ABSTRACT
Cave Hill, in the Shenandoah Valley of Augusta County, Virginia, contains a karstic groundwater system developed in intensely folded, interbedded carbonate and clastic units of the Conococheague Limestone (Cambrian). Groundwater appears to have circulated initially through porous clastic beds, coming in contact with soluble, impermeable, microcrystalline limestone.
Paleomagnetism preserved in cave sediments was used to determine the age of a cave. Reversals of the Earth’s magnetic field were preserved in fine-grained material (silt and clay) deposited under low energy conditions. Over 160 sediment samples from caves within Cave Hill were analyzed with a superconducting magnetometer. Samples were sequentially degaussed to reveal natural remanent magnetization. The resultant paleomagnetic vectors were statistically tested and corresponding virtual geomagnetic poles were correlated with known reversals in the Earth’s magnetic field. Magnetostratigraphic analysis suggests a minimum age for speleogenesis of Grand Caverns at 0.7-1.1 Ma. Based on elevations of cave passages above the present local baselevel (obtained by precise leveling surveys), the average rate of erosion for the Shenandoah region over this interval was 23–41 m/Ma. This agrees with published rates for other localities in the humid-temperate eastern U.S.
These results, together with earlier detailed geologic mapping and petrographic, lithologic, and hydrologic investigations of this study, suggest a chronology for development of the cave system. Cave origin was strongly controlled by bedrock composition, attitude of bedding, and joints and faults. Elevations of strike-oriented passages correlate well with former local baselevels; however, deep artesian groundwater flow may have been crucial to speleogenesis at Cave Hill. This is supported by the magnetostratigraphy and various geomorphic features, including lift tubes with dissolution scallops as paleoflow indicators, ceiling pockets suggesting mixing of waters of differing chemical composition, present-day phreatic passages well below the level of the nearby South River, and positions of former groundwater outlets of the major caves.
