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.
Sources:
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responses to the following:
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- 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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