The polygonal structures covering the tops and upper slopes of some treeless rocky islands in the White Sea are relic peatlands. They are composed of raised bog peat up to 80 cm thick. Originally, the peat was deposited by cotton grass communities similar to those of modern blanket bogs of Northern Europe. Later on, the peatlands grew dry and were colonized by dwarf shrub communities. Weathering rendered them their present-day polygonal structure.