A generally flat lying body of limestone of Lower Permian age outcrops over a 3 by 1 kilometre area on the top of Copper Mountain (Thornhill Mountain), 10 kilometres east-southeast of Terrace.

The deposit is underlain by siliceous and slatey argillites which strike 030 degrees and dip 45 degrees northwest. Triassic argillite and chert and basaltic to rhyolitic tuffs, flows and breccias of the Triassic Telkwa Formation overly the limestone to the south. The entire sequence is intruded by Upper Cretaceous granodiorite of the Coast Plutonic Complex to the southwest. A few bodies of skarn containing quartz and calcite with various calcium silicates occur in the limestone along the intrusive contact.

The limestone is medium to coarse grained and usually white with some grey streaks. A sample of randomly collected chips taken along 457 metres of limestone exposed in a roadcut contained 55.34 per cent CaO, 0.29 per cent MgO, 0.34 per cent insolubles, 0.10 per cent R2O3, 0.06 per cent Fe2O3, 0.01 per cent MnO, 0.03 per cent P2O5, 0.003 per cent sulphur and 43.49 per cent ignition loss (Minister of Mines Annual Report 1965, p. 265, Sample 3).

Limestone was produced from two small quarries near the south end of District Lot 2838 by Terrace Calcium Products between 1969 and 1982. A total of 2253 tonnes of limestone was quarried.