Abstract The boulbènes, Gleyic Luvisols, on old alluvial deposits of the Garonne basin (France) have two quite distinct types of structure within their argillic horizons. One type is prismatic; the other is vertic with large slickensides. They differ markedly in their mineralogy and shrinkage. On the second terrace of the Garonne, where the alluvial pebble bed lies deeper than 150 cm, the prismatic type occurs on the higher ground and the vertic type lower in the microrelief. An intermediate type occurs on the connecting slopes. On the lowest terrace of the Tarn the prismatic structure is present where the pebble bed is no more than 120 cm deep, whereas the structure is vertic where the pebble bed is deeper. To account for these differences we postulate that during the textural differentiation of the soil, illite weathered to smectite which then swelled and shrank, whereas the illite did not. On the higher ground and where pebble beds were shallow, the smectite weathered as a result of the strong acidification following leaching of cations. In the lower situations and over deeper pebble beds, as the hydromorphy was stronger, acidification was restricted and the preservation, or even the genesis, of smectite favoured vertic development.