A BRIEF GEOLOGIC HISTORY OF ROCKINGHAM COUNTY
By W. Cullen Sherwood
Dept. of Geology and Environmental Studies
James Madison University
CAVES AND CAVERNS
Of all the aspects of Rockingham County geology, probably none are more fascinating than the caves which abound here. Limestones are unique among rocks in having the ability to form caves which carry underground streams. Even a brief look around the Shenandoah Valley will show the observer that very few surface streams exist. The many streams normally found on the surface in other areas run underground in caves here. It is interesting to take a look back in time and see how these caves formed. Over thousands of years, as rain fell and soaked into the ground, it entered cracks in the underlying limestone bedrock and began to dissolve out small channels in the rock. As these channels grew larger, more water could enter the openings and underground cave systems were formed. Some of our caves have active streams flowing in them, while others have become dry as the groundwater forms more channels and moves deeper underground (see Figure 5).
|Figure 5. Evolution of cave and sinkhole. 1. Rain soaks through soil and starts to dissolve limestone. 2. Small wet cave carries underground stream. 3. Dry cave forms and groundwater forms another wet cave at a lower level. 4. Cave ceiling collapses forming sinkhole.
Dry caves with cave formations such as stalactites, stalagmites and columns are among the most beautiful of geological features. Many of the Shenandoah Valley caves have been commercialized and called caverns, but whether called cave or cavern they were formed by the same process. They draw thousands of visitors each year. Many other "wild" caves of near equal beauty occur throughout the limestone areas of the Valley and are known to but a few people. In addition to these dry caves, thousands of smaller wet caves carry much of the ground water available in Rockingham County. Local well drillers report the intersection of small caves to be a common occurrence when drilling in limestone areas. Many of the wells produce water from caves containing underground streams in the limestone.
Some of the older caves in Rockingham County have literally collapsed due to their size. This has occurred as a cave continues to grow larger and larger through time by dissolving away its limestone walls and ceiling. A stage will then be reached when the thin ceiling rock will no longer be able to support the weight of the overlying soil. The ceiling or roof of the cave then collapses and a sinkhole forms at the earth's surface. These depressions then collect surface water from the rain and snow and funnel it into the cave system. Sinkholes are a common sight in Rockingham County. They immediately notify the observer that a cave system has been developed in that area and continues to act as an underground drainage network. Sinkholes, like caves, only occur in limestone, consequently they are good surface indicators of which areas of the county contain limestone bedrock.*