Dwyka Group - ice sheets cover Gondwana
 

The Dwyka Group forms the lowermost and oldest deposit in the Karoo Supergroup basin. This basin extended across much of southern Gondwana and records 120 million years of geological history.

The rocks overlying the Natal Group is a thick unit of tillite that was deposited in a glacial environment by retreating ice sheets about 300 million years ago.

 

Ice cap covering Gondwana 300 million years ago

At this time South Africa was part of the supercontinent Gondwana, which was situated near the south pole and covered with ice. Rocks imbedded in the slowly moving ice sheets scoured and polished the underlying older rocks giving rise to glacial pavements. Striation directions indicate that ice flow was from north to south - valuable information when it comes to reconstructing Gondwana. The photo on the left shows a striation on the rock surface caused by the movement of ice and boulders about 300 million years ago.

Tillite is mostly a very fine-grained, blue-grey rock comprised of clay matrix with inclusions (or clasts) of many other fragments picked up by glaciers during their travels. The tillite in KZN often weathers to a characteristic yellowish colour. In the Durban area, cliffs near the mouth of the Umgeni River, and quarries in the Westville area offer the best exposures of tillite.

Striation
Dwyka Tillite

Striation - Loc.: University of KwaZulu-Natal, Westville Campus


A typical example of Dwyka tillite

Glacial Deposit

Picture adapted from: The story of Earth & Life (T. McCarthy & B. Rubidge)

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