The deposit consists of a limestone bed of the Upper Pennsylvanian to Lower Permian Mount Mark Formation, Buttle Lake Group (reassigned from the Sicker Group). It is exposed as an arc along the steep bluffs on the north and west sides of Horne Lake. The limestone is unconformably overlain by massive to pillowed basalts of the Upper Triassic Karmutsen Formation (Vancouver Group) and underlain by bedded tuffs and volcanic breccias of the Upper Devonian McLaughlin Ridge Formation (Sicker Group). Exposed thicknesses are up to 360 metres, as revealed on the south face of Mount Mark, north of Horne Lake. To the east, and west, of Mount Mark the limestone thins to less than 120 metres. The unit is folded into a broad northwesterly plunging syncline that is segmented by a series of steeply dipping faults.

The deposit is consists of medium to light grey, fine to coarse- grained recrystallized, yet well bedded bioclastic limestone, containing abundant crinoid remains. Thin sections display numerous whole and fragmented crinoid discs in a very fine grained limy mud matrix with minor secondary silica. At Mount Mark, the limestone contains minor thin chert beds in the upper and lower portions of the exposed section. In the middle of this unit, the limestone is interbedded with lenses and beds of argillite and tuff. Several gabbro sills intrude the limestone near the top of the section.

Development is limited to some mapping and sampling by B.C. Cement in the 1950's.