The Permian rocks in the central and southeastern expanse of the Lower Hunter Valley were formed when the area was underneath a shallow marine estuary. The remnants of this period has left an extensive network of coal seams that fuelled the early population boom of the Hunter Valley in the 19th century as well a high degree of salinity in the water table of much of the area. The further north and west, towards the Brokenback Range and the Upper Hunter, the more Triassic sandstone that can be found leading eventually to the carboniferous rocks that form the northern boundary of the Hunter with the New England Fold Belt and the foothills of the Barrington Tops.