Dyffrynnoedd Nedd a Mellte, a Moel Penderyn (grid reference SN921090) is a 421.1 hectare biological and geological Site of Special Scientific Interest on the border of Neath Port Talbot and Powys, Wales, notified in 1954.
[1] This site includes the wooded valleys of the rivers Nedd and Mellte and their tributaries above Pontneddfechan as they pass through a millstone grit and limestone plateau, and the hill of Moel Penderyn, which lies to the east.
The site is of special biological interest for its extensive and diverse semi-natural woodland, important populations of several flowering plants and supporting outstanding assemblages of mosses, liverworts and lichens.
More than 600 species of plant have been recorded from the site, including a very large proportion of the bryophyte flora of mid and south Wales.
The geomorphological features within parts of the valleys of the Hepste and Mellte are also of special interest.