The owners of the African Bird of Prey Sanctuary near Asburton have laid out a short nature trail on their premises. This is a pleasant stroll along a well-maintained path that more or less follows the contour of a spur reaching over the valley where the Mshwati Stream flows.
Please take your time to enjoy the stroll along the trail where you will find a number of caches. There is no admission fee to walk the trail. Allow an hour or two to walk the trail and enjoy the views (and find the caches).
After (or before) the trail, treat yourself to a stroll around the sanctuary where you can familiarise yourself with the raptors on display. The birds in the sanctuary are all captive bred or non-releasable. Brochures and a trail map are available at reception.
There is also a daily must-see Free-Flight display (twice daily on weekends).
This cache has been placed with permission of the Sanctuary owners.
The Earthcache
Along the trail (at the published co-ordinates) you will pass an isolated outcrop of blue-gray rock with some clasts (inclusions) in it. This rock is Tillite and is the focus of this earthcache. The rock was formed about 300 million years ago!
The Ashburton Tillites
Dwyka Tillite
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 Gondwana Ice Cap
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.
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.
Clast Formation
The clasts in the Tillite outcrop were formed as in the above image - loose pieces of other rock types were forced into the tillite matrix under great pressure and have remained there to this day.
KZN Geological History (in layman terms)
Approximately 1000 million years ago, subduction forces beneath the earth’s crust pushed the ‘Natal Metamorphic Province’ block into collision with the southern edge of the ‘Kaapvaal Craton’ block, forcing super heated rock upward into a mountain range that stretched many thousands of kilometres. This range was gradually eroded away to leave hard granite and gneiss deposits. An example of these granite remnants is the feature called “Old Baldy’ in the Valley of a Thousand Hills, just inland from Durban..
This was followed up by the ‘Natal Group Sandstones‘ (Table Mountain Sandstone) layer. This first sedimentary layer from 490 million years ago, is the result of rain and river erosion of earlier rocks in the highlands to the north east. The eroded sands were deposited atop the remaining Kaapvaal Craton and Natal Metamorphic Province basement. One can see these sandstone layers along the inner slopes of the Krantzkloof and Oribi Gorges.
300 million years ago, when Southern Africa was still part of the greater Gondwanaland landmass, it was situated much nearer to the South Pole, and much of the land was covered in thick glaciers of snow, ice and glaciers. These glaciers moved across the land in a south westerly direction, scouring the underlying bedrock and carrying within the glacier mud, pebbles and boulders. As these ice sheets retreated and melted they dropped their load of fine mud, boulders and pebbles into thick beds that hardened over time into the fine-grained blue-grey conglomerate called ‘Dwyka Tillite‘, These deposits can be seen in and around Durban, Umdloti, Ashburton and Umkomaas where they have been quarried for a 150 years for aggregate and building material.
Then during the Permian, Gondwanaland moved closer to the equator. South Africa, at the heart of this landmass was covered by a large warm inland sea as part of the Karoo Basin. Thick silt and clay beds built up interspersed with the vegetative remains of swamp plants. These sediments hardened into Ecca Shale and are darkly-coloured from their carboniferous plant content. These shales break down quickly and easily when exposed to erosive agents. One can see these beds in Cato Manor, just inland from the Berea Dune, where they constitute unstable hillsides unsuitable for the building of homes.
Earthcache Logging Tasks
In order to claim a find on this earthcache, please complete the tasks below and email the answers to me at my geocaching.com profile.
Task 1 - Measure the circumference of the outcrop to the nearest meter.
Task 2 - Examine the clasts in the outcrop and describe two of the largest inclusions in terms of shape, size, texture and colour.
Task 3 - What type of rock do you think the largest clast is made of?
Task 4 (Site Specific Question) - Near GZ there is a wooden sign about 1.2m high. What are the words on this sign?
Task 5 (optional) - A pic of you and your GPS close to GZ would be appreciated!
References
UKZN Geology - http://www.stec.ukzn.ac.za/geologyeducationmuseum/kzngeology/DwykaGroup.aspx
Other - https://grahamlesliemccallum.wordpress.com/2014/11/28/bay-of-natal-its-formation/