Description

Geology of the Uniondale area

Uniondale lies in the Kammanasie river valley, close to the river’s headwaters.  The main road (N9) from the south climbs the Potjiesberg pass to 900 metres above sea level.  The summit has some unusual geology of thick, flat topped rock ledges beneath which is very soft crumbly material, white in colour. As the route descends to the valley the town comes into view, and the distant scene is dominated by the river valley enclosed by rugged hills made of sandstone rocks named the Table Mountain group, 480 million years in age. Within the valley are softer rocks less resistant to erosion, the shales and muddy sandstones of the Bokkeveld group of rocks.  Standing in the valley are three prominent flat-topped hills dominating the vista.  These are “mesas”, erosion remnants of a one time widespread plain sloping gently from 1000 metres elevation in the north to 600 metres in the south, along the Kammanasie river.  These are the remains of the ancient African Surface, a landscape feature scattered throughout southern Africa.
 
The view from Uniondale Heights on approaching the town from the north on the N9 is similarly striking, and the three mesas are again first view of note.  The view of these mesas has been rendered in a landscape by Uniondale artist Gail Strever, a painting that captures the uniqueness of the scene. 

The African Surface originated through a very long geological event of 90 million years duration in this part on the country, and in surrounding areas. Erosion started with the breakup of the Gondwana supercontinent, and reduced the land surface to an undulating plain, with the erosion resistant sandstone and quartzite standing proud of the plain as impressive peaks; the most prominent peak of Table Mountain sandstone is Mannetjiesberg, visible to the north-west of the village.  A drive along the Uniondale Poort road towards Avontuur reveals the sandstone rocks close up; there are several stopping points where bedding (from when the rocks were laid down) and folding (caused by the great upheavals the rocks underwent) can be examined.

Over tens of millions of years, southern Africa was raised several hundred metres; these events renewed the erosive energies of the rivers to form the deep valleys, with the African Surface remnants seen today.

The weather was warm and wet for this long time, and thus the rocks on the plain became deeply rotted. This feature is seen on the southern entrance to the town.  Water and wind worked on the sandstone fragments on the surface on the plain and in time many of the fragments were rounded to pebbles and small boulders that lie on the present day flat surface.  While this surface was being formed, water carried silica and iron derived from the weathering into the rotted rocks below the plain, and cemented these to very hard duricrusts.  The narrow cliffs seen at the crest of the mesas are the cemented jumble of angular and rounded rock fragments. 

Weathering and erosion of the past few million years has removed the rotted rock and undercut the hard duricrust.  In this way caves have formed; the caves are of great significance in the history of land use by nature and man in the Uniondale area.

The caves are places of refuge for owls, swallows, bees, and small mammals.  In later years the caves became habitation sites for humankind, and are the places where many of the San paintings of the Uniondale area are preserved.  More recent use was by farmers, and sheep, goats and cattle were sheltered.  The caves are generally abandoned now, while there are efforts in place to preserve the art heritage of the San by limiting access to the sites.

Article submitted by Dr. Chris Lee, Blue Hills Escape, Paardekraal Farm