nappes

Nappes is the plural of nappe in French and appears in specialized contexts in geology and hydrology. The word nappe itself means a sheet, cloth, or layer, which gives a sense of a large, continuous piece of material spread over others. In everyday French, nappe also means a tablecloth.

Geology: In structural geology, a nappe is a large, coherent sheet of rock that has been moved far from its original position by compressional tectonics and thrust faulting during mountain building. These sheets can travel tens to hundreds of kilometres and cover substantial thicknesses, forming stacked units that reveal deeper rocks at the surface. Alpine nappes are among the best-known examples, including the Penninic, Helvetic, and External nappes, which record the tectonic history of the Alps.

Hydrology: In hydrology, nappe phréatique refers to a groundwater body. A nappe can be unconfined (nappe libre), where the upper boundary is the water table, or confined (nappe confinée), where the aquifer is overlain by an impermeable layer and pressurized. Groundwater nappes supply wells and springs, support surface ecosystems, and are affected by recharge, extraction, and contamination.

Nappes thus denotes different kinds of sheet-like formations in earth science, reflecting distinct processes: tectonic transport of rock in geology, and saturated subsurface water in hydrology.