Formation of the Drakensberg
The base of the Drakensberg Mountains had originally been part of the supercontinent Gondwanaland approximately 510 to 180 million years ago , This supercontinent included Africa, Australia, New Zealand, South America and Antarctica. The Drakensberg was originally part of a shallow depression fed by inland waterways. This formed an enormous inland lake. The sediments carried into this lake were generally deposited upon granite foundations which had formed almost 3 billion years ago. Today in areas such as Kloof Gorge we can still see small portions of these granite deposits.
Some 160 million years ago during the early Jurassic period, enormous internal pressures contributed to the cracking apart, breaking up and drifting away of the continents which once constituted Gondwanaland. This movement also resulted in enormous cracks in the crust of the African continent and it was through a series of these cracks or fissures that the massive basaltic lava flows which were to create the Drakensberg began. This was volcanic activity; however it was not an explosive eruption. It was rather, the thick lavas flowed and cooled, flowed and cooled, adding from 30 cms to 50 meters of lava at a time from the fissures which occurred roughly where Lesotho is today.
Throughout 20 million years these flows built up a deposit of basaltic rock over 1,5 kilometers thick in some places and covered an area extending from inland of present day Lesotho over most of KwaZulu-Natal as far as Mozambique and the Indian Ocean. The basaltic lavas were far from solid, being riddles with holes from trapped gas bubbles and tubes where the hot bubbles escaped upwards. Later these spaces were filled with white zeolite minerals, resulting in the white inclusions called amygdales, some of which are the beautiful blue, grey and white agates, chalcedony and quartz crystals. At the same time as the fissure flows were adding a massive cap over the softer sandstones and shales, other flows which could not reach the surface were being forced into underground cracks and crevices. Unlike the brittle basalts of the cap layers these underground lavas formed hard blue dolerite dykes and sills throughout the area. The resistant horizontal sills resulted in time if the formation of land forms, usually waterfalls.
Once the massive flows of basalt lavas had cooled they were immediately exposed to the erosive forces of wind, rain, ice, lightning, heat and drought. Where the elements broke through the basalt cap into softer layers, erosion was faster and caused steep sided valleys with churning rivers tumbling seawards. Through this process known as mechanical weathering the berg was formed. Mechanical weathering is the disintegration of rocks due to pressure. These pressures can be exerted through various channels, such as Exfoliation, Abrasion, Freezing and Thawing, Thermal Weathering and finally Salt Wedges.
The erosion backwards to the present-day escarpment was a relatively rapid process - about 200 kms in 140 million years, or an average of 1,42 mm per year. Even so the Drakensberg lavas are resistant to erosion and form the high peaks of the escarpment as well as the protective capping of the little Berg.
In order to log this cache the following questions must be answered
At waypoint 1: Look at the rock formation closest to you.
- Describe its coloring.
- Why do you think the colours are like this?
- Describe its shape
- Why is it shaped like this
At the final waypoint.
- Describe the size of the cave.
- How do you think the cave was formed?
Do not wait for my confirmation before you log the cache I will contact you if there are any problems.
References:
http://www.southafrica-travel.net/kwazulu/e2Drak1.htm
http://www.zulu.org.za/index.php?districthome+22++56948
http://www.zulu.org.za/index.php?districthome+22
http://www.buzzle.com/articles/mechanical-weathering.html
http://en.wikipedia.org/wiki/Gondwanaland