Studying peatland dynamics over long timescales requires a proxy for past peat cover. In this paper, we explore using cosmogenic nuclides to infer peat thicknesses averaged over 100 kyr timescales. We find that long-term peat thicknesses inferred from cosmogenic nuclides at an upland peatland site in the Belgian Ardennes are consistent with independent constraints, but exceed modern peat thicknesses. We attribute this discrepancy to peat degradation associated with historical land use.