basalt in the of columns which are formed by gradual cooling of magma. the gradual shrinkage of the rock creates hexagonal c

are basalt bollard boulder causeway close closeup color colour column columnar columns cooling creates dark detail formation formed geological geology giant gradual gray hard heritage hexagonal industrial lava magma majestic mine miner mineral outdoor pavement paving polygon popular post production rock scenic shape shrinkage source stone stoneware surface unusual volcanic which