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The basin is divided into two major physical units. To the east, rocks of the pre-Cambr ian 
Basement Complex (intrusive granites, extensive biotite-gneiss with pegmatites, metamorphic 
quartzites and phyllites) form what have been called "the High Plains of Hausaland". To the 
west, sedimentary rocks (sandstones, grits and shales) form the rolling ''Sokoto Plains". Although 
the plains display little relief, they represent several erosion surfaces which are separated by low 
scarps. The whole basin is traversed by shallow but broad valleys separated by inconspicuous 
watersneds, 
TABLE 1. MEAN MINIMUM AND MAXIMUM TEMPERATURES AT SELECTED 
STATIONS IN THE SOKOTO-RIMA BASIN, DEGREES C, 1951—1980 
Station Mean Min. Mean Max. 
Low Av. High Low Av. High 
Sokoto 20.6 21.0 22.6 33.4 34.9 37.4 
(1961) (1973) (1955) (1970) 
Katsina 17.0 18.9 20.2 32.1 33.6 34.8 
(1961) (1980) (1961) (1973) 
Gusau 18.4 19.3 20.2 33.5 34.0 34.0 
(1965) (1972) (1961) (1969) 
Source Meteorological Office, Oshodi, Lagos 
Surface soils in upland locations are fairly uniform, consisting of brown to light brown sandy 
loams, They are well drained, but with sand fractions of up to 75%, they are subject to wind 
erosion (Agyepong, 1984). In the lower areas, soils are more variable and range from clay loams 
to sardy-clay cloams, with browish clay surface soils over sand which are usually stratified. 
Organic content is higher, and these soils often have higher levels of moisture retention and ferti- 
lity; some are hydromorphic. The greater part of the basin lies in the Sudan Savanna vegetation 
zone (Fig. 1), and under natural conditions would consist of mixed combretaceous woodland 
with a dense understory of deciduous erect and scandent shrubs, with grasses limited to occa- 
sional open patches { Adejuwon, 1971). The woodland is now much altered by persistent human 
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