Since Uganda is within the tropics, it means that rates of chemical weathering are very high hence responsible for the formation of flat topped hills in the central region.

Ideal Conditions for laterisation

Gently sloping areas or depressions where regular flooding leads to the weathered layer becoming impregnated with iron or aluminium compound solutions. The iron and aluminium solutions originate from the leaching of rocks and older laterites along the surrounding slopes and are washed into the weathered layer by laterally moving ground water and surface drainage.

An alternating wet and dry savannah climate so that the exposed laterite hardens into a crust due to the wetting and drying which washes out the clay and crystallises the iron or aluminium.

The removal of top soil e.g. by destruction of the vegetation and in this context man plays a significant role.

Characteristic Features of Laterisation

a) Deep and intense chemical weathering of the surrounding country rock under humid tropical or sub tropical conditions.

b) Abundant supply and rapid decay of organic matter.

c) Mildly acid to mildly alkaline reaction which results from the rapid process of mineralisation aids the removal of combined silica and the accumulation of oxides of iron and aluminium forming a hard material. This process of desilication is crucial to the laterisation process i.e. "the process by which silica is leached and iron or aluminium are concentrated."

d) Decomposition of clays into Kaolinite

e) Formation of a stratified profile consisting of an indurated zone or laterilic crust, a mottled zone and a pallid zone overlying unwealhered, weathering or ordinarily weathered country rocks. This occurs under a fluctuating ground water level and on extensive plain lands.

As earlier mentioned under the process of hydrolysis, laterile is made up of Kaolin and oxides of iron and aluminium in varying amounts. Laterite is also made up of unweatherable minerals such as quarts. Generally laterites exist in form of hard layers but its structure may vary depending on the mode of Origin.

Where laterites exist "in situ", (i.e. mass of weathered rock that has not been removed by agents of transportation), it appears as a continuous mass with many small branching channels and cavities resulting from solution effects or from termite burrowings. Later, secondary deposits of iron and aluminium oxides fill up these channels and cavities and become hardened. These massive laterites are commonly found in the older erosion surfaces and remains best developed on Buganda surface where they may attain a thickness of up to 10 metres.

In East Africa laterites are found around Kampala. They are characterised by being flat topped hills and are separated by broad valleys some of which contain papyrus swamps. This landscape is commonly known as the "Bugfimla Landscape" or Catena. That is in hilly or mountainous areas soils are closely related to relief so that in only one climatic region different soil types occur below the plateau hill slopes and the valley floor. Hence the term Catena is used to describe any regular repetition of soil sequences down a slope due to topography and change in parent material.

Types of Rocks, Weathering and Slope Development

The diagram below illustrates the Buganda Catena

Many of the heavily populated parts of East Africa are located a way from these erosion surface.

Economics Importance of Laterites

a) Lateritic soils are often resistant to ploughing and mechanical cultivation especially if they are capped by duricrusts.

b) Lateritic soils are poor in mineral salts due to prolonged leaching which has in the past removed mineral salts from surface layers of the soil to deeper layers. This partly explains why tropical soils of this nature are not very productive.

c) Laterites are stable and can therefore be quarried for construction of roads and buildings.

d) Where the mineral content is high it can provide seams of valuable mineral ores such as iron and bauxite.

e) The slopes of lateritic hills provide some pasture for animals since these areas tend to have sparse population. However, due to scarcity of building space, man is seriously encroaching these slopes for construction surfaces e.g. Mulago, Naguru and Kireka hills around Kampala.

ii) Formation of Inselbergs and Plains

An inselberg is an island hill or mountain formed by weathering and erosion and rising abruptly from a plain or from relief. Inselbergs are commonly associated with granitic rocks. They are remnants of older higher peneplains which have been removed except for these island hills that have proved more resistant rocks within the basement complex.

a) Types of Inselbergs

Bornhardts and Domes

These are steep sided dome shaped hills formed mainly in massive crystalline rocks such as granite or gneiss.

b) Castle Kopjes

These are steep sided piles of massive crystalline boulders. Such hills often *have a castle-like profile hence their name.

A number of inselbergs are undoubtedly composed of more resistant rocks than the surrounding plains and for this reason appear as upstanding residuals. But the majority are formed in rocks identical to the adjacent plains and they seem to be clearly related to the origin of the plains.

Origin of Inselbergs and Plains

Two theories help to explain the origin of plains and inselbergs.

a) Pediplanation (Parallel Slope Retreat and Pediment extension) in Arid Regions

According to L.C. King, hill.slopes are initiated as valley sides after a stream erodes vertically along its bed, or as a result of earth movements. As the destruction of an older and higher plain takes place along the scarp face,

Types of Rocks, Weathering and Slope Development ⁷⁷ a new plain develops but is separated by a steep slope. At the scarp where rivers fall from the original higher surface to the lower one, erosion is active and in conjunction with weathering and mass wasting, causes the gradual retreat of the scaip as shown below.

PARALLEL SLOPE RETREAT AND PEDIMENT EXTENSION

At the foot of the scarp, a rock surface of low gradient called the pediment slopes gently down to the plain (new surface). On the pediment erosion is active and sheet flooding is the main process. This is sufficient to cause the extension of the pediment a process known as scarp retreat. The greater erosive activity on the scarp helps to maintain the knick i.e. the sharp break of slope between the two surfaces i.e. the new and old surfaces.

As the process of scarp retreat goes on, the pediments are gradually extended until all that remains is a residual hill rising above the plains and inselbergs, the final remnant of an original higher planation surface. Initially the inselbergs would be large and of bornhardt variety but eventually they are broken down into castle Kopjes by weathering attacking the joint planes. Finally when inselbergs are worn away, the pediments coalesce to form a continuous plain called a pediplain. The diagram below shows stages of pediplanation.

b) Deep Weathering and Stripping (Etchplanation) in humid regions

Deep weathering by chemical decomposition produces thick layers of water which accumulate on top of fresh unweathered rock. This results in an irregular basal surface.

Deep weathering is then followed by etching or stripping episodes when parts of the weathered mantle are removed, thereby exposing parts of the basal surface as hills called bornhardls or inselbergs. The stripping episode starts with streams cutting into the weathered layer due to: i) Either a fall in base level due climatic change or ii) Uplifting may initiate a new cycle of erosion.

The overall effect is to rejuvenate the activities of streams. Once the bornhardts are exposed surface, weathering and erosion begin to attack the curved joint systems leading to exfoliation thus forming castle Kopjes.

Examples of inselbergs are prominent in Northern Uganda e.g. Parabong in Kitgum District is about 2,000 ft above surrounding plain. Other examples include Labwor on the Moroto-Kitgum district. Elsewhere examples can be cited at Nakasongola in Luweero district, in Teso, and from Nabingola in Mubende district to Kyenjojo in Kabarole along Kampala-Fort Portal road.

 

Another evidence of granitic landscape is the Arenas of Ankole. These are areas which have circular amphilheatre-like appearance and are surrounded by steep inward facing slopes.

Other examples of inselhergs are found in Central Tanzania (Dodoma). In Kenya they are found around Maragoli hills and Voi hills. Other places in Tanzania includes Iringa, Morogoro and Bismark Tor in Mwanza (Sukuma land).

Weathered rock is stripped away leaving fresh unweathered rock as inselberg bornhardts.

c) Deep weathering continues. Subaerial erosion attacks bornhardts to form Castle Kopjes.

Economic Importance of inselbergs:

a) They can be quarried to provide materials in road construction and building houses.

b) In some cultures stones from inselbergs are used for grinding food stuffs or for spreading them to dry.

c) Inselberg landscapes generally inhibit agriculture because soils from these areas are very old (they are remnants of the basement complex) and therefore of limited fertility.

d) They may present impressive scenery such as castle Kopjes, the arenas of Ankole and can he tourist attractions.

e) Locally they may induce relief rainfall.

f) They pose communication problems i.e. roads have to go round the hills.

iii) Formation of Karst Landscapes

These are landscapes associated with limestone and dolomite rocks. Because these rocks are soluble in acidulated water, (carbonic acid) they form distinctive landscape known as Karst.

Characteristics of Karst Scenery

a) Surface drainage is intermittent or absent. Streams that exist rarely flow for long distances but disappear underground through solution hollows.

b) There are outcrops of bare rugged rock and steep sided valleys.

c) There are numerous network hollows and residual hills of various sizes.

d) An underground network of caves and water courses exist.

The action of carbonation working along the bedding planes and joints in the rock leads to the formation of a series of land forms as illustrated below.

UNDERGROUND CAVES AND RELATED FEATURES

(a) Limestone Pavements

This is a bare rock surface that is criss-crossed by many grooves where solution has worked along the joints. It appears as a number of narrow ridges called clints divided by hollows known as grikes which may be up to one metre deep.

(b) Dolines

These are shallow depressions or hollows with gently sloping sides generally circular or oval in plan. They originate from water percolating underground at the intersection of major joints. The rocks are slowly dissolved and a small basin is formed which is then enlarged later.

(c) Sink holes

This is a deep hole with nearly vertical sides leading to an underground cave system. Some result mainly from surface solution and subsurface collapse.

A sink where a river disappears underground and is lost from surface drainage is often called a swallow hole or sink hole.

d) Underground Caves

These are natural underground chambers in the rock usually joined to the land surface by a system of interconnecting shafts and galleries. The main factors in cave formation are solution effect, together with mechanical erosion by underground streams and also occasional rock collapse. Ground water circulating in the zone below the water table dissolves limestone and forms cavities by attacking joints and bedding planes. As the water table falls, Vadose water percolating down through the rock continues the process of enlarging the cavities. Finally rivers taking underground courses may increase the size of the cavities by stream erosion.

In many caves there are stalactites, stalagmites and other calcite deposits. A stalacdte is a mass of calcite (crystalline calcium carbonate) hanging down from the roof of a cave, while the stalagmite grows up from the floor. These deposits form as water seeps through cracks in the rocks and drips from cave ceilings and walls. Carbon dioxide escapes from the water causing some of the dissolved calcium bicarbonate to change back into calcium carbonate.

Examples of limestone pavements with numerous gorges, sinks, dolines and other depressions are widespread near Kiomoni and Kange on either side of Mkulumuzi valley in Tanzania.

iv) Formation of Residual Clays

During hydrolysis water becomes a reactive agent especially with silicates leads to the decomposition and breakdown of feldspars which leave behind a residue of clay e.g. Kaolinite, which is used for manufacture of plates and cups. Hydrolysis also leaves a residium of another type of clay known as montmorillonite which is the basis for clay industry at Kanjansi Clay Works situated on Kampala-Entebbe road.

Exposure of Minerals

v) Prolonged chemical weathering under successive erosive phases have also helped to expose minerals e.g. The Tororo volcanic plug is the basis of cement industry in Uganda.

vi) Frost weathering in upland areas of East Africa besides facilitating the processes of soil formation e.g. along Mts. Kenya, Kilimanjaro and Ruwenzori also has created impressive glacial features that contribute significantly to tourism.

Movement of Materials along Slopes

Slopes originate from a number of ways e.g. from faulting, coastal cliff formation, valley erosion by streams and glaciers and moraine deposition. Material is moved down slopes by two main processes namely, by mass wasting and down wash. These two types of movement remove material that has been weathered and thus enhancing continuous weathering.