Freshwater deposition of the upper Paleozoic, Gondwana tillite in the Pensacola Mountains is indicated by low δ values of +4.9 to +6.1 permil δ0 1 8 (SMOW) and -1.8 to -15.9 permil δC1 3 (PDB) in primary sedimentary calcite within the tillite. In contrast, Cambrian marine limestone from the Pensacola Mountains contains heavy isotopic abundances that are entirely characteristic of marine deposition.