ABSTRACTDuring a week in late Octorber 1986 the formation of ridges of pumice by the movement of a mat of the floating fernSalvinia molestaMITCH was studied at the southern shore of Lake Naivasha, Kenya. The height of the ridges or ramparts was about 0.1 m, the width 0.1–0.5 m and the length c. 100 m. The resemblance to ridges or ramparts formed by floating ice in Arctic and cold-temperature areas in clear. It is assumed that the pumice ramparts in the Lake Naivasha were very ephemeral features.