In its middle section, the South Nahanni River flows through four deep, meandering canyons incised through ranges of the southern Mackenzie Mountains. The downstream canyon, First Canyon, contains fossil solution caves of shallow phreatic type developed in limestone. The stratigraphy of the most extensive caves indicates that there was an interglacial climate between 300 000–200 000 years B.P., (from U Decay Series dating of stalagmites), which was preceded by a 'First Canyon Glaciation'. There has been no subsequent glaciation of the canyon and its environs. The position of the caves above the River, plus morphological evidence from cutoff meanders and a tributary canyon, indicates that since the First Canyon Glaciation the river has been entrenching into a rising anticlinal arch and that the maximum postglacial uplift of the arch has been not less than 1600 ft (486 m). The other three canyons of the South Nahanni River are also of antecedent type and are probably of similar age with a similar history.