The Piedmont landscape of Maryland: A new look at an old problem

Abstract

AbstractThe Piedmont upland of Maryland has been variously interpreted as a peneplain, a series of peneplains, a surface of marine planation, and a landscape in dynamic equilibrium. These different perspectives of landform evolution are related to different scales of time and space. Both equilibrium and episodic erosion features can be recognized in the modern landscape. An equilibrium condition is suggested by adjustment of first and second order streams to rock structure and lithology, entrenchment of some streams against gneiss domes, altitudinal zonation of rock types around gneiss domes, correlation of lithology with overburden thickness on uplands, decreasing overburden thickness on uplands related to decreasing degree of metamorphism of crystalline rocks, and correlation of secondary mineral assemblages with subsurface drainage and slope. The long‐term episodic character of erosion is suggested by clastic wedges on the adjacent Coastal Plain, an upland of low relief that truncates non‐carbonate rocks of different lithologies, isovolumetric chemical weathering of alumino‐silicate rocks, clastic deposition in marble valleys, and weathering profile truncation by modern drainage.The Maryland Piedmont may have been an area of positive relief subject to subaerial erosion since Triassic and possibly Permian time. The upland surface preserved in the eastern Piedmont developed by the Late Cretaceous. In the interval from the Late Cretaceous to the Late Miocene, low input of terrigenous sediments to the Coastal Plain, dominance of marine sedimentation, and spotty evidence of saprolite formation on crystalline rocks, suggest that the Maryland Piedmont was an area of low relief undergoing intense weathering. Incised valleys were formed during a cycle of erosion probably initiated in the Late Miocene and extensive colluvial sediments were deposited on hillslopes by periglacial processes during the Pleistocene.