The physical environment
11 Sep 2009

The geological formations present in the Pietermaritzburg area consist simply of an ancient (1 000 million-year-old), solid, but greatly contorted Basement, and a succession of more recent and much less deformed sedimentary cover rocks overlying the Basement. Intrusions of molten rock here and there penetrated, some 180 million years ago, these formations along fissures, gradually to solidify as dolerite in the form of vertical 'dykes' and horizontal 'sills' .

The Basement is not exposed in the immediate vicinity of the City, being found rather in the valleys to the east of Pietermaritzburg, and particularly in the Valley of the Thousand Hills. The more recent cover rocks resting on the Basement lie horizontally, or may be inclined gently to the west. These rocks may be divided into three groups, the Natal Group, the Karoo Sequence and the most recent deposits, which belong to the Pleistocene Period.

The Natal Group is the lowest rock group and consists of sandstones, formerly rather well-known to local inhabitants as Table Mountain Sandstone, or TMS. This sandstone was originally erosional material deposited over the area by large southward-flowing rivers; after sufficient time and compaction the sediment was transformed into a sandstone plain above the Basement. Long-term subsequent erosion of this plain left many resistant remnants or 'outliers' in the region, the most prominent being Table Mountain itself. 'Conformal' summit levels surmounting the Valley of the Thousand Hills, and including that of Table Mountain, are also very noticeable remnants of this former plain.

Above the Natal Group sandstones are two formations belonging to the Karoo Sequence, i.e. the Dwyka Formation and the Ecca Group.

The Dwyka Formation provides evidence of huge continental ice sheets, at least 1 000 m thick, which moved from the north-east over the region about 300 million years ago, planing the underlying surface and depositing glacial sediment referred to as till (which later compacted to tillite). The undulating landscape east of the City is underlain by glacial debris while a veneer of the tillite on a polished glacial pavement is to be found on Table Mountain.

A period of less rigorous climate followed the glacial episode so that the deposits overlying the Dwyka Formation had their origins in rivers flowing from the glacier, and huge rive r deltas, rather than in ice. This new sedimentary succession (in its compacted form referred to as the Ecca Group) was deposited on top of the glacial deposits following the melting and gradual withdrawal of the ice around 250 million years ago. Following compaction of the new sedimentary material it consolidated into the well-known horizontally-laminated shales of the Pietermaritzburg area.

The Ecca Group is customarily divided into three formations, the Lower, Middle and Upper Ecca. The Lower Ecca or Pietermaritzburg Formation consists of shales and siltstone. Such material is used in the local manufacture of bricks - an iron oxide content giving the familiar red brick colour to many of the City's buildings. The shales are often exposed in road and railway cuttings as well as in excavations for swimming pools. Not all of the shale is suitable for the construction of crazy paving or buildings, but, should it have been hardened by being in contact with intrusive dolerite dykes or sills, or should it not yet have been weathered, then its hardness is such that it may be used in various ways. The shale foundations and walls of some of the older buildings of the City have been constructed from such hardened shales.

Next above the Lower Ecca are the Middle Ecca (Vryheid Formation') sandstones and shales. These sometimes occur as outcrops in the area overlooking the City, for example near World's View and in the kranses of Otto's Bluff. Sandstones were quarried at Sweetwaters in earlier days for building-stone while the Cascades near Queen Elizabeth Park are on sandstone beds.

The Upper Ecca (Volksrust Formation) shales and sandstones are generally poorly exposed in the area; they underlie much of the Cedara-Howick region. The most recent deposits of material in the local area belong to the Pleistocene Period (beginning about 1 million years ago). Alluvium, for example, is found along the flood plain of the Msunduze River; the floodplain is particularly wide in the Edendale area. Former landslides have created hillside scars while associated unconsolidated debris (talus and soil) has created the hummocky terrain in the Queen Elizabeth Park and Country Club area. Such landslides occur when water seeps through the pervious Middle Ecca sandstones to meet the underlying impervious plastic, clayey Lower Ecca shales. The N3 double carriageway in the vicinity of Queen Elizabeth Park has suffered more than once from such slipping. Railway geologists strongly recommended avoiding the area for the reconstruction of the Natal Main Line and instead the SATS went to the great expense of building the Cedara tunnel at depths where this would not be a problem.

Above: Two dolerite dykes.

Above: Wall of snuffbox shale in Loop Street, demolished in 1983.

The dolerite sills are most commonly noted in the Ecca Group of shales and sandstones. One resistant sill forms the crown of Swartkop, another that of World's View, while still another provides the raw material for a major quarry near the Greytown road. Loose dolerite boulders (with which many local gardeners must come to terms) are formed by 'spheroidal' weathering. Penetration of air and water, along the cooling or tension joints and crevices of sills of rock, weaken these areas and the continuous weathering leaves rounded residual cores of, often, excellent rockery boulders. Along such joints in the dolerite and adjacent baked or 'indurated' shale, water may percolate to well below the water-table, often to be extracted from boreholes in times of drought.

Above: A dolerite quarry.

River erosion by the Msunduze and its tributaries in the Pietermaritzburg area has created the roughly 8 km x 12 km x 300 m basin in which the City is situated. There is a gap to the south-east of the City through which the Msunduze flows to join the Mngeni River. The basin is surrounded by dissected spurs and rounded hills and is one of many such basins in the Natal Midlands. A well-defined sinuous escarpment rises 300 to 400 m above the City on the west and north-west sides; it is situated along the outcrop of Middle Ecca in a region where dolerite sills cap the sandstone and shale. These resistant sills are mainly responsible for the high ground in the area. Erosion in the basin has left a number of ridges between the Msunduze and the Dorpspruit. The spur runs from Fort Napier through the centre of town and tapers off. The soils occurring in and around Pietermaritzburg owe their distinctive characteristics to an interplay of geological, climatic and topographic factors.

Left: Taking out shale to put in a swimming-pool in Scottsville.

On the drier side of town (south and east of the Dorpspruit) three main categories of soil may be recognized : those derived from shale and from dolerite, and alluvial soils . In soils derived from shale, the topsoil is generally light grey-brown with a loamy texture. It sets very hard when it dries out. The subsoil may be partly weathered shale or impervious clay or a gravelly partly-cemented layer of iron concretions (ouklip or nkubanei. Either too hard or too sticky, the soils are not easy to manage. Mulches may provide temporary help but termites, warmth and humidity soon break these down. The soils are quite fertile, seldom needing more than nitrogen fertilizer for vegetable or flower gardens.

Soils derived from dolerite are usually clayey. They may be dark, chocolate or red coloured, depending on drainage. The dark variety may contain a clay called montmorillonite which, if subjected to extremes of wetting or drying, expands and contracts considerably, often cracking or shifting foundations of houses as well as providing a natural underground pruning action (which may prove fatal) on tree and other plant roots.

Such 'black turf soils may be used by groundsmen in the preparation of cricket pitches. The redder soils are physically better, being well-drained and quite fertile.

Alluvial soils are deep, somewhat silty soils found on the floodplain of the Msunduze and along the banks of its tributaries. They are fertile and easily irrigated. On the wetter side of town the soil pattern changes considerably. Given the cooler temperatures and higher rainfall, more leaching and weathering of soils have occurred. They are therefore deeper and fairly porous. Topsoils are enriched with humus and hold water well. Fertilizer or manure may be required because the leaching causes chemical infertility. In most cases the soil is acidic so that lime will be needed to counteract this, except where acid-loving plants such as azaleas and hydrangeas are grown.

Numerous factors give Pietermaritzburg its particular weather and climate - its latitude (300S), altitude (658m at the City Hall), distance from the sea, and, particularly, the local topography. Fig. 5 gives a summary of the main climatic statistics, taken from various sources. Variations of rainfall across the area are to be expected, especially in view of the marked change in altitude between the south-east and the north-west of the City. In particular there is a steady increase in the mean annual rainfall total from less than 800mm in the drier south-east to above 1 100mm on the wetter north-west. Variations from year to year are also very noticeable (for example 575 mm in 1941 and 1 533mm in 1917 at the Botanic Gardens), and the available rainfall data do not indicate either a long-term increase or decrease, or wetter and drier cycles.

Right: Pietermaritzburg's rogue storm, 12 February 1978.

Most of the rain is associated with the passage of large-scale disturbances such as depressions and cold fronts along the Natal coast which bring to Natal a cool southwest airflow, or with the frequent thunderstorms of spring and summer - either local convectional storms or those which originate in squall lines extending right across the country.

Some 60 per cent of the rain falls with an intensity exceeding about 2 or 3 mm/hour and between 20 per cent and 40 per cent with intensities exceeding 25 mm/hour.

Most high intensity rains, of whatever duration, begin between 16hOO and 17hOO. About 26 days during the year experience rainfalls greater than 10mm and thunder will be heard on 60 days a year.

Average monthly rainfalls are given in Fig. 5. The most rainfall recorded at the Botanic Gardens in anyone month was 468,9mm in March 1925. On 14 January 1947 a record 246,6 mm was recorded; the next highest was 117,3mmon 13 January 1915. Relative humidity (RR) data are seldom collected and are in any case meaningless unless considered against temperature. The RH values in Fig. 5 provide some idea of the fall-off in values as the temperatures increase to midday; the actual amount of water vapour in the air may or may not change over the same period.

If all the water vapour in the air over the City were to condense, the depth of the liquid water would be about 27mm (37mm in summer, 13mm in winter). If the vapour were not continuously replenished, the rainfall would completely dry out the air in about ten days. Only about 10 per cent of the rain comes from locally evaporated water; thus the planting or removal of trees and the presence of dams such as Midmar can have no effect on rainfall, since local rainfall is from water evaporated some ten days previously from areas which could be hundreds if not thousands of kilometres away.

Temperature variations also occur across the City. Suburbs on the cooler north or north-west side owe their lower temperatures in part to higher altitudes and in part to their earlier sunsets each day . More detailed variations are related primarily to the ventilation provided by local air movements. Thus temperatures at the sheltered Botanic Gardens site are lower than those at the exposed Ukulinga Farm site in winter because of earlier sunsets, but are higher in summer because of the comparative lack of ventilation.

Fig. 5 gives temperature data at the Botanic Gardens; the highest temperature ever recorded there was 44,4°C on 18 January 1966, the lowest -50C on both 2 July and 11 July 1934.

Temperature and humidity combine to produce a 'humiture' figure, or comfort index. Such data have not been mapped across the City but it is certain that it is the (high) humidity values in the basin rather than high temperatures that produce the feeling of discomfort in the City on many a summer day, especially when ventilation is low and the air is stagnant.

The humiture index may be obtained from temperature (0 °C) and relative humidity (%) data using a specially designed chart. There are probably between 3 and 5 days during the average summer in Pietermaritzburg when out-of-doors humiture values reach or exceed the dangerous 110 mark.

Pietermaritzburg faces wind from four main directions, although not with equal frequencies. (see Fig. 4, right) The main daytime wind in the City is from the east or south-east, a direction up the Msunduze valley. The southeast wind may not be the only local wind controlled in its direction by the delineation of the river valley; other southeast winds are linked to large-scale anticyclones ridging south of the country. They can bring prolonged rainfall to Natal. The south-east wind over Pietermaritzburg is more frequent in summer than in winter and has a greater speed on average in summer.

During the night the wind direction is mainly from the west or north-west, again largely along the Msunduze valley. It is also from the north-west that the unpleasant Berg wind blows. This wind is not related to the direction of any local valley but rather to a large-scale weather situation covering perhaps one-third of the country. Berg winds usually blow between April and September and bring with them temperature rises of perhaps 5 to lOoC. Fortunately when Berg winds die down they are often replaced by far more pleasant conditions, with a cool, cloudy movement of air from the south-west. Winds from the north-east are more common at the coast than at Pietermaritzburg but they do sometimes penetrate inland. The winds are related to the large anticyclones over the southern Indian Ocean.

 

Fog or smog?
by Owen McGee

Commonly during the early evenings of winter and especially when the sky is clear, when conditions are calm and when humidities are high near the surface, the land cools off rapidly through long-wave outgoing radiation. This cooling is communicated to the adjacent air which then increases in density and slips down sloping ground to form cold air pools in any depression, such as river valleys (locally the Msunduze and its tributaries are ideal sites) . With the cooler air now below, and warmer air aloft, a stable stratification known as a temperature inversion is established. Should the cooling continue to the dewpoint temperature, some of the water vapour in the air will condense to form a mist or fog.

The next morning the oblique rays of the rising sun will have difficulty penetrating the air, as will a car's full-beam head lights. This drop in visibility is therefore a natural occurrence and is not caused by 'pollution' . In a natural way, too, the mist or fog will lift by mid-morning, evaporate completely, and the vapour will be ready to start the whole cycle again towards sunset. The stable stratification may persist all day, though in an altered or weakened form.

However, should pollutants be released into the stable air, especially from low-level sources, the visibility may be reduced even further because many of the pollution particles are 'hygroscopic' - that is, they have an affinity for water which results in their growing larger by extracting water vapour from the air . In this way a pall of smoke is added to the air and becomes trapped in the inversion layer.

Thus when the Pietermaritzburg basin is supposedly shrouded in 'smog', part will be anthropogenic (smoke) and part natural (fog)- hence the word 'smog' . The ratio of smoke to fog over Pietermaritzburg is unknown, but the monthly average smoke levels recorded at the Municipality's six stations leave little room for complacency on the part of householder or industrialist. The only other atmospheric constituent monitored at present is S02 - an invisible substance which, when combined with water in the atmosphere, produces an acid.

In addition to these two (the dust and S02) there are undoubtedly many other pollutants present in the air . It is probably t rue that at present the average citizen overreacts, particularly in w inter, to the visible inversion layer, and is blissfully unaware of t he possibly far mo re injurious, and virtually un monitored, invisible pollutants. Bearing in mind local topography, the present level of pollution, and the incompleteness of the pollution picture, one should perhaps firmly endorse the conclusion reached by a CSIR ventilation study: 'all industries subject to smoke control should be situated at least 100 metres above the floor of the Pietermaritzburg Basin and, better still, above the 762 metre (2500 ft) contour line .. .' (Liebenberg, 1976).

Source: Pietermaritzburg 1838-1988: a new portrait of an African city, edited by John Laband and Robert Haswell (Pietermaritzburg: Univerversity of Natal Press and Shuter & Shooter, 1988) pp.6-10.