The land is of the fertile Mercia Mudstone Group, formed of red layers of mudstone and siltstone of Triassic age (see Keuper marl), with peat at the Somerset Levels, giving good pasture on the Mendip Hills. The edge of the hills has carboniferous limestone that has been exposed on the Black Down Pericline due to the erosion of the overlying Triassic dolomitic conglomerate. The dolomitic conglomerate and limestone is used in local buildings and walls. A quarry for conglomerate stone was on the north side of Windmill Hill. Mines in Dolebury Warren have sought lead (galena) with silver, iron (haematite), ochre (limonite), manganese (for glass making) and zinc carbonate (calamine); the latter for brass manufacture. Important features like Dolebury Warren and Burrington Combe were formed by the action of water. Like Cheddar they may be collapsed caves or created by meltwater rushing off the hills during the end of glacial periods.