During the period from about 320 to 270 million years ago, the Gondwana landmass was partly covered by ice as the supercontinent migrated over the South Pole. The extent of the vast ice is known from the presence of widespread deposits of similar-aged glacial sediment. Known as the Dwyka tillite in South Africa, this sediment was derived from melting glaciers and ice sheets. It was deposited into a large inland sea, forming the unusual tillite we see today at the base of the Karoo sequence.
The moving glacier carried a mixture of rock debris from boulders to fine clays. Movements within the ice caused some of these to reach the sole (underside) of the glacier. As the ice was grinding over the rocks below, “rock flour” or till was produced. The weight of the ice crushed them down and the clays help to stick the till to the underlying rock. The melting ice sheet and glaciers thus left behind large piles of till which was later compressed into tillite.
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. Very characteristic of tillite rocks is the way they weather into vertical teeth-like structures, sometimes referred to as "tillite daggers", commonly found in the rock ridges and outcrops around Matjiesfontein which is what this EarthCache is about.
Matching beds of ancient tillites on opposite sides of the south Atlantic Ocean provided early evidence for continental drift. The same tillites also provided the key evidence for the Precambrian Snowball Earth glaciation event.
To claim “Found it” you must email me satisfactory responses to the following:
Any logs not accompanied by an email will be deleted.
- Send me a picture of you/your party with your navigation device taken with this feature in the background.
- By looking at the way these rocks eroded, why would you say this is often referred to as “tombstone weathering”?
- There are number of similar examples of tillite rock in this area, some a little further down the road towards Sutherland and others along the N1. As you drive off, look out for such an example along the roadside and send me its GPS coordinates (and a picture if possible).
- Why would you say is the tillite of the Dwyka Group regarded as one of the “delights of the Karoo Supergroup”?
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