Both these lakes lie in the Otavi mountain-land, which consists of a thick succession of well-stratified dolomite and limestone about 700 million years old. Being carbonates of calcium and magnesium, these rocks are soluble in water, especially if they contain some carbon dioxide. The rocks are criss-crossed by a system of solution channels that have generally developed on joints, fracture zones or bedding planes, which become partially filled with groundwater. Now and then big cavities are exposed by weathering, or the roof caves in when it becomes very thin, as in the case of Otjikoto and Guinas. These solution channels lead away from them, although the two lakes need not necessarily be directly connected. The lakes are fed by water seeping through porous rock from southern Owambo.
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