Terrain, soils and geology comprise the
foundation of the physical environment of the CMA. Its distinctive geology gives rise to
various types of soils that support indigenous fauna and flora as well as human
agricultural practices. The geology is in turn shaped by hydrological and weathering
processes (Moon and Dardis, 1988), creating a distinctive terrain. Collectively soil,
geology and terrain offer opportunities and constraints for human utilisation of the
physical landscape of the CMA.
|
|
| Contents of CONTEXTUAL INFO on
Terrain, Soils and Geology: |
|
|
| Terrain |
Back to top |
The CMA has a varied terrain ranging from low-lying sandy plains to rocky
mountains. The most dramatic terrain is located along the Cape Peninsula, a spine of
mountains extending some 56 kilometres from Table Mountain in the north to Cape Point in
the south (Map 1). The Cape Peninsula mountain chain is a series
of peaks, rising to Maclear's Beacon (1038 m) on Table Mountain and dropping dramatically
to the sea in many parts of the Peninsula. Topographical features are varied and include
narrow flats, kloofs and gorges, cliffs, rocky shores, wave-cut platforms, small bays and
sandy and gravel beaches.
The plains of the Cape Flats extend from the Cape Peninsula to the Hottentots Holland
Mountains in the east and Atlantis in the north (Background: Map 2).
The undulating plains and koppies of the CMA are sandy and interspersed with some larger
koppies, with a diversity of slopes and hollows, landforms and drainage lines. False Bay
is characterised by long stretches of sandy beaches and coastal dunes. This varied terrain
leads to a diversity of habitats and micro-climates.
|
| Soils |
Back to top |
Soil, a term used in the broad sense to include surficial material, is the
product of parent material (geological rock type), slope, and a variety of chiefly
climate-driven processes interacting over time. The inorganic component of soil can be
regarded as the product of mechanical and chemical weathering. Within the CMA, geology and
slope (see Background: Map 3), and therefore soil, are closely
allied. The soils of the CMA, which form the basis for all habitats, can be placed in
three groupings (McVicar, 1991):
|
- Shallow, acidic, sandy soils derived from
Table Mountain Group sandstone. Such soils occur on, or close to mountain slopes. They are
nutrient poor, have poor water retention properties and although they provide an
apparently inhospitable medium for plant growth, fynbos vegetation has adapted to these
harsh growing conditions.
- Deeper, sandy, calcareous soils of the
low-lying areas. They are less acidic than the previous type and their nutrient status,
while still low, is higher than the previous group. The low-lying nature of the
environment in which they typically occur implies that these soils are often subject to
waterlogging during the winter months.
- Soils which have derived from the weathering
of parent material which is relatively rich in clay. Typically, these soils are found on
granitic footslopes to the north east of the CMA (Paarl, Stellenbosch areas) as well as on
Malmesbury Group shales in the Swartland to the north of the CMA. Because they contain
significant proportions of clay, their nutrient status and water holding capacity is
superior to the previous two types. This group displays the widest variety in terms of
depth, profiles and mineralogical make-up. However, for the sake of convenience, they are
grouped here as one.
|
| As indicated in Background: Map 3, other surfaces in the CMA comprise the following:
- Bare rock surfaces and very thin sandy
veneers;
- Wetlands and marshy areas;
- Alluvial soils along drainage lines (in some
cases, brackish);
- Talus, rock debris and scree (boulder-rich
slope deposits, for example Oudekraal); and
- Built and paved areas.
|
| Geology |
Back to top |
There are three main rock types in the CMA, namely shale, sandstone and
granite (Map 2). The dominant rock type underlying the soils
throughout most of the CMA is from the Malmesbury Group (Theron et al, 1992) and is
broadly classified as a type of shale. Shale derived soils (clays) are most easily seen in
the Tygerberg area, on the Peninsula (Signal Hill) and near Somerset West.
The two other main rock formations are sandstone from the Table Mountain Group and
granites from the Cape Granite Suite (Theron et al, 1992). Sandstone is most easily
recognised on the slopes of Table Mountain while granites are highly visible on Chapman's
Peak and on Lion's Head.
In general, weathered sandstone provides the acidic and low nutrient soil on which
Mountain Fynbos and Sand Plain Fynbos thrive. Granite weathers to form various coarse
sands and, given the appropriate geomorphic conditions, kaolin. The calcareous
marine-derived soils contain a varying amount of decomposing shell matter (Theron et al,
1992) and is rich in calcium-carbonate. All three rock types (shale, sandstone and
granite), as well as their numerous derivations are found in the CMA. There are deposits
of limestone on the Cape Flats, Kaolin in the southern Cape Peninsula and minerals such as
silica on the dunes on the Cape Flats and Malmesbury hornfels in Tygerberg.
|
| Opportunities and constraints |
Back to top |
The terrain (relief), geology and soils of the CMA are inextricably
interwoven (Map 1 and Map 2, as well
as Background: Map 3). Typically, geology controls both slope and
soil. A number of morphological units, classified on the basis of slope, rock type and
surficial material can be recognised. They are here simplified into three primary
groupings.
|
| Steep mountainous areas |
Back to top |
The upper areas of the Cape Peninsula mountain chain and Hottentots
Holland range have very steep slopes, and in many places sheer cliffs. Where soils do
occur, they are very shallow and nutrient poor. It is these areas which contribute to the
inherent physical beauty of the region. They are in themselves a primary tourist
attraction, as well as serving an important recreational function. Under the right soil
and moisture regimes, they support a number of rare plant species, for example the red
disa. In terms of constraints, they form major obstacles to infrastructural development
(roads, railways, pipelines). They cannot, and should not, be utilised for building
purposes.
|
| Mountain footslopes |
Back to top |
Mountain footslopes are found in the Cape Peninsula mountain chain and
Helderberg, and the steep to undulating hills (Tygerberg) of the northern CMA and the
extreme eastern CMA. This morphological unit has in common moderate to steep slopes and,
depending on their exact location in the landscape, either granitic or shale bedrock.
Soils are typically clay-rich, with a moderate nutrient status. Many fynbos species,
including a variety of proteas, previously thrived on these slopes. However, because of
their agricultural potential, these areas have been heavily exploited, particularly for
viticulture. From a mining perspective, there are pockets of commercially exploitable
kaolinite in the Cape Peninsula, and the metamorphosed shales of the Tygerberg provide a
source of aggregate for construction purposes.
In urban areas, many of the steep granitic slopes have been taken over by upmarket
housing. This should be a cause for concern due to: the aesthetic impacts and the
engineering requirements when building on these slopes; injudicious development can and
does lead to erosion, slope failure and the movement of mud, rocks and boulders. From an
aesthetic perspective, the development of the suburb of Plattekloof on the west flank of
the Tygerberg and the development on the Helderberg are typical examples.
|
| Flat, or slightly undulating low-lying areas
|
Back to top |
Flat low-lying areas occur in three regions, namely the Cape Flats, the
Sandveld region of the West Coast, and the undulating landscape of the north-north-eastern
parts of the CMA. The first two are overlain by significant depths of Quaternary,
calcareous sand, to the extent that bedrock geology is not mapped. From an agricultural
perspective, where irrigation water is available, such areas can be used for intensive
market gardening and potato cultivation. However, it is these areas which are currently
experiencing an explosion of urban development. This includes the informal, and low-cost
formal housing of the Cape Flats, and the middle income development of the West Coast
(Table View) as well as developments such as Century City. Such areas do not place any
particular constraints on construction. In fact, the nature of the sandy substrate
facilitates the digging of foundations and the emplacement of infrastructure (pipes,
cables). However, the low-lying nature of these areas and their proximity to the ocean
implies a high local water table and, in specific areas, frequent waterlogging and
localised flooding in the winter months. Landfill sites, typically sited in such areas in
the past, are prone to seepage and groundwater contamination.
The third region undulating, north-north-eastern area of the CMA is underlain primarily by
shale. A variety of soil types occur. They are generally nutrient rich, fertile and
amenable to agriculture. Wheat and, in places, vines are grown. Provided they are
correctly managed (contour ploughing, for example) they are not particularly vulnerable to
erosion and will provide sustainable crop yields. However, construction should be
undertaken with caution and only after thorough examination of the substrate.
|
Back to top |
|
