The Circle S mine is located in the Bigfork chert, a thin bedded, dark grey, cryptocrystalline chert.
The cherts normally occur in thin to medium beds and are usually highly fractured. The limestones occur mostly as interbeds in the chert and typically weather to a soft brown layer.
The chert is interbedded with varying amounts of black siliceous shale, calcareous siltstone, and dense, bluish gray limestone. The interbedded siliceous shales occur in thin to thick sequences and are often pyritic. The limestones occur mostly as interbeds in the chert and typically weather to a soft brown layer