A limestone lens of the Middle Triassic Brooklyn Formation outcrops on the Kettle River 6.5 kilometres west-northwest of Midway and continues northeastward up the west slope of a hill for approximately 700 metres, where it becomes overlain by conglomerate. Exposures along Highway 3 and the Canadian Pacific Railway reveal an east-west width of 600 metres. Bedding generally strikes northwest and dips northeast despite some folding and faulting.

The lens is comprised of mixed, medium grained, light grey to white limestone and fine grained, black limestone with some interbedded greywacke, argillite and light grey chert. The limestone is cut by numerous fractures along which the limestone is commonly bleached white. Numerous dykes have intruded the limestone. A chip sample taken along 4.3 metres of limestone exposed in a road cut along Highway 3 contained 52.10 per cent CaO, 0.37 per cent MgO, 6.70 per cent insolubles, 0.52 per cent R2O3, 0.80 per cent Fe2O3, 0.07 per cent MnO, 0.18 per cent sulphur and 40.12 per cent ignition loss (Minister of Mines Annual Report 1960, p. 143, Sample 1).