Cardiff is built on reclaimed marshland on a bed of Triassic stones; this reclaimed marshland stretches from Chepstow to the Ely Estuary, which is the natural boundary of Cardiff and the Vale of Glamorgan. Triassic landscapes of this part of the world are usually shallow and low-lying, consistent with the flatness of the centre of Cardiff. The classic Triassic marl, sand and conglomerate rocks are used predominantly throughout Cardiff as building materials. Many of these Triassic rocks are purplish, especially the coastal marl found near Penarth. One of the Triassic rocks used in Cardiff is "Radyr Stone", a freestone which as its name suggests is quarried in the Radyr district. Cardiff has also imported some materials for buildings: Devonian sandstones (the Old Red Sandstone) from the Brecon Beacons has been used. Most famously, the buildings of Cathays Park, the civic centre in the centre of the city, are built of Portland stone from Dorset. A widely used building stone in Cardiff is the yellow-grey Liassic limestone rock of the Vale of Glamorgan, including the very rare "Sutton Stone", a conglomerate of lias limestone and carboniferous limestone.