GEOLOGICAL FEATURES OF THE GREATER WOODMEAD ESTATE

Geological Setting

The most prominent feature alongside the road leading from the entrance of the Country Club to the parking area are a cluster of large rounded boulders known as Dombeya Rocks. They lie in amongst the pristine bush and trees on the right hand side of the road after the first major bend. As golfers might know, similar clusters of rock, (sometimes represented by only one or two boulders) such as the well-known Castle Rock between the third green and fourth tee on the appropriately named Rocklands course, are often associated with circular stands of natural vegetation known as Copses. These are particularly noticeable in the Wilds East and Wilds West areas and between a numbers of the fairways on both courses.

The boulders are comprised of one of the most important rocks on our planet, namely granite. Granite forms the bedrock of the entire Greater Woodmead Estate (GWE) and the outcrops and boulders we now see are merely windows peeping through the much younger cover of sand, soil, clay and ouklip, all derived in one way or another from the weathering of the granite.
The granite on the GWE forms a small portion of a much larger circular body of granite extending from immediately below the prominent Linksfield – Parktown ridge in the south, almost to Centurion in the north. This geological feature is known as the Johannesburg granite dome and the northern suburbs of the city, as well as our two golf courses have been built amongst the many hills and valleys formed by the much more recent erosion of this dome by the Sandspruit, Braamfontein spruit, Jukskei and other rivers.

The granites of the Johannesburg dome represent the ‘basement’ onto which were deposited a succession of younger, mainly sedimentary, ‘cover’ rocks. These cover rocks have been tilted and eroded and now form the Linksfield Ridge – Parktown escarpment which is composed of Witwatersrand sediments which dip south off the dome and which host the conglomerate reefs of the world’s greatest goldfield.

The granites of the Johannesburg dome and their analogues world-wide constitute the ancient, ‘Basement’ nuclei of all the world’s continents and range in age from about 2, 5 to 3, 5 billion years.

 

The Tree and Environmental Walk

A number of sites on the newly established Tree and Environmental Walk provide excellent exposures of granite where one can observe and study these ancient rocks including their important link with plants, animals, birds and insects. The most common form of granite to be seen on the GWE is homogeneous potassium rich variety comprised mainly of the minerals quartz (a light glassy translucent mineral) and feldspar (a white opaque mineral). Other minor constituents include mica. It is typically pinkish to light brown in colour as can be seen on the outer surfaces of the innumerable boulders scattered throughout the GWE. This rock formed three thousand two hundred million years ago (3, 2 billion years) from molten magma well below the surface of the earth. Smaller areas of even older granite (3, 4 billion years) are also to be seen at various localities within the GWE. The latter variety of granite is termed migmatite or simply put, mixed rock. It occurs as inclusions or remnants within the younger potassium rich granite and can be recognized by its banded appearance. The bands which in some instances are distinct and in others rather diffuse and generally folded or undulating are comprised of grey granite and darker bands richer in iron and magnesium bearing minerals. The later rock is thought to be an altered volcanic rock which formed part of the earth’s primitive crust. The huge perched boulder at Dombeya rocks is composed of this type of granite and folded bands which have been etched out by weathering processes are beautifully displayed.

The youngest form of granite to be seen on the GWE are narrow intrusive veins up to 50 cm wide composed of large crystals of quartz and feldspar in a rock termed pegmatite.

As granite formed at considerable depths within the earth’s crusts and under great pressure, the rock is now out of equilibrium with its surroundings being exposed on the surface of the earth due to uplift of the Johannesburg dome and removal of several kilometres of overlying sedimentary and other rock formations.

The release of pressure due to the removal of these overlying rocks has produced a system of fractures called joints. They are fundamentally important in the weathering of rock as they effectively dissect large blocks into smaller ones thereby greatly increasing the surface area on which weathering can take place. Low platforms of granite known as exfoliation domes or Flat Rocks with overlying stacks of rounded boulders (castle kopjes or Tors) as seen in the photograph, have been formed by weathering along different joint sets. The most prominent of these joints are a series of horizontal slightly convex planes or expansion joints which form the domical platforms such as the one in the photograph.

All that remain of older disintegrated platforms are small hills of rounded boulders perched above the platform. These rounded boulders have been formed by a process of spheroidal weathering. A rounded shape is produced as weathering initiated along the sub-horizontal as well as along vertical joint planes attacks exposed rock from all sides simultaneously with decomposition being more rapid along the corners and edges of the block. As the decomposed material spalls off along a series of concentric shells in a process known as exfoliation (onion skin weathering) the corners become rounded and the block which often forms part of a cluster, is reduced to an ellipsoid or sphere, often with a totally unaltered core referred to as a corestone. Scattered granite corestones are the most obvious geological feature of the GWE.

 

Granites, Weathering and the Earth Life Link

The joints or fractures form openings of variable width in the granite and some of the vertical ones often extend for several meters into the bedrock. Rain water seeps into these vertical openings which also contain nutrients from the chemical weathering of the granite. Plant growth commences with the root systems, particularly of the larger trees, penetrating for many meters down the joint planes to the water source. As the roots grow and expand they start pushing the rock further apart leading to its eventual breakdown and the formation of the corestones described above. Examples of this phenomena are to be seen in many parts of the Greater Woodmead Estate including a few fine examples between several of the fairways.

Termites also require water and moisture to live and thrive and the joint planes are ideal reservoirs of this precious commodity. As a consequence, termite mounds are frequently found in close proximity to areas of granite outcrop with prominent jointing and tree growth. They contribute to the ecosystem by, for example, by advancing the decomposition of dead wood. As tree growth proliferates in such areas aided by birds carrying seeds of different plant types, circular features, often centred around clusters of granite boulders, the Copses mentioned above, form. They are particularly prominent on parts of the estate, more especially along the Tree and Environmental Walk and provide fascinating insights into the earth life link.

The Tree and Environmental Committee have to be congratulated in drawing our attention to the wonders of nature to be seen at our Club. We are particularly grateful to them for their efforts in introducing a more holistic approach, including geology as an important component in the understanding of our environment.

Richard Viljoen

 

Recommend
Share
Tagged in