Because of insulation by supraglacial debris, the dynamics of debris-covered glaciers differ from those of 'clean' glaciers. Thus, changes in debris cover have to be taken into account when interpreting glacier fluctuations in terms of climate forcing. Three large glaciers in the Mont Blanc massif were investigated for the period since the termination of the 'Little Ice Age' (LIA) using historical and scientific documents of the eighteenth and nineteenth centuries. Although debris cover was limited to its frontal area until the 1840s, Miage Glacier exhibited a continuous debris cover in the first decades after the LIA, increasing to its present extent by the 1930s. On the Mer de Glace, a partial debris cover formed by coalescence of two of the four medial moraines at the close of the LIA; until 1890, these moraines have formed the present Veine noire ('Black vein'). In contrast, Brenva Glacier had a continuous debris cover at the end of the eighteenth century, possibly following a rock avalanche in 1767. For glaciers like Mer de Glace and Miage Glacier, the close of the LIA represents a threshold marked by a rapid change in state from 'clean' to debris- covered; for others, long-term debris accumulation or frequent rock avalanching (e.g., Brenva Glacier) mask the climate control of a slow post-LIA expansion of debris cover.
