Blocks of limestone and dolomite up to tens of metres across occur near the base of the Lower Devonian (Siegenian–Emsian) Stuart Bay Formation at six sites on eastern Bathurst Island. These blocks occur in groups of up to 30. At the two localities with the greatest number of blocks they are disposed in two or three roughly linear groups reflecting their occurrence on bedding planes. The blocks are mostly wackestones and floatstones and they contain abundant fossils of the reefal biofacies of which stromatoporoids and corals are most prominent.The blocks have weathered from a matrix of finely laminated deep-water siltstone. Most of the blocks are unbedded but where bedding attitudes can be measured they are discordant with that of the siltstone and those of neighbouring blocks. Although they have been described as bioherms that grew in place, the evidence indicates that they are allochthonous blocks derived when several catastrophic events such as earthquakes disturbed a Devonian reef tract developed on the western flank of the Cornwallis Fold Belt.