Milk Lake, a small carbonate playa covering some 300 square metres, is located 31 kilometres due north of Clinton and 19 kilometres west-northwest of Seventy-mile House.
The lake is one of several semi-evaporitic playa lakes in the "Green Timber Plateau" area (EMPR Bulletin 4), a semi-arid plateau area averaging 1130 metres elevation which is part of the Cariboo Plateau. The area is underlain by alkaline plateau basalt flows of the Miocene to Pleistocene Chilcotin Group, mantled by a thin cover of glacial till and glaciofluvial sediments. Annual precipitation averages between 300 and 400 millimetres (EMPR Paper 1991-1).
This ephemeral lake is largely underlain by a central mudflat forming a hard flat surface of pale grey carbonate mud, comprised mostly of hydromagnesite. A shallow pit dug near the center of the playa encountered 80 centimetres of massive grey mud comprising magnesite and hydromagnesite underlain by cream coloured magnesite mud. The central mudflat is rimmed by a peripheral mudflat, a few metres to 20 metres in width, containing a mixture of massive to crudely bedded siliciclastic detritus, precipitated magnesium carbonates (magnesite, hydromagnesite and dolomite) and organic matter. The peripheral mudflat is bounded by glacial deposits that rise abruptly from the shoreline on all sides of the lake.
In general, the magnesite content increases downward at the expense of hydromagnesite. In two instances, hydromagnesite forms 25 to 30 per cent of the total carbonate, 10 to 20 centimetres below the central playa surface. Dolomite occurs in the southern peripheral mudflats in association with magnesite, 40 to 80 centimetres below surface. The carbonates of the central mudflat are relatively pure. Four samples contained 1.2 to 5.4 per cent acid insoluble matter comprised of clay, plagioclase silt, diatom debris and organic detritus. Non-carbonate content was found to be higher in the peripheral muds and consisted of plagioclase, quartz and clay minerals.