The Assiniboine and Qu'Appelle valleys formed as spillways and meltwater channels, mostly within partly re-excavated ancestral valleys of probable Early Wisconsin age. It is unlikely this region was glaciated in Mid-Wisconsin time and the Late Wisconsin glaciation was relatively weak resulting in only partial infilling of Early Wisconsin valleys with alluvium, drift, and ice. The alluvial fill below the valley bottoms commonly includes a thick (100 to 200 ft (30.48 to 60.96 m)) lower unit mostly of Mid-Wisconsin and Late Wisconsin age, and a thinner (60 to 100 ft (18.28 to 30.48 m)) upper unit of Late Wisconsin and Recent age.The modern valley system formed between 15 000 and 12 000 years ago. The Qu'Appelle Valley was cut to its maximum late glacial depth more than 14 000 years ago and the Assiniboine Valley about 12 000 years ago. Subsequent infilling of most parts of the valleys with alluvium and colluvium has continued at a gradually diminishing rate to the present.