Ironmask Lake is the first and largest of a series of small undrained lakes occurring at intervals along the Trans-Canada Highway 1/97 for nearly 6 kilometres to the west. The lake is about 500 metres long by 91 to 152 metres wide. The eastern end is covered with tailings from the old Iron Mask mine (092INE010) situated on the hillside above; the remainder was dry in October 1937, except for numerous shallow pools distributed over its central portion. The depression in which the lake lies is flanked on the north by a low hill underlain by greenstone of the Upper Triassic Nicola Group and fresher porphyritic mafic volcanics of the Eocene Kamloops Group. The south is flanked by the slopes of Iron Mask Hill underlain by diorite of the Cherry Creek unit of the Late Triassic-Early Jurassic Iron Mask batholith. The basin of the lake is closed at the west end by a low divide.

In October 1937, the surface of the lake was heavily encrusted with a white efflorescence of dried salts, but no permanent layer of crystal was observed. Beneath the encrustation was soft black mud containing, in places, sparsely disseminated mirabilite crystals and smelling strongly of hydrogen sulphide. A number of 1.2 metre deep auger holes were drilled in a zone within 12 metres of the shore due to the soft mud surface. A bed of solid crystal was intersected in three holes near the margin of the tailings dump, beneath 0.9 to 1.2 metres of mud. The thickness of the bed was not determined. The lake was partly drilled several years ago (early 1930s) and it is understood that an area of 2 hectares was estimated to be underlain by a crystal bed from 1.5 to 3.6 metres thick, containing approximately 45,355 tonnes of salts. The depth of overlying mud ranged from 0.9 to 1.l5 metres.

In 1937, collecting a sample of permanent crystal was not possible but a brine sample taken from an old pit near the west end of the lake analysed 27.63 per cent total solids with a specific gravity of 1.240 at 16 degrees Celsius. The composition of the solids was: 35.8 per cent Na2SO4, 61.8 per cent MgSO4, 2.0 per cent Na2CO3, 0.4 per cent NaCl and trace CaSO4. A sample of surface crystals that formed on the lake later in the season yielded 94.2 per cent Na2SO4, 5.1 per cent MgSO4, 0.5 per cent Na2CO3 and 0.2 per cent insoluble (Bulletin 4). The magnesium sulphate mineral is assumed to be epsomite.