[The photo is from the exposed part of the bornhardt just above Eagle Rock. That *is* me -- OzGuff -- standing atop the tor.]
A 'bornhardt' is a type of inselberg, that is a remnant of a previous landscape near the end of an erosional cycle in an arid region. An 'inselberg' gets this special name if it is dome-shaped. They are named after German geologist Bergassor Bornhardt, who made a study of the geology of southern Africa.
Unlike inselbergs which develop in faulted sedimentary rocks, bornhardts are granite or gneiss domes which are particularly resistant to weathering. Bornhardts often form below ground, as chemical weathering and freeze/thaw weathering work their way into joints in granite covered by regolith. Sometimes, there is a particularly large resistant block, perhaps a particlularly intact unit of a batholith. The other blocks weather more quickly because the joints are closer together. Eventually, the regolith erodes to the point where the bornhardt is exposed. Some bornhardts will have a rocky pediment around the base, while others will stick straight up from the regolith. If the block weathers into smaller blocks before it is exposed, a pile of hill-sized boulders called a 'tor' will appear instead.
Top: Before
Bottom: After
Once exposed, the bornhardt is exposed to further weathering processes: water and wind erosion, exfoliation, further jointing assisted by salt crystal growth and frost cycles. Around the base of the bornhardt will be a talus slope of its own debris. Eventually the bornhardt will be reduced to a small isolated rocky hill called a 'koppie' or 'kopje'.
Examples of bornhardts include Stone Mountain in Georgia, Looking Glass Rock in North Carolina, Castle Rocks and the City of Rocks in southwestern Idaho, Spitzkoppe in Namibia, Domboshawa in Zimbabwe, and Poondana/Brazil Rock in South Australia (as well as many others in south Western Australia). By stretching the definition, we can include Uluru (Ayers Rock) as a bornhardt, although it's made of conglomerate.
Not far from Eagle Rock is an example of a 'tor', which is a large, free-standing residual mass (rock outcrop) that rises abruptly from the surrounding smooth and gentle slopes of a rounded hill summit or ridge crest. So the tor sits on the bornhardt. And at the top of this tor is a rock basin, somewhat similar to swirl holes one sees at the edges of rivers. But it has been a long time since a river flowed through this area at this elevation. The rock basins are formed over thousands of years by water freezing, with the ice breaking off grains of the granite as it expands.
In order to get to the Eagle Rock area, park at the end of Shumont Road, which is off of NC 9. Please do NOT block any driveways. Go through the gate with the negative signage, and stay to the right. [The negative signage is there to dissuade you from parking inside the gate. Which you should NOT do.] Follow that road to the next gate, go around it, and follow the "road" all the way to Eagle Rock.
At the listed coordinates you should be at the top of the tor. Be careful as you climb up and be careful as you look around. You should find a discolored depression -- the rock basin -- at the listed coordinates. Take note of this, as you will have to answer a question about it. Also take note of your elevation. To your east you should see another rocky bald area -- the obvious exposed portion of the bornhardt and Eagle Rock. Estimate the distance from the tor to the bornhardt. Climb down from the tor and make your way to the bornhardt. Use the strong ropes to get yourself up. Explore the bornhardt, taking note of the two heart-shaped discolored depressions. Take note of your elevation. Be careful while on the bornhardt. Be careful while getting yourself down from the bornhardt.
In order to log this earthcache as a find, email to me the answers to the following questions:
1. What is the approximate elevation of the tor?
2. What is the circumference of the rock basin found atop the tor?
3. What is the approximate distance between the tor and the obvious exposed section of the bornhardt to the east?
4. What caused the two heart-shaped discolored depressions on the bornhardt?
5. What is the approximate elevation of the the bornhardt?
6. Investigate the base of the tor. What evidence is there, if any, to suggest that the tor is getting taller due to erosion of the ridge crest floor or if the tor is getting shorter due to erosion from its surface? Explain. Please try to stay on the uphill side of the tor, as the downhill side is very down the hill.
And though it is not a requirement, any photos of you and/or your group at/on/near/under the tor and the bornhardt will be much appreciated! Enjoy your time in the Eagle Rock area. And please be careful!
[The photo of Eagle Rock was taken from the tor looking back towards the bornhardt. Thanks to Rabid Chipmunks for the photo.]
FTF HONORS GO TO sawmiller and jonesdt!!!
