Geological history

Understanding the Magaliesberg's geological history—comprising four key stages—is essential to appreciating the geology at Hamerkop.

Stage 1

Over more than 300 million years, numerous layers of sediment were deposited one upon another on the bed of a shallow sea. Primitive algal mats established in some of the muds. This was followed by a process called “lithification” (the formation of rocks, by means of compaction, increased temperature and crystallization).

Depending in their composition, the sediments were transformed into mudstone, shale and quartzite. Of these, the quartzites (often crystalline) are the hardest and most erosion-resistant and the mudstones are the softest.

Stage 2

Around 2,000 million years ago, the Bushveld Igneous Complex (BIC) formed north of the Magaliesberg when a massive body of molten igneous rock rose up from the mantle and intruded the Magaliesber sedimentary layers. The sedimentary rocks were then further altered by heat and pressure, (a process called metamorphism). In the north, the molten igneous rock assimilated the Magaliesberg sediments.

These rocks then sank into the BIC, while in the south, the sediments were uplifted like the prow of a sinking ship (Carruthers 2000). These processes created the characteristic northward-dipping dip slope and the steep southern scarp slope of the Magaliesberg Mountains.

Stage 3

Mainly as a result of the BIC, further igneous intrusions occurred in the Magaliesberg, when molten igneous rock, called diabase intruded the metamorphosed sediments.

This further metamorphosed the shales, forming a metamorphic rock called hornfels. Where the diabase intruded between the bedding, sills were formed and where it intruded the near vertical fractures, dykes were formed.

Stage 4

Once the above processes were complete, the Magaliesberg was subjected to millions of years of erosion. this involved the preferential weathering of the rock, where the softer rock eroded more quickly than the harder rock.

This resulted in a number of distinct features:

The hard quartzite rock, which is observable everywhere as in situ bedding, or loose blocks and scattered rocks. These are the result of joint erosion and gravity.
The rocky outcrops and ridges, comprising the quartzites, are more resistant to erosion.
The deep kloofs result from the erosion of softer dykes by the flowing streams.

Geological observations associated with Hamerkop

This section describes some geological features observable in the field and how they relate to the geological history. Where appropriate, the stage of the geological history is noted.

1. View from the N4 highway (stage 2)

Approaching Hamerkop from the east along the N4, the gently north-sloping quartzites and kloofs of the Magaliesberg are seen to the south. To the north are the anorthosite rocks of the BIC, which spans about 65,000 km². Since the BIC is one of the largest platinum repositories on earth, this area has been devastated by open-cast mining and enormous, poorly rehabilitated spoil dumps.

2. Buffelspoort Dam area (stage 3)

Approaching Buffelspoort Dam from the old main road, reddish-black diabase boulders are visible on the left (stage 3), and the dam wall is built into the quartzite ridge.

3. Beyond the Buffelspoort Dam

The surrounding farmlands and game reserves are situated on ‘recently deposited’ soils, which are probably underlain by shales.

4. Road cutting before Rietfontein turn-off

In the road cutting just before the Rietfontein turn off, one can see a relatively fresh exposure of a sequence of Magaliesberg shales and quartzites.

5. Hamerkop buildings

The Hamerkop buildings are all built from Magaliesberg quartzite, (although the slate paving on the stoep probably came from the Pretoria geological formation).

6. From the terrace

The beauty of the Magaliesberg topography, which formed as the result of the underlying geology and the subsequent erosion, is visible:

Most impressive is our kloof, formed by stream erosion of softer diabase that was intruded into vertical joints in the hard north sloping layered quartzite. Aerial photos clearly show how many of the kloofs follow jointing patterns in the rock. (All stages).
Vegetation patterns on both sides of the kloof align with the underlying shale, showing clear geobotanical relationships. The Peglerae aloes, which only grow in certain geological bands, are another example.
When sitting on the terrace and looking closely at the stonework on top of the terrace garden bed, a small grey stone with shiny flakes of mica is visible. This is hornfels, probably metamorphosed by a diabase sill intrusion, (stage 3).

7. Path down to the dam (stage 3)

A diabase sill (stage 3) outcrop is visible on the path down to the dam. In the same vicinity there are hornfels boulders, indicating the heat-altered zones where diabase intruded. Similar rocks can be seen along the approach road.

8. Into the Kloof

The northward-dipping quartzite layers and their joint patterns are clearly visible.
More impressive, however, are ripple-marked quartzite beds.
Beautiful examples of these on boulders in the kloof (and elsewhere) indicate that these sediments were deposited at the bottom of a shallow sea, (Stage 1).
Also evident in the kloof and elsewhere, are rocks exhibiting the fossilized algal mats, formed in the shallow sea. (Stage 1).

9. Out and about

Examples of coarse recrystallized quartzite can be observed higher up the northern slope (possibly stage 3, but more likely stage 2).
In many parts of the Magaliesberg, erosion has channelled rainwater runoff into porous basins, termed “groundwater catchment areas.” This water often emerges lower down the slope as seepages or springs of beautiful, clear water. Some of these are accessible from Hamerkop.