GEOLOGY

The whole reserve is underlain by rocks of the Klipriversberg formation of the Ventersdorp system. These volcanic rocks are Basalt and Andesite.

To the north of the reserve lie the quartzites and conglomerates of the upper Witwatersrand system which dip underneath the lavas.

To the south, beyond the Klip River lie the Dolomites of the Transvaal system.

Both the Wits and Transvaal systems were laid down as sediments in shallow seas or deltas and were originally flat lying. They were pushed up by the granite dome to the north of Johannesburg .

The lavas were placed by numerous volcanic fissures over the whole of the areas of the Transvaal and Free State during a period of extreme vulcanism.

Topography

The Topography of the area results from the Geology. The quartzite and lavas are hard, weathering resistant rocks which thus form hills. The Mondeor Valley is formed by the weathering of the weaker contact zone between the Witwatersrand sequence and the Ventersdorp lavas.

The Bloubosspruit valley formed by a tributary of the Klip River cutting back through the hills and finally capturing the stream along the Mondeor dip.

It must be remembered that after the lavas were deposited, the area was covered by rock to a depth of at least 10 km. The pressure of this weight of rock formed quartzites from the sands and gravels of the Witwatersrand system. The present land surface is a very recent surface not more than 5 m illion years old.

Age of the Rock

• The Wits system is about 2.8 thousand million years old
• The Ventersdorp system is about 2.4 thousand million years old
• The Transvaal system is about 2.1 thousand million years old

The pebbles in the Witwatersrand conglomerates and the gold came from even older rocks which were being eroded at that time.

Variations in the lava

The lava forming the hills is not uniform but consists of many kinds of volcanic rock. These are ordinary lava flows often marked by white inclusions (like pebbles) at the top. These are Amygdales, bubbles caused by the release of gasses in the molten rock which are frozen into position and later fill with silica (quartz). Tuffs made up of volcanic ash, fossilised pumice and agglomerates of volcanic cinders can also be seen. At a point near the sewer pipe Collumnar basalts can be seen where the cooling lava shrank to form columns. In places angular crystals in the lava called Phenocrysts are seen. These indicate that the lava was cooling and crystallising before reaching the surface.

Vegetation

The soil on the lavas is a clay-rich alkaline soil which is extremely fertile in comparison to the acid sandy soil formed on the quartzites to the north. The ecology of the two environments is affected by this on the lavas. The dominant trees on the clay-rich alkaline soils are:

• Acacia Caffra
• Acacia Karroo
• Rus .Leptodicta
• Cussonia Paniculata Etc.

• Protea Caffra
• Magaliesmontanun (Stam Vrugt)

Uses

• The lavas are quarried for building aggregate to the south west of the reserve.
• Due to their massive structure these lavas have weathered to round boulders and were the primary building material for the iron age ruins in the reserve.