Definition - What does Hardpan mean?
In the world of wine, hardpan refers to the dense layer of soil, such as clay, which is impermeable to water. It is the layer of dense soil that is found below the uppermost topsoil. Such soil composites consist mainly of soil deposits that bind and fuse together. The most commonly known hardpan is clay, however, hardpan may come in forms of matrices from iron oxides and calcium carbonate or dissolved silica.
WineFrog explains Hardpan
Hardpan is a type of soil composition which may be found directly beneath the topsoil layer. It is a layer of soil which does not allow water to freely flow. In the case of vineyards with hardpan, this is often seen as an advantage. The topsoil layer of most vineyards is of low nutrient quality and drains well. If such soils have a hardpan layer, this is advantageous as it can retain water for longer periods. This aids in the vines' survival for dry farming and during periods of excessive drought.