Abstract <p>The mineralogical composition of the clay fraction of marsh soils on the tombolo, a bridge between the mainland and a small island on the Pomorsky coast of the White Sea, has been studied. The soils on different parts of the marshes are characterized by different sequences of sediments and soil horizons embedded in them. Mica minerals (“illites”), kaolinite, chlorite, and mixed-layer illite–vermiculite and chlorite–smectite were present in the clay fraction of all soils. Variation of mineralogical composition of clays was noted both between soil horizons and between the studied profiles. In our opinion, the variation is determined both by sediment composition and soil weathering of minerals, which confirms the idea of T.A.&#xa0;Sokolova that the formation of mineralogical composition of clays in soils on sediments redeposited by water has a complex character. The main weathering process in marsh soils was the degradation transformation of chlorite and, possibly, mica with the formation of smectite packages. These processes are spatially associated with the aeration zones of the marsh soils, at least periodically under oxidative regime.</p>