Deep wells sunk for artesian water beneath Christchurch penetrate three formations. The youngest, the Christchurch Formation, consists of marine, estuarine, and fluviatile deposits of which at least the upper part was formed during the post-glacial rise of sea-level. It extends down to more than 200 ft below present sea-level, and radiocarbon dates record part of the rise of sealevel, from — 73 ft 9400 years ago (S83/501) to — 12 ft 6100 years ago (S84/523). Other dates, together with topographic and soil evidence, are interpreted as indicating the 'Subsequent period of development by the Waimakariri River of a fan of gravel over the Christchurch area, probably completed about 2,000 years ago when sea-level was a little higher than at present. The aggradation surface of this fan is termed the Yaldhurst Surface.