Carmel National Nature Reserve

Pant-y-Llyn turlough occupies a small depression on the northern perimeter of the South Wales Coalfield at Cernydd Carmel.

This depression represents a glacial channel formed along the Betws Fault where displacement has brought Carboniferous Limestone into contact with older impervious Devonian rock.

The basin fills to a depth of about 3 m during late autumn and remains full until the following summer when it empties completely, thus reflecting the characteristic behaviour of turloughs.

This is because limestone can be dissolved away by rainwater, which becomes mildly acidic by picking up carbon dioxide as it passes through the atmosphere.

[1] The local landscape is a mix of traditional agriculture and quarrying activity, with ancient woodlands, and field systems.