[2] The designated area covers 12,770 ha (49 square miles) of countryside surrounding the upper reaches of the River Tweed between Broughton and Peebles.

The general sense of containment created by the narrow valley is given interest and variety by the inter-relationship of woodlands, sometimes shelterbelt, hedgerow or plantation, and both deciduous and coniferous, with farmland which ranges from roughgrazing on the hill, through parkland, pasture and arable to riverside meadows.

The valley floor widens at each of the confluences of the Holm, Lyne and Manor Waters to give longer views into these tributary glens and the higher summits at their heads, and then narrows again at Neidpath where the castle guards the defile between Caedmuir and the Meldons.

[12] The river has a very diverse ecology due to the mixed geology of its catchment area,[13] and is also designated as a Site of Special Scientific Interest (SSSI).

[5] There is a further SSSI within the boundaries of the NSA, the Tweedsmuir Hills, which form the largest area of mountain plateau in Scotland outwith (i.e. outside) the Highlands, and which hosts nationally important populations of breeding birds, mosses, liverworts and arctic–alpine plants.