A limestone lens of the Middle Triassic Brooklyn Formation is exposed along the Thimble Mountain side road, 2.25 kilometres east-northeast of the road's conjunction with Highway 3, 2.5 kilometres southeast of Eholt. The lens continues southwestward across the west peak of Hardy Mountain for 1.2 kilometres. The deposit varies up to 400 metres in width.

The lens consists mostly of light grey, medium grained, well fractured limestone. Beds of cherty argillite occur in the limestone near the eastern margin of the deposit. A sample of chips taken at 3.0 metre intervals across the south end of the lens contained 38.29 per cent CaO, 0.91 per cent MgO, 25.62 per cent insolubles, 2.18 per cent R2O3, 1.76 per cent Fe2O3, 0.04 per cent MnO, 0.06 per cent P2O5, 0.04 per cent sulphur, 31.23 per cent ignition loss and 0.12 per cent water (Minister of Mines Annual Report 1960, p. 143, Sample 7).