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Thursday, 24 March 2011

Geology

Had to attend a church AGM last night so I could not keep my promise re the Drakensberg geology, but here it is.

 In Pearse's book "Barrier of Spears I found a diagramatic cross cut of the Drakensberg. Starts of with a layer called the Upper Beaufort beds, which reaches up 1250 meters. This layers is said to be 190 million years old. The next layer is called the Molteno Beds, from 1250 meters to 1500meters. This layer is said to be 180 million years old. Then follow the Red Beds, from 1500 meters to to about 1600 meters and is said to be 170 million years old. Following this up to 1750 meters Cave Sandstone, and 160 million years old. These layers are sedimentary rock.

About 150 million years ago a period of intense volcanic activity took place. This volcanic activity threw up immense amounts of basaltic lavas which reach up to about 3300 meters.In its molten state the basalt was full of gas bubbles. As the basalt cooled it left cavities in the basalt. These cavities filled up with minerals over time which then crystallised. A lump of basalt resembles a piece of fruit cake filled with agates, rose-pink amethysts, calcite, chalcedony, quarts and other crystals.

Basalt is soft and crumbly, but fortunately a  at the time of the the volcanic activity successive intrusive flows of hard dolerite was forced between the layers of basaltic lavas. These layers of dolerite form a supportive skeleton. Without them the Berg might have eroded away long before we got to see it. Forces of nature, rain, wind, frost, etc. sculpted the Berg over millions of years to the dramatic mountain we see today.

The Drakensberg is the source of most of South Africa major rivers rivers, both those flowing east or west. Basalt acts like a giant sponge soaking up the more than 2000mm of rain that fall on the Berg in summer, and snow in winter.  

Mont-uax-sources (source of spring) is the source of three major rivers. The Orange River which flows westward to the Atlantic Ocean, a tributary of the Vaal River which flows throw South Africa's industrial heartland land and eventually joins up with the Orange. Then there is the Tugela river which flows eatwards to the Indian Ocean in a much shorter journey of several hundred kilometers.  The Tugela River starts its journey with a dramatic plunge of 800m, 600m of which is a sheer drop. The mouths of the Tugela and the Orange are separated by more than 1500km.

The Drakensberg stretches southward from the Sentinel in the north to its southern most point Xalanga (place of vultures) a towering mass of cliffs in the north eastern part of the Eastern Cape Province. (this are is home to hundreds of kilometers of wild trout streams). From the Sentinel, northwards the Drakensberg  stretches for another 600km. This part is not so dramatic as the southern part, but there are parts in The Mpumalanga Province which is quite as spectacular. (Mpumalanga, Zulu for place where the sun rises. Mpuma rises, Langa the sun).

Hundreds of streams tumble down the face of the Berg, flowing eastward, cold, clear and clean providing a home for both brow and rainbow trout. Because the gradient is so steep, the parts habitable by trout is short, because they soon reach an altitude where they become to warm. But short or not there is more than enough trout water to keep the most avid fly fisher happy for a life time. These streams are all freestone streams.

Sources: Southern Africa: land of beauty and splendour. (1978) by T. V. Bulpin
          
             : Barrier of spears. Drama of the Drakensberg. (1973) by R. O. Pearse




Next blog I'll say something about rock art, Bushmen, fauna and flora.

4 comments:

Desiree said...

This is so interesting! My brain struggles to comprehend timescales of 190 million years...heck, even 2 million has me looking perplexed! The geology sounds truly amazing and I'd no idea the annual rainfall is as high as 2,000mm. No wonder it's the birthplace of 3 of our major rivers.

Thank you for this marvellous tutorial, Phillip!

John said...

An interesting read Phillip. Looking foreward to some pics of these streams.

Gaelyn said...

Interesting and relatively young geology. I showed your nephew John some blue agate in the basalt that he said he'd never noticed before.

Jo said...

Hi Phillip, I love the two books you referenced from. I have Barrier of Spears at my home in Marquard. Have a great weekend. We're off to Nairobi on business on Sunday and return on Tuesday. Love and blessings, Jo