A 300 to 900-metre thick succession of dolostone (dolomite) with minor limestone and sandstone forms rugged peaks and cliffs just east of the Ospika River in the vicinity of Williston Lake.

The succession is comprised of the Lower Silurian Nonda Formation, the Silurian and Devonian Muncho-McConnell Formation and the Lower to Middle Devonian Stone Formation. Extensive folding and thrust faulting causes the sequence to outcrop in a series of parallel bands extending north-northwest from the east arm of Williston Lake (formerly the Peace River) for at least 110 kilometres. Individual bands vary up to 63 kilometres in length and 9.3 kilometres in width.

The Nonda Formation consists of 200 to 660 metres of massive, medium grey weathering dolostone containing ubiquitous quartz silt and sand. Chert nodules and silicified fossils are also common.

The overlying Muncho-McConnell Formation is comprised of light grey weathering, massive cliff-forming dolostone with a variable sand and silt content that generally increases upwards. Some brecciation is evident. The unit is 410 to 564 metres thick in this region.

The Stone Formation consists of a lower 254 to 463-metre thick member comprised of banded light grey and brownish dolostone and sandy dolostone overlain by an upper 100 to 150-metre thick member containing medium to thickly bedded, light grey dolomitic siltstone. dolomitic sandstone, dolostone and sandy dolostone. The dolostone is medium to fine grained.

The most significant exposure occurs along the Bernard anticline, centered 20 to 28 kilometres east of the Ospika River. Here, the dolostone outcrops along the syncline over a length of 11 kilometres with widths of between 7.5 and 9.3 kilometres along most of its length.