All Fall Down EarthCache

An Earthcache in Krantzkloof Nature Reserve, close to Splash Rock on the Yellow Trail.

This Earthcache will inform you of a significant geological event in the Krantzkloof gorge.

General Geology of Krantzkloof Nature Reserve

The Krantzkloof Nature Reserve’s breathtaking scenery is the result of the deep incisions of the Molweni and Nqutu and Nkonka Rivers – tributaries of the Umgeni – into the Kloof plateau. This plateau comprises erosion-resistant Natal Group sandstone, which forms the reserve’s orange-red, iron oxide stained cliffs which overlie a bed of basement granites.

River Sediments – The sediments which form the Natal Group sandstone were transported by river systems which flowed from an active mountain belt to the north, about 490 million years ago. At this time the earth lacked plant cover and the oceans were dominated by faunas such as trilobites (extinct arthropods), brachiopods (now mostly extinct clam-like creatures), echinoderms (ancestors of starfish and sea urchins) and corals.

Basement granites – Beneath the Natal Group sandstone are even older granite gneisses. These were formed in the roots of ancient mountains about a thousand million years ago at temperatures above 600 degrees centigrade, 10 to 20 kilometres beneath the earth’s surface. These mountain peaks, similar to those of the Alps or Himalayas of today, were gradually eroded until the basement granites were brought to the surface. Further erosion formed a flat surface onto which the Natal Group sandstone was deposited.

These granites are seen as huge dome-shaped boulders along the Molweni River bed and on hillsides above the river. They are coarse-grained crystalline rocks comprising the minerals, feldspar (which is milky pinkish-white), quartz (which looks glassy) and a few dark specs of a brown mica called biotite.

Dolerite intrusions – Between 300 and 180 million years ago a thick sequence of sediments (known as the Karoo Supergroup) was deposited on the Natal Group sandstone. This was followed by a massive upwelling of heat, known as a hot spot, from deep within the earth that melted rocks beneath the Gondwana supercontinent.

Molten rock (magma) forced its way through fissures in the earth’s crust and erupted as basalt lava flows on the surface. These fissures are known as dykes (if vertical) and sills (if horizontal). Magma which crystallised more slowly in the dykes and sills is coarser grained than basalt and is known as dolerite. This is a blackish crystalline rock consisting of feldspar and pyroxene.

The numerous narrow and deep crevices that extend into the sides of the Krantzkloof’s cliffs, often a few metres wide and up to 50 metres deep are dolerite dykes which have weathered and eroded from the more resistant sandstone.


Sandstone
 

Sandstone (sometimes known as arenite) is a clastic sedimentary rock composed mainly of sand-sized minerals or rock grains.

Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow, red, grey, pink, white and black. Since sandstone beds often form highly visible cliffs and other topographic features, certain colours of sandstone have been strongly identified with certain regions.

Weathering of Sandstone (and other rock types) is mainly by wind, water and mechanical methods.

Rock formations that are primarily composed of sandstone usually allow percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs. Fine-grained aquifers, such as sandstones, are more apt to filter out pollutants from the surface than are rocks with cracks and crevices, such as limestone or other rocks fractured by seismic activity.

Quartz-bearing sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts.
 

Sandstone Boulders and Rocks

The rocks are continually being broken down and the sandstone cliffs eroded away by wind and rain. The evidence is all around in the form of fallen rocks and boulders scattered on the lower slopes of Krantzkloof.

The sandstone cliffs of Krantzkloof are hard and resist weathering. The cliffs erode by overhang collapse, where the lower, softer layers of the Natal Sandstone Formation erode away leaving sandstone overhangs, and gravity eventually causes large blocks to come crashing down. One of the most impressive of these is the rock close to where you are standing.

There are many massive boulders and large rocks on the lower slopes of Krantzkloof and these can be seen from many vantage points along the Yellow and Orange trails. There are also occasional slope failures caused by heavy rains, where the thin soil layers on the lower slopes slip down, taking plants, even trees, along with the slip. Several of these have occurred in Krantzkloof over the millenia.

The Earthcache

The published co-ordinates lead you to an area of Krantzkloof known as Splash Rock, on the banks of the Molweni River, shortly after the nKonka River and Uve Rd stream join and just before the Nkutu triver joins the Molweni.

Close to where you are standing is a large boulder.  Examine this boulder and it soon becomes apparent that the mineral composition of the boulder is vastly different to the surrounding rocks.  The obvious inference is that this boulder fell from a nearby cliffside many thousands or millions of years ago.

Note:  Krantzkloof Nature Reserve is run by Ezemvelo KZN Wildlife and a nominal admission fee (R20 in November 2014) is payable to enter the reserve.  This can be paid at the main picnic site in Kloof Falls Road.

Note 2:  There is an altitude difference of approximately 300m between the parking co-ordinates and the published co-ordinates.  Ensure that you are equipped for this hike with sufficient water, snacks etc.

Logging Tasks

In order to claim a find on this earthcache, please complete the logging tasks below and email your answers to me via my gc.com profile or direct to technonut[at]stars.co.za .
 

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