London. The ground around the monument is pitted with various holes and craters. Many of these are dolines or swallow holes. These are formed when rain falls on the highly acidic topsoil. The water increases in acidity as it percolates through the topsoil then dissolves the underlying chalk. Eventually there is nothing but topsoil above the caverns so formed and the familiar shape of a doline is created when the topsoil collapses into the cavern beneath. Dolines are usually shaped like a teardrop cut in half vertically and laid down horizontally. Within of the monument there are three dolines which were formed by the chalk being dissolved in a vertical fissure in its structure. These dolines are vertically sided and are shaped like wells. The two which opened up in 1956 are some deep. The one which opened in 2006 is not quite vertical and its depth is unknown. Geodesy The Hardy Monument was the origin (meridian) of the 6 inch and 1:2500 Ordnance Survey maps for Somerset and Dorset. References External links Hardy Monument information at the National Trust Monuments and memorials in Dorset National Trust properties in Dorset Buildings and structures completed in 1844