an engineering geological characterisation of tropical clays - GBV

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neighbouring metamorphic areas and deposited by rivers and/ or surface run-off into the
present area. Alteration of kaolinitic red soils deficient in alkali and alkali earth metals (Na,
K, Mg, Ca) could have also caused the formation of the respective aluminiumsilicates of the
heavy minerals. The altered soils were then eroded, transported and deposited in the lower
areas of black clays.
The black clays generally exhibit sharp to fairly sharp contacts with the underlying volcanic
rocks. Cases of patches of black clays overlying red soils were observed in the northern and
north-western sections of the study area. In other parts of the study area, laterite was
commonly found underlying the black clays. In addition, areas with black clays having been
removed by erosion show the underlying rock weathering through red. The above four
observations contradict the suggestion that the black clays could have been derived from the
underlying volcanic rocks by weathering.
The neighbouring areas further eastwards are underlain by metamorphic rocks, and commonly
exhibit a line of quartz stones between the black soils and rock (Scott, 1963; Saggerson,
1991). However, no quartz veins have been observed within the black clays; and this is
contrary to what would be expected if the soils had actually developed in situ, suggesting
once again that the black clays may have not derived from the weathering of the underlying
rocks. In the present study area underlain by volcanic rocks, pieces of quartz stones are found
randomly imbedded in the black clays. This could point to the possibility of the quartz stones
and other materials as having been transported from the surrounding areas into the present
area by erosive and depositional processes.
The general morphology of the area of occurrence of black clays in the present study area and
neighbouring areas further eastwards is roughly basin-like. The eastern and western margins
of this basin tend to follow the 5200 feet contour, while the northern margin is at an altitude
of about 4800 feet. The altitude of the southern boundary of the soils is up to 6200 feet in the
vicinity of Ngong hills. The soils are found occurring on both flat sites and appreciable slopes.
However, isolated patches of black clays occurring on high grounds are also found; and are
possibly part of the same earlier and larger formation, which later underwent erosion leaving
behind only relics of the soils.
The properties and characteristics of black clays discussed above serve to conclusively
suggest that these soils are not wholly developed in situ. A more plausible explanation as to
the genetic origin of the black clays would be based on the existance of a large basin in their
present areas of occurrance, prior to the formation of the soils. The basin was bounded by the
Machakos hills to the east, the Ngong hills to the south and the high ground of the eastern
flanks of the Rift Valley to the west. The northern margin of the basin was most probably
associated with the Ithanga-Kakuzi hills. The lowest point of the basin was most probably
around Athi River Township. The basin was filled with water during a wet cycle, forming a
lake. Materials of colluvium and alluvium from the surrounding hills were later deposited
into the lake. Portions of the basin within and/ or in the vicinity of metamorphic rock areas
were therefore characterised by more sandier variants of black clays than areas of the basin
within volcanic rock areas and/ or next to volcanic hill masses. This is also a possible
explanation for the presence of sandier variants of black clays in the current study area, which
is underlain by volcanic rocks. In addition, there is a high possibility of volcanic ash having
also been deposited into the lake during this period; and this could account for the high
presence of smectites in the black clays, as detected during mineralogical (X-ray diffraction)
analyses performed on the soils in this study (Table 5.9).