This is a quick Earthcache which can be found just south of Bedford, on the Grahamstown road. It is a small quarry, visible from the road. You do *not* need to enter the property to see what is needed to answer the questions for this cache.
Background
Dip and dip direction
When describing rocks that have been folded or tilted, there are two very important things used to describe them: dip and dip direction.
Dip is the angle from horizontal that a dipping bed makes. It is measured in degrees using a clinometer from 0° being flat, to 90° being vertical.
Dip direction is the direction measured from north at which the maximum dip is measure.
Together, these are reported in a variety of ways, but the easiest way is as follows: a bed dipping 25° to the north-east can be recorded as 25/045 N. 75° dip to the west is 75/270 W. (The direction is usually added as a check that the direction is correct.)
Apparent and True dips
When looking at layers of rock from the side, it is possible that you are able to see the real (true) dip. However, it is more likely that the dip is only an apparent dip.
The true dip can only be recorded along the profile with maximum dip, while the apparent dip will be some other angle. The following sketch should hopefully make things clearer. Here, the blue is the real angle of the orange prism, while the red is an apparent one.
If one looks at dipping beds in the right way, then the apparent dip can be zero, making it look like you have completely flat beds, as is the case in places in the following sketch:
What you can see
The quarry at the location has two walls well-exposed. They have wildly differing dips, because one is apparent and one is true (or at least truer). So, although it looks like the rocks are dipping differently, they really are not.
The quarry is located in the Daggaboersnek Member of the Balfour Formation. This is a late Permian (about 255 million year old) river system which is part of the lower Beaufort Group of the Karoo Supergroup. It comprises mostly mud and siltstone, with some tabular sandstone bodies in places.
To log this cache:
Please send a message with the answers to the following questions. Logs without answers will be deleted.
- Estimate the difference in dip between the north and east walls of the quarry.
- Which wall is closer to the true dip?
- In which direction (N, NE, E, SE, S, SW, W, NW) are the beds in this area dipping?
- What large man-made structures are visible to the west?
References
Oghenekome, ME (2012) Sedimentary environments and provenance of the Balfour Formation (Beaufort Group) in the area between Bedford and Adelaide, Eastern Cape Province, South Africa. MSc Thesis, University of Fort Hare. url: http://contentpro.seals.ac.za/iii/cpro/app?id=6228073686345914&itemId=1004354&lang=eng&service=blob&suite=def