Grand Caverns is one of thousands of discovered caves in Virginia's Shenandoah Valley. This is due to the karst topography of the region.
Karst topography is the result of mildly acidic groundwater acting on soluble bedrock -- here, limestone, or calcium carbonate. As rain falls through the atmosphere, carbon dioxide dissolves into the water, forming a weak carbonic acid solution. The water percolates through the soil, and when it reaches the limestone bedrock, the rock dissolves, creating a solution in the water of calcium carbonate and potentially other trace minerals. If the rock is unbroken, the surface dissolves and may form a sinkhole over time. If there are cracks or fractures in the bedrock, the water will seep into and widen the cracks. Over time, the cracks get larger, and eventually an underground drainage system forms, leading to cave formation.
As a cave grows larger, the water level may drop and the cave may begin to fill with air. As groundwater seeps into the cave and comes into contact with the air, some of the carbon dioxide may discharge, causing the water to be supersaturated with the minerals that dissolved into it. As calcium carbonate and other minerals precipitate, they form the speleothems (cave formations) that make limestone caves so interesting to explore: stalactites, stalagmites, shields, flowstone, cave coral, columns, drapery, straws, and so on.
It is difficult to know the rate at which cave formations form and grow, as it can vary by water flow, relative acidity of the water, water and air temperature, and other factors. Several sources we consulted on the internet indicated that stalagmites can have an average growth rate of 0.13 mm per year but can be as much as 3 mm per year.
Without a consensus rate, or specific measurement of a particular formation, it is impossible to have a good estimate as to how quickly or slowly a cave formation forms. It is, however, easy to see when cave formations stop growing, as some have here in Grand Caverns due to human intervention.
Grand Caverns is relatively unique among the other caves in the Shenandoah Valley. Limestone is formed from sediment, so it naturally forms in flat layers over time. Here, however, tectonic forces pushed the limestone bedrock, so that the layers are nearly vertical. This may be the reason why Grand Caverns has an unusually high number of shield formations, flat disks of rock hanging from the cave wall or ceiling.
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.
Grand Caverns may be the oldest show cave in the United States. It was discovered by 18-year-old Bernard Weyer, who was looking for one of his traps. The cave was opened for tours in 1806. There are actually three separate caves here in Cave Hill, but only Grand Caverns, the largest, is open to the public.
Although the cave is now protected, earlier visitors carved or signed their names on cave formations. During the Civil War, the cave was visited by troops on both sides of the conflict. One clearly visible signature was left on September 26, 1864, by Captain W.W. Miles, 14th Pennsylvania Cavalry. According to our tour guide, he was killed in action approximately a month later.
LOGGING THIS EARTHCACHE
You will need to take the tour to enter the cave. Before or after the tour, there is also an exhibit near the cave entrance that is worth visiting.
To log this earthcache, email us or send us a message and copy and paste these questions, along with your answers. Please do not post the answers in your log, even if encrypted. There's no need to wait for confirmation from us before you log, but we will email you back if you include your email address in the message. Group answers are fine; just let us know who was with you.
1. The name of this earthcache: Grand Caverns Earthcache.
2. Describe the shield formations.
a. How do they compare to stalactites?
b. Why do you think shields formed here instead of stalactites?
3. The tour will take you by a formation called George Washington's Ghost:
a. What type of cave formation is "George Washington's Ghost?" Describe its appearance.
b. Is the formation still growing? If so, how fast? If not, why not?
Photos of your visit aren't required, but they are always appreciated!
Sources:
Wikipedia
Grand Caverns homepage
Show Caves of the United States
The Virtual Cave
This earthcache was placed with the permission of the City of Grottoes.