Sections of a geological formation that exhibit similar hydraulic properties (see hydrogeology), regardless of their composition. These units may be very extensive. The region of Alberta containing the Athabasca tar sands, for example, covers approximately 67 260 km² and comprises three hydrostratigraphic units. These consist of Cretaceous and Holocene sediments with alternately vertical and horizontal water flow, Upper Devonian strata with predominantly horizontal flow, and formations lying beneath evaporites where water flows horizontally towards a fault zone. The concept of hydrostatic units is important in planning, when the movement of groundwater must be predicted, and in tracing the source of groundwater pollutants.