Fifty-four 230Th–234U ages for 36 different speleothems from mountain karst areas of the Rocky and Mackenzie Mountains cluster into five distinct age groups. Speleothems of the two oldest groups are particularly massive and ornamented. Several samples exhibit complex growth histories with successive periods of deposition separated by periods of interrupted growth and (or) erosion. Attributing episodes of deposition to interglacials and nondeposition to glacial periods, interglacials are recognized at (i) present to about 15 ka; (ii) 90–150 ka; (iii) 185–235 ka; (iv) 275–320 ka; and (v) ≥ 350 ka. These interglacials correlate well with periods of high sea stand and warm climate observed in the marine paleoclimate record. It appears that during the broad period 275–400 ka interglacial conditions were strongly predominant in the mountains, producing effects suggestive of a 'great interglacial'.