Radnor Forest () in the county of Radnorshire, Wales is a rock dome composed of Silurian shales, mudstones and limestone in Mid Wales. It is a forest in the medieval sense of the word (an area of land set aside for hunting). It lies within the Welsh Marches region since Offa's Dyke lies nearby to the east. The area lies to the north of the Black Mountains and to the east of the Cambrian Mountains. Hergest Ridge is immediately to the south-east of the area (just across the English border into Herefordshire), near the small town of Kington. The highest point in the area is Rhos Fawr a broad plateau which reaches , and is equipped with a trig point to mark the summit. A similar plateau adjoining to the east, Black Mixen is the only Nuttall to have a communications mast (a radio transmitter) on its summit. Geology The massif is formed from argillaceous rocks of Ludlow age, i.e. late Silurian around 425 million years ago. The strata are exposed around Harley Dingle and Whinyard Rocks. In 2020 the area was in course of being geologically mapped for the publication of the 'Knighton' 1:50,000 scale geological map by the British Geological Survey, no such work having been undertaken in this area since the middle of the nineteenth century. Site of Special Scientific interest Part of the area is a designated and protected SSSI, with rare saxifrage and other plants. It covers the highest peaks and a rocky gorge near the centre. The vegetation of the summit ridges is fairly uniform, and is dominated by a mixture of dwarf shrubs, heather, crowberry, and bilberry. Mixed with these dwarf shrubs are hare's-tail cottongrass, purple moor-grass, sedges such as Carex binervis and a number of mosses and lichens. The dwarf shrubs are very well grown and suggest the area has suffered little from fires in the recent past and that grazing pressures are light. Purple moor-grass is but thinly scattered and does not form the extensive stands so familiar only a few miles west. Peat pools support common cottongrass Eriophorum angustifolium. The steep-sided dingles and rock outcrops of the Forest's edge support rich plant communities, since these eastern Silurian shales are slightly calcareous. Mossy saxifrage and rock stonecrop are common on some cliff faces, with marjoram and a number of interesting mosses and liverworts. A variety of typical upland breeding birds are