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This paper describes the microaggregate structure of residual clay soils of the red earth order in East and Central Africa, examined by low-power microscopy, and the effects of gradual moistening and sudden wetting of the soil. Results showed a markedly angular microstructure termed fragmental, resulting from simple cleavages in the soil mass, a microgranular structure involving minute spherical bodies, either discrete or in small compound aggregates, and two types of intermediate structure. The several types of microaggregate and associated profile characteristics are described in detail, and are compared with those studied more recently in West Africa. The microgranular soils also contain larger pellet bodies originating as residual cores of rotted rock. Present evidence for the distribution of these soils is outlined and possible factors in their occurrence are reviewed, including age of land surface and termite activity.?Enquiries about this paper should be sent to R. Webster, Rothamsted Experimental Station, Harpenden, Herts AL5 2JQ.

Summary Development of the Soil Pattern As an outcome of an ecological reconnaissance this account provides a record of the occurrences of two major regional types, a system of consolidated sand dunes and derived sands, referred to here as the Chestnut Sands, and a wide assortment of stony truncated ‘desert pavement’ soils or soil remnants described as Stone Mantles. The latter involve areas both of basement complex and of lava origin. Briefer observations are also given on other soils of lesser extent, including flash‐flood, alluvial and lacustrine deposits. Although a present‐day desert or semi‐desert, the south Turkana region bears the stamp of successive erosion cycles, which have largely influenced its soil pattern. Modified remnants of the original end‐Tertiary surface are probably discernible in the higher‐lying stone mantles about the central hill ranges (Section 2, Introd.). Residual stone mantles of this class appear to be the last truncated remnants of an ancient soil cover. Ancient alluvial and lacustrine deposits, the former of conjectured and the latter of established Pleistocene age, have provided the material for other stone mantle areas (Section 2 (b)). The present surface of all these has been affected by more recent erosion and deflation. On top of these several types of stone mantle, or the stony soils from which they originated, seems to have been deposited the deep mantle of the Chestnut Sands, blanketing at one time the greater part of the Turkana plain. These dune sands are regarded as a mainly aeolian drift, deposited during a period of active arid erosion, though their materials may have been derived from older surfaces (Section I, Introd.). They must be subsequent to the deposits which they overlie, and may possibly be compared with the Goz or Kordofan Sands of the Sudan, which have been very tentatively referred to the late Pleistocene (Andrew, 1948). As with the Goz Sands, the dunes of the Chestnut Sands presumably became fixed under less arid conditions. Finally, in common with the rest of the Turkanalandscape, they have been subjected to the present régime of brief seasonal flash‐flood erosion and intervening wind action. Their formerly extensive dune structure has been greatly modified and from many areas they appear to have been removed wholesale. The redeposited Chestnut Sands, the flash‐flood deposits and sandy riverine alluvia are the outcome of this latest phase of erosion. At the same time tracts of comparatively recent lacustrine sands and possible deltaic silts (Section 4 (b) have been left in the east of the region as a result of the progressive retreat of Lake Rudolf. The influence of the resulting soil pattern on the vegetation of the region is very briefly indicated, with reference more particularly to the principal shrub components.

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