A belt of limestone of the Hadrynian Cunningham Formation up to 1500 metres wide, extends northwest from the Cariboo River for 16 kilometres, passing just east of Roundtop Mountain, located about 22 kilometres southeast of Barkerville. The limestone is exposed along the crest of the Cunningham anticline. Overlying chloritic schists, phyllites and quartzites of the Yankee Belle Formation outcrop along the flanks of the belt. The belt is truncated by faults to the northwest and southeast. The unit is at least 150 metres thick in the vicinity of Roundtop Mountain. The Yankee Belle and Cunningham formations are part of the Upper Proterozoic-Cambrian Cariboo Group.

The belt is comprised of fine grained, black to dark grey limestone that is commonly bleached to a light grey to cream colour where fractured and cut by quartz veins. The limestone is commonly altered to ferroan dolomite. The upper 15 metres of the unit is composed of tightly folded limestone thinly interbedded with chloritic and argillaceous layers. Nodules and irregular masses of chert are sometimes present. Spherical to ellipsoidal pellets of ankerite or ferroan dolomite up to 5 millimetres in diameter form up to 40 percent of the rock just east of Roundtop Mountain. In thin section, the rock displays up to 5 per cent in detrital quartz-muscovite grains.