A type of Crustose Lichen, possibly Caloplaca Brattiae grow on basalt blocks near a lake in Holland. The particular solidification of lava results in the usually hexagonal shape of basalt blocks. The interlocking angles of the different blocks and the hardness and durability of the material made it very appropriate for using it as a levee covering. Nowadays it is mostly replaced by concrete except for some historical areas.
see also: Jetty Lichen