The hydrogeological map indicates aquifer potential in generalised terms using a threefold division of geological formations: those in which intergranular flow in the saturated zone is dominant, those in which flow is controlled by fissures or discontinuities and less permeable formations including aquifers concealed at depth beneath covering layers. Highly productive aquifers are distinguished from those that are only of local importance or have no significant groundwater. Within each of these classes the strata are grouped together according to age or lithology. The 1:625 000 scale data may be used as a guide to the aquifers at a regional or national level, but should not be relied on for local information.