Holey Rocks!
This EarthCache is located about halfway along the Maalgat Trail which follows the north bank of the Matjiesrivier River for about 1km from Sanddrif to the wonderful Maalgat Rock Pool.
The correct meaning of Maalgat is ‘grinder hole’ – the reason for which will become clear below. It is sometime incorrectly called Malgat which means ‘mad hole’ and also known as Hippo Pool.
To Reach the EarthCache Location: first collect your Cape Nature permit from Sanddrif Reception which is located at Dwarsrivier Farm @ S 32 30.237 E 19 15.468 and clearly signposted off the gravel road from Algeria to the northwest or Mount Ceder to the south-east.
It is open from 08:00-17:00 daily except Sunday/Public Holidays 09:00-12:00 and 16:00-18:00. Permits are R50 per person but free for Wild Card holders and are for a given day.
Return to the main road and head right to the signposted junction @ S 32 29.749 E 19 16.004. Turn left (north) here and follow the track for just over 1km to cross the drift over the river. Turn right and follow Maalgat signs passing through the top of the Sanddrif campsite to the signposted smaller dirt road starting just beyond the ablution block @ S 32 29.274 E 19 16.229. Follow this for some 500m to the end/parking area at S 32 29.453 E 19 16.315. The clearly signposted trail heads east from here.
To Claim the EarthCache
Please make the necessary observations on site and answer the following questions, submitting answers to the CO by GC message from my Profile page or email (forshaw.chris@gmail.com):
- Name three requirements for potholes to be formed.
- Name three typical characteristics of potholes.
- What factors influence the rate of growth of potholes?
- What are the approximate dimensions (diameter & depth) of the largest pothole seen?
- What things do you see in the potholes?
- What do you estimate is the height of the old river bed where the potholes are located above the current river level? What is this dramatic difference in height due to?
- (Optional but appreciated) Take a photo of yourself and your favourite pothole – maybe even standing in it!?
Near the cache location the trail passes along the edge an extensive (approx. 75m long x 30m wide) area of exposed smoothly weathered sandstone slabs overlooking the river several metres below. Of course, during the heavy winter rains the river will experience high flow rates and its level will rise considerably – to such an extent that the drift on the main road 5km downriver may become impassable (see video here).
On these slabs you will notice several extraordinary large and smaller smooth-sided, mostly circular holes extending down into the rock – these are potholes! One description of a pothole is ‘an exceptional dynamic micro-fluvial erosional landform of the river bed’.
Water shapes the land through a combination of weathering, erosion, transport and deposition. A river pothole is a cylindrical, bowl-shaped, or irregular hollow - usually deeper than wide - in a rocky river bed. It is formed by either the grinding action of sediment whirled around by stream eddies or the force of fast flowing water. They vary in depth & diameter from a few centimetres to several metres.
They are usually found in fairly homogenous rock in the upper or middle courses of rivers at higher 
altitudes where - at least periodic - turbulent water flow and sufficient potential (or gravitational) energy cause vertical erosion in the bed of the river with the river channel cutting into the bedrock – as can be clearly seen at Maalgat.
Long grooves parallel to the water flow carved into the bedrock floor may lead from one pothole to the next. As the river continues to erode its channel through the bedrock the size and shape of grooves and potholes changes.
The process of pothole formation
The riverbed in the upper course of a river is typically rough with a large load* which is mainly transported by traction (ie. the action of being dragged along a surface) along the river bed.
[*bits of eroded material – mainly rocks – which the strong water flow moves downstream until they are deposited on the river bed]
When flowing water encounters the static bedload (ie. a deposited load), it is forced over this obstacle and downcuts behind it in swirling eddies** which force water down onto the bedrock causing erosion and over time creating small depressions.
[**circular movements (currents) of water causing a small whirlpool, similar to the wind eddies caused when air flows over a tall building]
As these depressions deepen, pebbles can become trapped in them – initially when still fairly shallow this may be temporarily but later when deeper, permanently.
As a result of the eddies, the pebbles spin around in the same spot and the constant abrasion of the pebble against the river bed, begins to drill down into the depression and wear away the bedrock into a small basin-type hollow just a few cm in diameter.
Over time and through positive feedback with increased eddies (turbulence) and erosion the hollow deepens
to an established pothole which becomes progressively more circular, wider & deeper – usually deeper than it is wide – and with smooth sides which are also usually spirally grooved.
Smaller bedload becomes trapped in the hole and continues to abrade its sides with the rocks developing into smooth pebbles as they wear down. The rocks can only do this if they are of harder rock than the river bed.
Once started, the feature is self-reinforcing as the hole creates an even stronger eddy. This in turn creates greater erosion forces and accelerates the process.
The rock being spun in the hole is called a grinder and once it wears away another falls in to take its place and continue the process. As long as the grinder continues spinning around in its abrasive path, vertical erosion continues to take place.
If there are many eddies near each other, this erosion can cause multiple adjacent potholes to merge to create much larger potholes – as can be seen at the spectacular Bourke’s Luck Potholes in the Blyde River Canyon in Limpopo @ S 24 40.455 E 30 48.648 (see photos here and in the Gallery). Here the strong flow of the river has over many years rapidly eroded the sandstone bed to form a deep gorge in the river valley.
In the case of a waterfall or cascade, the rocks at its foot are swirled around by the water coming down from above and potholes may form leading eventually to the formation of a rock pool such as that at Maalgat some 250m downstream. During high flows, such as flood events, larger rocks may be pushed over the edge of the waterfall to fall down into the pothole helping to deepen it and the pool further.
Several factors are known to affect the rate of formation of riverbed potholes including a) geological factors such as the composition and state of the bedrock (including presence and size of faults and fractures) and load; the resistance of the bed and supply of sediment, and b) hydrological factors like stream power – a product of the density of water [1g/cm] x acceleration due to gravity (9.8m/s2) x discharge (ie. rate of flow) (m3/s) x channel slope. See here for an illustrated description of stream power.
It is not known when and how long it took for these potholes to be formed but given the current height of the riverbed compared with its previous height where the potholes are, it is clear that this happened a long time in the past – probably several 1,000 years ago.
See here for a short video showing a deep pothole in a relatively small river gorge.
The largest potholes can reach several metres wide and up to 15m deep! See here for drone video of the huge Bourke’s Luck Potholes in the Blyde River Canyon, Limpopo.
Sources used:
River Potholes: Modern and Ancient by Robert E Diffendal Jr, School of Natural Resources, University of Nebraska 1988 (available here)
Potholes - a short article on the Keweenaw Geoheritage, Michigan Tech website
River Landforms – paragraph on this page on the British Geographer website
River Landforms – paragraph in this piece by Alex Jackson (on GeographyAS Notes website)
Acknowledgement:
Thanks are due to Richard Fuggle (Richard.F), retired UCT Professor of Environmental Studies, for kind and careful review of the draft cache description.