Recent highway developments in the north-east of Nigeria have enabled a comparison to be made between black cotton soils of residual and lacustrine origin. Those residual clays derived from weathered basalt in the Biu area are rather less plastic than the lacustrine or lagoonal clays of the Chad basin to the north and have markedly higher silt:clay ratios. Montmorillonite is the predominant clay mineral in both groups but the residual clays have significantly higher relative proportions of quartz to kaolinite. Organic contents are low for both groups of clay. (TRRL)
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Ackroyd et al. (1986) studied this question.
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