Terrestrial oncoids, up to 4.3 mm long, are common in sinkholes that penetrate the dolostones of the Oligocene–Miocene Bluff Formation on Grand Cayman and Cayman Brac. The vaguely laminated terrestrial oncoids, which generally lack a nucleus, are formed of detrital material (micrite, clays, dolomite), calcified filaments and spores, and insect fragments. The abundant, diverse assay of microorganisms includes six different types of filaments and five different types of spores, which can be attributed to fungi, algae, and (or) cyanobacteria. Thin mats of mucus are commonly associated with the microorganisms. The microorganisms contribute to the formation of the terrestrial oncoids by (i) calcification of the filaments and spores, (ii) trapping and binding detrital material, and (iii) binding with their mucus detrital material to the surface of the oncoids.Identification of terrestrial oncoids relies on (i) the recognition of a microbial assemblage, (ii) the demonstration that the microorganisms played an active role in the their formation, and (iii) evidence that they formed in a terrestrial setting. If the microorganisms cannot be recognized because of diagenetic changes, identification must rely on the overall texture of the rock and its stratigraphic setting. Recognition of terrestrial oncoids is important because it provides additional evidence that a succession has been subaerially exposed. Terrestrial oncoids also indicate that the strata in which they occur are close to an unconformity.