| I have earned GSA's highest level: |
 |
An Earth cache is a special type of Virtual Cache that is meant to be educational. Therefore to log a find you must demonstrate that you have learnt something from the site and experience.
Send your answers to us in an email via our profile page.
Any logs not accompanied by an email will be deleted.
Logging Tasks:
The listed co-ordinates will take you to a point where you will have a good view of the vertical cliff face across the river.
- Describe the colour of the rocks forming the cliff face and are there any layers visible.
- If layering is present what is the layering orientation.
At waypoint 2 S33 42.510 E24 36.956 you will also have a good view of a striking rock formation across the river.
- Describe this rock formation in terms of layering.
- What is the orientation of these layers.
- Why do you think the orientation of the two cliff faces is so different.
- How do you think this narrow kloof was formed between the mountains.
River Evolution
The Kouga and Groot Rivers which both joined the Baviaanskloof River downstream (at some stage in their history) becomes the Gamtoos River, which is a major Eastern Cape River. This is a most curious element of river evoluton: river capture or river piracy. It is an aspect of geomorphology unusually well illustrated in this part of the world.
We know that rivers grow by headwater advancement. Starting as a trickle in a barely perceptible valley on a hill slope, the stream deepens the valley as it grows, cutting back endlessly, either from the coast or from a bigger river to which it is a tributary. In time it divides the hilly massif in two and keeps going.
Assuming a river has cut through soft rocks to start with, it not only cuts back, but it cuts down too. Even if it encounters harder rock as it erodes back, it strives to maintain the base level at which it has started. Any zones of weakness, such as faults or close-spaced joints in the hard rock, will be exploited in the quest to maintain the starting base level, even if it constricts valleys into narrow kloofs or gorges.
Let’s assume it has split the hilly block or ridge and emerged into lower ground on the side, a valley formed by a contemporaneous or old river. This other river, perhaps having started at a slightly higher level or flowed over harder formations, is not as deep. The river we’re following meets up with it. Being deeper it ‘captures’ the headwaters of the new river, channeling them into its own course and making the term ‘piracy’ entirely appropriate.
In the southern part of the Eastern Cape, the main wide valleys are defined by the east-west grain of the fold belt and parallel rift faulting. We should not forget the important role played by the alternating hard and soft component formations: the Table Mountain Group being mainly extremely resistant to erosion; the Bokkeveld Group easily eroded and the Witteberg Group moderately resistant. Roughly perpendicular to the main tectonic fabric are tight gorges formed along secondary fault zones and joint clusters. The map below shows how river capture has drastically re-landscaped the Eastern Cape.
As you are about to emerge from the Baviaanskloof, it’s interesting to note how the Kouga joined the Baviaanskloof River (the’pirate’) at some stage in its history, having previously entered the sea just north of Cape St Francis as part of the Krom River system. Soon after leaving the Baviaanskloof Reserve you encounter the Groot River. This, too, has been captured by the ‘proto-Baviaanskloof River’, previously having been a main part of the Sundays River system, which flows into Algoa Bay far to the east.
Geology
Over the last 450 million years, the land was further buried, deformed, uplifted, faulted and eroded. The Cape Fold Belt Mountains were pushed up about 270 to 230 million years ago. The Gondwana supercontinent disintegrated about 170 million years ago, and continents drifted slowly apart to their present configuration. Down faulted basins rapidly filled with sediment.
Folding
Sedimentary layers are pushed upwards by forces within the earth’s crust forming great mountain ranges. Sediments are eroded from the continents by wind, ice and running water and deposited on the ocean floor. They are usually deposited in layers, with each new layer burying the previous one. Sometimes a layer of mud will bury a layer of sand, or the other way round. We can see these different layers in the rock record. As the sediments build up there is enormous pressure on the underlying layers. Water is squeezed out, and certain chemicals in the sediment may act as a kind of cement. Gradually, over millions of years, the sediment turns into solid rock.
While sediments are building up on the ocean floor, the enormous forces, which are at work in the earth’s crust, may act to bend or fold the layers. The folded sediments will, in time be lifted up by the forces to form vast new mountain ranges. The mountains of the western and southern Cape were formed in this way. This is why they are called the Cape Fold Mountains.
Faulting
Forces within the earth can cause the crust to ‘break up’ along a fault or weakness lines so that mountains can be formed. The forces responsible for the folding of the Cape Fold Mountains are linked to the early stages of the break-up of Gondwana. The folding happened before the actual break-up.
Rocks are softened by weathering, and then eroded. The result is the rugged mountain peaks, steep cliffs and deep valleys which are common to the Cape Fold Mountains.
The mountains, although mediocre in height by world standards, remain extremely majestic and dramatic to the eye. This is due in part to numerous geological factors; the ranges usually have few to no foothills and rise directly from the valley floor.
The mountains are not particularly ancient, despite their old-looking appearance. They are considered middle-aged in geologic terms.
Acknowledgements
Geology off the beaten track Nick Norman.