Swelling, cracking, high activity clay (smectitic) soils are found on basic parent materials in semi-arid to sub-humid climates, especially in lower lying landscape positions.
They are pedologically charismatic, exhibiting slickensides (see photo), haploidization and pedogenic aggrandizement, with a tendency for the solum to become inverted (gilgai micro-relief is an extreme expression).
Typically dark coloured (sometimes red or grey), base-rich and chemically fertile, they are physically challenging to flora, fauna and crops and anathema to engineers but can be highly productive under careful husbandry, especially when irrigated.
They support sweet natural veld and are strongly buffering towards water and chemical substances. Smectitic clay is favoured as bulli for cricket pitches and as an impermeable liner for dams and landfills. (Soil forms: Rensburg, Arcadia)