Geology of the Cave
Let us look briefly at the geological history of the area and at how the caverns were formed. In a process that took millions of years, enormous thicknesses of sediments consolidated themselves into limestone, shales, and sandstones. These were deposited in horizontal beds in great interior seas that occupied the Appalachian Trough, extending form what is now Canada to the Gulf of Mexico. After the seas receded, this huge mass of sedimentary rocks was folded, fractured and displaced by great thrust faults, producing vertical uplift. During and after this and successive periods of uplift, thousands of feet of the rock were eroded and removed by prolonged activity of rains, surface streams, groundwater, and other geologic agents. In areas underlain by weaker rocks, such as limestone and shale, broad valleys were formed, whereas the more resistant rocks, such as sandstone and quartzite, were left standing as ridges. Thus were the present Alleghany Mountains and the Valley of Virginia formed. The Blue Ridge Range to the east was formed by the same process, but it is older than the Alleghenies, and is composed largely of volcanic-formed, or igneous, rocks.
In the Valley, surface water penetrated the weak, soluble limestone, usually though crevices in the folded and fractured rocks. Solution occurred; underground streams developed and carved out channels of considerable size. When the water table was lowered, the streams disappeared and large underground rooms, or caverns, remained. Thereafter, groundwater seeping though the earth and dripping into the caverns evaporated, forming particles of calcium carbonate. Stalactites, which hang form the ceiling and stalagmites, which grow upward from the floor, are formed in this manner. When a stalactite and stalagmite come together, a column is formed. ¹
In addition to the standard formations including but not limited to stalactites, stalagmites, columns, and flowstone, Grand Caverns is considered unique for having:
- Vertical bedding- the caves of the Shenandoah Valley were formed in mostly horizontal limestone layers. At Grand Caverns the layers of bedding are vertical, not horizontal, turned on end by tectonic or pushing forces
- Cave Shield Formations- Grand Caverns is known for the abundance of shield formations found within, the most in the eastern United States. It is a geological mystery as to how these shields are formed. One theory for the formation of these shields is that water is forced out of a crack in the cave wall. As the water is forced out of these cracks, the calcite crystallizes and a plate begins to grow. There are many other theories regarding the formation of shields.
Example of Vertical Bedding
Example of Shield Formation
¹ UVRPA, “The Story of Cave Hill”, 2nd Edition, 2010
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