The Nantlle Valley (Welsh: Dyffryn Nantlle, IPA: [ˈdəfrɨn ˈnantɬɛ]) is an area in Gwynedd, North Wales, characterised by its numerous small settlements. The area is also historically important geologically, and featured in one of the most contentious disputes of the 19th century, between the 'Diluvialists' who believed in the Biblical flood, and the ‘Glacialists’, who supported the Glacial Theory, which was substantially established by studies of the drift sediments on Moel Tryfan. Between 85 and 90% of the population of the Nantlle Valley speak Welsh as their first language.