of block fields, tors and sorted nets on South Mountain, and thermal contraction polygons within the adjacent Great Valley more convincingly support the presence of Late Pleistocene permafrost. This study contributes yet another piece of supporting evidence for the long-standing notion that paleoperiglacial climates and processes have heavily impacted the Late Pleistocene geomorphic evolution of the South Mountain-Great Valley margin in the unglaciated Central Appalachians.