Boulders around Keetmanshoop

**missy111984** · 18th March 2009, 01:36 PM

OK this goes out to geologists and earth scientists out there.. I have been living in keetmans all my life, but there is something that has always facinated me... IN and around the town you can see all these big boulders, 80cm to 1m in length and approx.. 60cm in circumference.. dark brown, stacked on top of each other to form a small hill sometimes as high as 6meters,,how did that come about

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**Galaxy** · 19th March 2009, 03:40 PM

Interesting question! Well, from what I can gather these are apparently dolerite boulders. Dolerite is a fine-grained basaltic rock. According to Wikipedia the "vast areas of mafic volcanism/plutonism associated with the Jurassic breakup of Gondwanaland in the Southern Hemisphere include many large diabase/dolerite sills and dike swarms. These include the Karoo dolerites of South Africa, the Ferrar Dolerites of Antarctica, and the largest of these, indeed the most extensive of all dolerite formations worldwide, are found in Tasmania."

These dolerites formed when sills have formed very characteristic flat-topped koppies. Becuase the dolerite is harder than the surrounding sedimentary rock, it resists weathering, thus forming flat tops on hills. The boulders can also be strewn around on slopes. The boulders tend to be sub-rounded and brown to red-brown from the oxidation of ferro-magnesium minerals.

Hope I'm on the right track here. Any budding geologist will probasbly be able to coonfirm this, or alternatively correct me.

**missy111984** · 19th March 2009, 04:47 PM

thanx. that is some interesting information. this area is such a unique landscape and most of the people never bothered to ask. I noticed in the first picture you posted that the rocks are a bit less rounded then the once i have seen.. where is it from. I noticed you also mentioned they are called koppies because in keetmans we call the koppetjies.

**Galaxy** · 19th March 2009, 04:54 PM

You're welcome. I hope the explanation is right, not being a geologist myself. The picture is apparently from an area closer to the Fish River canyon. Do you have pictures of the boulders you could share here/