The wet rockhead areas are zones where natural, very gradual dissolution of salt deposits has occurred over a timescale of millions of years. Several hundred metres of salt deposits have been dissolved due to continued natural dissolution of the upper surface of both the Upper Keuper Saliferous Beds and Lower Saliferous Beds since the Triassic period some 145 million years ago. However some localised features can be attributed to post-glacial dissolution. Such subsidence hollows are well developed above the Upper K in the Middlewich/Sandbach area where the wet rockhead is closer to the surface. Crater subsidence features are prevalent in some areas and are attributed to the dissolution of salt deposits beneath freely draining sandy superficial deposits, causing subsequent ground movement. The presence of some linear subsidence hollows within Cheshire predates any brine-pumping activity and is confirmed by the thickness of peat in some instances and by topographic evidence such as the use of such features as field and parish boundaries. It is likely however that many such hollows may have been deepened and lengthened by the subsequent extraction of natural brine along the line of the historical hollows.