Reports
Chemical and physical erosion in the South Mountain Anticlinorium, Maryland
1975, Godfrey, A.E.
Information Circular 19
Abstract
Catoctin and South Mountains, together with the intervening Middletown Valley, constitute the major portion of the Blue Ridge Province in Maryland. They are the two limbs of the South Mountain Anticlinorium and are underlain by the quartzites and phyllites of the Weverton and Loudoun Formations. The Middletown Valley is underlain by metabasalt of the Catoctin Formation and granodiorite gneiss.
The presence of stone streams, boulder deposits and scarps ·suggest that the area has been eroded by periglacial processes that are presently inactive. Boulders were split from cliffs, incorporated into the soil, and moved down slopes and along stream bottoms by solifluction. Fluvial transport was also active but did not extend as far upstream as it does today.
The Middletown Valley was not affected by this periglacial weathering because the deep soils covering 80 % of the Valley probably protected the bedrock from frost action. Also, the area may have been just below timberline.
Under the present climate, frost action is no longer the dominant erosive process. Chemical weathering is preparing the bedrock for fluvial transport. Geochemical data indicate that the Catoctin Metabasalt is weathering three to four times faster than the Weverton and Loudoun Formations.
It appears that the Middletown Valley is presently being lowered faster than the surrounding ridges. Conversely, during the Pleistocene, the ridges were probably being lowered more rapidly than the Valley. When the climate is relatively warm, chemical processes of erosion are dominant, while in colder periods, physical erosion prevails. Thus, it appears that since Pleistocene time changes in climate have modified the type of erosion process and have resulted in an increasing difference in elevation between the Middletown Valley and Catoctin-South Mountains.
