As you continue on the Alum Cave Trail past Arch Rock and Inspiration Point, you will soon see the focus of this EarthCache. So what are you looking at when you arrive at the posted coordinates? Is this a cave or is this a bluff? and how can you tell? So let's figure this out together.
As you left Inspiration Point, the trail continued for about 500 yards before you see the steep staircase built from logs. At the top of the stairs you will notice that the soil changed from a black gritty consistency to a gray powder. This is the entrance point to the Alum Cave Bluff. The bluff is about 90 feet tall and 10 feet deep.
But contrary to its name, Alum Cave is not really a cave. Instead it is a massive ledge of black slate that is jutting out from the face of a high bluff. Iron sulfide can be found in traces of the slate. So as the rainwater trickles in the crevices of the rock, it forms a weak solution of sulfuric acid that breaks down the slate. This decomposition causes the deep layer of dust that covers the floor of the "cave" bluff. There are areas of the site that are more exposed to direct rainfall and runoff and you can easily see the salts.
The bedrock at Alum Cave Bluff is the Anakeesta Formation, which is a dark gray phyllite. It is rich in carbon and pyrite, but doesn't have alum in it.
Side note: Alum Cave was named so because of supposedly encrustations of alum that were found in the walls of the bluff. They thought that alum and saltpeter could be mined from this location. Saltpeter was very important during the Civil War. Geologists have found out that there is actually no alum nor saltpeter in the rocks!
Logging tasks:
1. Post a picture of yourself (face not required) or a personal item at the "Alum Cave" bluff.
2. Explain in your own words how this bluff formed.
3. Compare the soil at the bottom of the stairs to the top of the stairs. What is the same and what is different?
sources: "A Natural History of Mount Le Conte" and "Secondary Sulfate Minerals from Alum Cave Bluff: Microscopy and Microanalysis"