Gravitational sag, or sackungen (Zischinsky, 1969), occurs along the east slope of Short Hill Mountain. A 0.5-km-long by 100-m-wide sackungen of quartzite of the Buzzard Knob Member(-Cwb) of the Weverton Formation occurs west of the intersection of Virginia State Routes 758 and 852 (pl. 1, ref. loc. 19). The gap exposes 10 to 13 m of northwest-dipping right-side-up quartzite on top of southeast-dipping overturned quartzite, which forms the dip slope west of the gap and a continuous dip slope near the contact with the Catoctin Formation. The leading edge of the sackungen can be traced continuously from end to end; the strike of west-dipping upright beds forms an arc above a linear northeast-striking belt of overturned beds on the dip slope.
A spring in the gap at the sackungen discharges more than 200,000 gallons per day for the Brunswick, Md., municipal water supply (Southworth, 1990). A causal relation between ground water impediment by a diabase dike and increased pore pressure along bedding and joint planes may have initiated the sackungen.
A sackungen is interpreted along the sole of a bedding-parallel thrust fault southeast of White Rock. Quartzite of the Buzzard Knob Member of the Weverton Formation is displaced eastward over a continuous strike belt of the Swift Run and Catoctin Formations that forms the hogback ridge. This area is locally known as the "sand pit," and a 61-m-deep water well in quartzite saprolite discharges more than 75 gallons per minute (Brutus Cooper, Loudoun County Department of Environmental Resources, oral commun., 1990).
A rock block slump of the Catoctin Formation occurs at the end of Virginia State Route 852 near the Potomac River. Approximately 25,000 m3 of phyllite and metabasalt moved 20 m down slope. West-dipping slaty cleaved rock is exposed at the toe; detachment was along cleavage parallel to the slope. Hemlock trees growing on the rock block slump suggest that the block has been stable for more than a century.
A small landslide immediately north of U.S. Route 340 on the east side of South Mountain lies at the contact of Catoctin Formation and Weverton Formation. The landslide is composed predominantly of colluvium of quartzite and weathered phyllite and is active during wet periods.
U.S. Geological Survey, U.S. Department of the Interior
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Last modified 08.29.00