Lithalsas, mineral-based permafrost mounds with segregated ice in their cores, are found in the Great Slave Lowlands, Northwest Territories. They are formed in (glacio)lacustrine deposits of Glacial Lake McConnell and ancestral Great Slave Lake left as water levels receded during the Holocene to the present level of Great Slave Lake. Ground ice conditions were examined in one 700-m long lithalsa adjacent to Highway 3, 30 km west of Yellowknife. Samples obtained to depths of 8.4 m from 17 boreholes revealed domed layers of silts, sands, and clays in the lithalsa, with related layers appearing horizontally beneath an adjacent peatland. Segregated ice lenses over 0.1 m thick occurred in clays below 4 m depth. δ18O values of lithalsa ice indicate modern water sources for the ground ice. The ages of organic material recovered from the lithalsa imply permafrost aggradation initiated growth of the feature within the last 700 to 400 years.