Prior to glaciation the Cox River flowed through the Pūkio Stream valley, discharging into the Esk River, a lower tributary of the Waimakariri River. This route was later blocked by a series of terminal moraines deposited by the Cox glacier during the Pleistocene period. The river then created a new outlet through to the main Poulter valley via a rocky gorge, known as McArthur Gorge, which now contains the East Branch of the Poulter.[3]