The extensive coastal exposure of massive underground ice at Peninsula Point, southwest of Tuktoyaktuk, Northwest Territories, is believed to be intrasedimental ice. The ice grew beneath a frozen diamicton during the downward aggradation of permafrost. The water source was probably glacier meltwater, with low negative δ18O values, that flowed, under a substantial pressure, through permeable unfrozen sands. Evidence for a high water pressure is shown by ice dikes, which extend upward from the massive ice into the superincumbent diamicton. The diamicton was frozen when the dike water was injected, as proven by the chill contacts and petrofabrics. The diamicton – massive ice contact is a conformable contact with features characteristic of downward freezing. The continuity of δ18O and δD profiles from the top of the massive ice downward to a depth of 10 m into the underlying frozen sand demonstrates a common water source for the massive ice and interstitial ice in the underlying sand. A similar continuity of δ18O profiles has been determined from three drill holes at another site 15 km northeast of Tuktoyaktuk, Northwest Territories. The ages of both the diamicton and massive ice at the Peninsula Point site are uncertain, because of unexplained differences in published radiocarbon dates.