sentences:  Geologically, the aquifer is part of what is known as the Yarragadee Formation, which is a relatively thick geological unit in the Perth Basin.Formed during the Jurassic Period, the Yarragadee Formation is composed primarily of non-marine fluviatile feldspathic, poorly sorted sandstones which are porous and poorly cemented, hence allowing for considerable groundwater reserves.It grades from a shale-siltstone dominated base to a cleaner sandstone in the upper portions of the formation, probably representing increased subsidence or filling of the basin during the late Jurassic.