Reports
Equilibrium, cyclicity, and problems of scale - Maryland's Piedmont landscape
1979, Cleaves, E.T. and Costa, J.E.
Information Circular 29
Abstract
The 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. Different perspectives of landform evolution are viewed as having their basis in different scales of time and space. Both equilibrium and episodic erosion features are 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, altitude zonation of rock types around gneiss domes, correlation of lithology with overburden thickness on upland landforms, decreasing overburden thickness on upland landforms related to decreasing degree of metamorphism of crystalline rocks, and correlation of secondary mineral assemblages with internal drainage and slope. A long term episodic character of erosion is suggested by clastic wedges represented on the Coastal Plain, by an upland of low relief that truncates non-carbonate rocks of different lithologies, by isovolumetric chemical weathering of alumino-silicate rocks, by clastic deposition in marble valleys, and by weathering profile truncation by modern drainage.
The Maryland Piedmont may have been an area of positive relief subject to subaerial erosion since Triassic and posssibly since mid-Ordovician. The upland surface preserved in the eastern Piedmont originated as a peneplain that probably evolved by Late Cretaceous. In the interval from Late Cretaceous to Late Miocene, low input of terrigenous sediments to the Coastal Plain, dominance of marine sedimentation, and spotty evidence of saprolite development on crystalline rocks, suggests that the Maryland Piedmont was a terrain of low relief undergoing intense weathering. Incised valleys were formed during a cycle of erosion initiated in Late Miocene (?), and are younger than the upland surface. The upland surface was lowered and modified by periglacial processes during the Pleistocene.
The Piedmont landscape in Maryland is multicycle (it has relic features). Modern landform modification is by fluvial erosion. The geomorphic processes affecting the Piedmont from Late Miocene to Holocene apparently have varied in intensity and dominance.
