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Swakop River Whale-backs

February 9, 2018

 

Many of Namibia’s westward flowing ephemeral rivers have cut deep valleys into the landscape in search of equilibrium, which is usually the sea.  We use many of these river valleys on our riding safaris and each has a particular geological feature:   in Damaraland the Ugab with its “squeezed toothpaste-like folds; the giant erosional feature that is the Fish River Canyon (Desert Canyons Safari); the uplifted schist layering of the Kuiseb Canyon and the Swakop River with its dolerite dykes (whale-backs), both on the Namib Desert Safari.

 

Image right is the Kuiseb Canyon, Namib Desert Safari 

 

 

 

 

 

The rivers that formed this landscape are no longer active enough to dramatically change the landscape.  During ice ages sea levels drop as water is frozen thus creating a steeper gradient for rivers resulting in an increase in their erosional power.  When sea levels rise, rivers can stop flowing altogether forming huge sedimentary deposits as it evidenced by the Home silts in the lower reaches of the Kuiseb River.

 

 

Image left, Fish River Canyon, Desert Canyons Safari

 

Image below, Swakop river whale-backs (Dolerite Dykes)

 

 

 

Image left, Namib Desert Safari, Swakop River.

 

The Swakop River’s whale-backs are magmatic dykes formed from molten volcanic intrusions of dolerite into the surrounding softer granite.  The volcanic activity that formed these dykes possibly occurred some 500 million years ago when south America and southern Africa collided to form the super continent of Gondwana. 

 

 

 

Wind, water, chemical exfoliation and extremes of temperature have eroded the softer granite and other substrate while the more resistant dolerite remains as a curious geological feature many millions of years old

 

When these ephemeral rivers do flow, their power can even overwhelm the sea, pushing the shoreline out to sea and depositing huge amounts of silt into the ocean.  Image right shows the mouth of the Swakop River in the 2010/2011 flood....imagine trying to ride down the river in that!

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