A fault bounded upper Mississippian-Permian limestone unit of the Horsefeed Formation (Cache Creek Complex) extends northwestward from Opal Lake for 12.5 kilometres to Chastot Creek, just east of Nahlin River, about 106 kilometres north of the community of Telegraph Creek.

The limestone unit widens northwestward from 300 to 2300 metres. A narrow, 6 kilometre long mass of chert of the Mississippian-Triassic Kedahda Formation (Cache Creek Complex) lies infaulted within the limestone near its southwest margin. The limestone is also in contact with upper Mississippian-Permian Cache Creek Complex ultramafic rocks to the southwest and altered basaltic flows and pyroclastic rocks of the Mississippian-Pennsylvanian Nakina Formation to the northeast.

The limestone band generally consists of pale grey weathering, pale to medium grey, massive, unsorted limestone breccia containing angular fragments up to 5 centimetres in length. The fragments are comprised of various carbonates including pale brownish grey, porcelaneous, very fine grained limestone, fusiliferous, calcarenitic limestone and fine grained black limestone. The limestone is in places recrystallized and heavily veined with calcite.