**6. The influence of geology**

The general influence of lithology on soil erosion processes is manifest directly by the resist‐ ance of the denuded bed rocks exposed to the flow of water and affected by the character of parent materials whose properties are given by the bed rock. The direct effect of bedrock is also manifest in the properties of the soil forming parent materials which conditions the principal properties. Some geological materials are vulnerable than others to aggressive en‐ ergy of the rainfall and runoff. High erosion risks match with units of weak unconsolidated geological formations. This is more pronounced when such geological units coincide with medium to long and even very long slopes with marked gradients.

clay content, level of soil organic matter (SOM) and sesquioxides such as Al and Fe oxides, clay dispersion ratio (CDR), mean-weight diameter (MWD) and geometric-mean weight diameter (GMD) of soil aggregates all influence soil erosion hazards in southeastern Nigeria. SOM, Al and Fe oxides control dispersion and flocculation of the soils. In the event of very aggressive rainfall, the soil inherent properties often combine with the physical forces of rainfall to pro‐

Gully Erosion in Southeastern Nigeria: Role of Soil Properties and Environmental Factors

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Erodibility varies with soil texture, aggregate stability, SOM contents and hydraulic proper‐ ties of the soil. Igwe [22] claimed that the soil dispersion ratio (DR) and the clay dispersion ratio were good indices of erodibility. The soils with high water-dispersible clay (WDC) in southeastern Nigeria often create problem in that in tilled land use, mud flow and soil loss from runoff cause major alteration in the stream flow within watersheds causing severe en‐ vironmental challenges. Soil crusting, sealing resulting from aggregate breakdown are sec‐ ondary problems arising from deposited sediments. The large particle sizes are resistant to transport because of the greater forces required to entrain these large particles while the fine particles are resistant to detachment because of their cohesiveness. Aggregate stability and associated indices have been shown to be most efficient soil properties that predict the ex‐

In other parts of the world the use of aggregate stability indices in predicting soil erodibility have shown reliable information on the extent and degree of soil erosion [23, 24]. In Western Europe, Le Bissonnais [25] indicated that the mean-weight diameter (MWD) of soil aggre‐ gates was a very reliable soil property that could show the erosion potential of the soil in the sense that MWD predicts soil erodibility. Therefore aggregate stability and MWD are very

duce soil erosion in the soils.

**Figure 3.** Gully cutting

tent of soil erosion.

In Nigeria, Ofomata [1] classified the potential erosion susceptible areas based on underly‐ ing geology. He indicated that areas of high susceptibility correspond to geological regions of weak unconsolidated sandy formations while least susceptible areas are within the con‐ solidated tertiary to recent sediments. Also in southeastern Nigeria, the classical gully sites are located in the False-bedded sandstone, Coastal Plain sands, Nanka Sands and the Bende-Ameki Formations. These are all sandy formations which have more gullies than their Shale formation counterparts. In these formations, there exist the sites of worst catastrophic soil erosion in the whole of sub-Saharan Africa. The geology therefore plays direct and indirect influence on the gully formation. The indirect effect is on the soil formation and the nature of soil which contribute significantly to erosion processes. The influence of soil process on soil erosion often referred to as erodibility is the subject of discussion in the next section.
