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12 O' Clock Hole - TMS EarthCache

Hidden : 12/21/2010
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Geocache Description:

Here there is a small rock shelter in the Table Mountain Sandstone (TMS) where you can see evidence of how these rocks were deposited.

Sandstone is a sedimentary rock. This means it was deposited as sediment (dirt), and later, under pressure, turned into rock by a process known as ‘lithification’. These sands were laid down in an alternating pattern that could be the result of either river or beach deposits, possibly both. At this locality, the layers alternate between planar bedding and cross bedding.

Planar bedding typically occurs when sediment is in a low velocity environment. This means that the sand was not moving quickly when it was laid down. This can happen in a slow moving river bottom, a mud flat, or at the bottom of a lake or ocean. This kind of deposition results in mostly flat pancake-like layers, one on top of another.

Figure 1: How planar bedding forms

These layers form natural breakage (or cleavage) planes that the resulting rock will easily fracture along. Often the surfaces exposed reveal the kind of depositional environment that the rock formed under. Two common environments that can be seen in the TMS are desiccation cracks and ripple marks. At Waypoints 1 & 2, a few meters from the shelter, you can see examples of these tell-tale exposed surfaces.

Figure 2: Modern examples of ripple marks and desiccation cracks

Cross-bedding occurs when sand grains move more quickly as they do in waves or wind. This can happen at a beach coast, in dunes, or along the edges of a braided river channel.

Figure 3: How cross-bedding forms

If planar bedding resumes, as it does here, the top of the cross bedding is eroded off and topped by the flat pancake layers again. These alternating environments result in changes in the rock strata that can be seen in their profile. This is the origin of the kind of cross-section you can see at this earthcache locality, and many other places around the mountain.

Figure 4: A cross section showing the alternating patterns of deposition

These rocks do not contain any fossils, and as a result these layers are very clear. However, the lack of fossils make it difficult to interpret the environment these rocks formed in. One clue lies in the colour of the rocks. When sediment is formed at or near the surface, the abundance of oxygen in the air will often oxidize the iron in the sediment, giving it a maroon or reddish colour. Sediment that forms further away from the atmosphere, in a more anaerobic (lacking in oxygen) environment, will typically have a grey/black or greenish colour.


  1. At Waypoints 1 or 2, what kind of surface is preserved?
  2. What kind of depositional environment does this reflect?
  3. Given the colour of the sandstone, what can you say about the environment where these sands were deposited?
  4. Why would the presence of animals, like clams for example, effect the resulting bedding patterns?
  5. Optionally, Take a photo of a feature you find interesting at the site, or of your team at the site and post with your log. If you do photograph either waypoint, please don’t indicate which one!

Sources and further reading:
Compton, J.S. 2004. The Rocks & Mountains of Cape Town. Double Story, Cape Town.
Ripple marks - Wikipedia, the free encyclopedia
Cross-bedding - Wikipedia, the free encyclopedia
Lithification - Wikipedia, the free encyclopedia
Sedimentary Structures

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