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After we have identified Hakskeen Pan as the right place to run BLOODHOUND SSC we might have a
look into the evolution of the pan and find out why the surface is as flat and hard as it is, and where
all these annoying pebbles come from.
Mud and salt pans usually occur in clusters, and the Hakskeen Pan is the biggest in such a cluster of
more than fifty larger and hundreds of smaller ones north of Upington in the Northern Cape
Province. Pans are always found in arid areas with flat topography, with low annual precipitation,
long dry seasons but occasional flood events. Only episodic rainfall results in ephemeral river flow
during the rain season in summer. The rainfall in the area does not exceed 200 mm per year and
often is considerably less. Sometimes the rainfall is not intense enough to allow surface run-off for
several years. However, summer thunderstorms may be quite dramatic and much of the annual rain
may fall in a few hours. Then the rivers run and the pans get flooded. What is great for the wildlife in
the Kalahari is counteracted by the intense heat of the South African summer, and within days or
weeks the water in the pans will evaporate, leaving behind a layer of clay, sand, pebbles and salt.
But how do the pans form in the first place, and how do they develop over time?
Pans owe their existence to the combination of a number of geomorphologic, geological and
climatologic factors that favour their formation and development. Important pre-requisites are a flat
terrain, semi-arid or arid conditions with seasonal (but then intense) rainfall, the right kind of rock
type in the subsurface, and some tectonic events.
Pans develop best on relatively soft rock types that are easily weathered and eroded, in order to
provide the mud, sand and pebbles that we later find in the pans. The Hakskeen Pan formed in soft
shales of the Ecca Formation and in Dwyka Formation tillites. Tillites are poorly consolidated
moraines, which are rocks that form at the bottom of glaciers. They consist of components highly
variable in size, including clay, silt, sand, and larger rock fragments. Strange to see such rocks in the
middle of a desert, but in the in the upper Carboniferous and lower Permian periods, about 300
million years ago, the area that now is the Kalahari was close to the South Pole. The rocks of the
Dwyka and Ecca Formations probably were overlain by further sedimentary rocks that have been
removed by erosion since.
Compared to the age of the Ecca and Dwyka rocks, the river systems and processes that have led to
the formation of the Hakskeen Pan are geologically young and probably not older than Late
Pleistocene and Holocene in age. Hence, the pan may have existed only for the last 10-20 000 years.
River systems in flat, arid terrains show poorly organised geometric patterns with meandering river
beds changing directions frequently. Watersheds are often large and poorly defined in shape. In
order to form a pan some event needs to disrupt the drainage of such a terrain and allow water to
gather in a certain place. Such an event could be tectonic movements that lift part of the area. In flat
terrain not much uplift is needed to reverse the flow direction and effectively stop water drainage
from the area. Another option is the arrival of migrating dune fields that might have a similar effect.
Once the water drainage is disturbed, shallow lakes may form.
Depending upon the intensity of the rainfall and the amount of water flowing, the rivers will erode
mud, sand, and pebbles downstream and into the lake where all carried material comes to rest.
Since further flow is not possible the water currents within the lake are very weak and the sediment
load in the water can settle on the ground in a quite environment. This forms perfectly flat sediment
layers at the bottom of the lake. Subsequent evaporation will remove the water fairly quickly. The
then dry and flat plains are exposed to desert winds which remove part of the sediments, leaving
behind shallow topographic depressions. This is important in order to make space for new sediment
brought in by the next year’s floods.
In addition to wind activity another, controversially debated explanation for the removal of
sediment from pans has been proposed. The availability of water in the pans may attract grazing
animals. Wet clay-rich sediment may stick to their hoofs and will be carried away from the pan when
the animals leave. This process has been seen as insignificant in effect, given the large dimensions of
some pans, such as Hakskeen. However, early western arrivals in Southern Africa have reported on
springbok herds of hundreds of thousands of animals, and therefore antelope-assisted pan
propagation might not be entirely out of the question.
The “hollows” in the ground, may they be formed by wind or antelopes or both, attract the
formation of centripetal drainage systems, along of which new sediment is provided. The satellite
image of the Hakskeen Pan shows such a concentric arrangement of river valleys on the western
side. The eastern border of the pan is formed by Kalahari sand fields which appear to delay or block
the pan’s propagation to the east.
The intensity of annual floods will determine the kind of sediment that can be transported in rivers
and be deposited in the pans. Weak water currents in the rivers will be able to carry clay and silt,
and perhaps sand, but not coarse components like pebbles and boulders. The abundance of pebbles
in the pan, however, indicates fairly intense flash floods in the recent past.
After evaporation of the water in the pan, the sediment starts drying out, often forming
characteristic mud cracks during contraction during water loss. When water evaporates and the
sediments in the pan dry out, the clay binds together. This forms the coherent, hard and flat surface
that BLOODHOUND needs for high-speed driving.
Remaining loose sand, silt and clay are typically blown out by the desert winds but the heavier
pebbles and boulders are usually left behind in the pan. Such pebbles can be hazardous for highspeed driving and the BLOODHOUND team has taken all pain to remove them from the race track.
The blown-out sand, clay and silt often form a characteristic type of dunes, the so-called lunette
dune. These crescent-shaped dunes typically are found along the margin of the pan in down-wind
direction. The horns of the crescents point up-wind. Clay and silt help to consolidate the dune and
provide substantial cohesion between the particles. This prevents lunette dunes from migration.
Hence, other than pure sand dunes, lunette dunes are stationary.
In the Hakskeen Pan there are a number of large blocks of carbonate-rich, layered rock, which have
not come there by transport in flash floods but have formed in-situ. These crusts of silt, sand, clay
and carbonate minerals form by the rise and fall of the groundwater level beneath the bottom of the
pan, which influences the alkalinity and can promote the precipitation of carbonates. This forms
layers and irregularly shaped bodies called calcrete, a fairly hard rock type that cannot be removed
by desert winds. Where the softer material around such calcrete blocks is blown out, the cemented
calcrete might stand out of the ground, forming the only topographic anomaly in the otherwise flat
plains of the pan.
Dr Steffen Büettner
Geology Department Rhodes University Grahamstown South Africa