Humans impact on soils by damaging the structure of the soil (the shape of the soil peds). When the peds of soil are rounded it gives soil a crumby structure, which is the best structure for soil fertility. Overgrazing occurs when large numbers of animals are allowed to graze in one area for too long. In the Sahel region, animal ownership is considered the main criteria for defining wealth. Nomadic farmers allowed too many animals to graze the land than it had the ability to support, resulting in desertification. This is where the soil cannot sustain the herd numbers and is therefore gone beyond its ‘carrying capacity’. Herders then moved to marginal areas, which made the situation more dire. Alongside this nomadic farmers were gradually replaced by more sedentary farming practices and villages were established. Wells were sunk to provide water for animals; however, these wells used up groundwater further reducing soil fertility in the region. With an increasing population, more pressure on the land meant that farmers were forced to farm the land more intensively and remained in the one area for a longer period. Once the grazing animals removed the vegetation, the soil was exposed to the weather (wind erosion , also known as Aeolian erosion) resulting in soil erosion and eventual desertification.