An aquifer in which a ground water body is separated from the main ground water below it by an impermeable layer (which is relatively small laterally) and an unsaturated zone. Perched aquifers are common in glacial outwash, where lenses of clay formed in small glacial ponds are present. They are also common in volcanic depositional sequences where weathered ash layers of low permeability are sandwiched between high permeability basalts. Water moving downward through the unsaturated zone will be intercepted and accumulate on top of the lens before it moves laterally to the edge of the lens and seeps downward to the regional water table or forms a spring on the side of a hillslope.
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