The Giant's Causeway in northeast coast of Northern Ireland, is an area of about 40,000 interlocking basalt columns, the result of an ancient volcanic eruption.  The majority of the columns are hexagonal, but some have four, five, eight or even ten sides, measuring approximately 12 inches wide. The tops of the columns form stepping stones that lead from the cliff foot and disappear under the sea.  The Giant’s Causeway has often been described as the Eighth Wonder of the World and was declared as Ireland’s first World Heritage site by UNESCO in 1986.
 
 



 
Sixty-one million years ago, geological activity caused a series of volcanic eruptions, and molten lava flowed from cracks in the ground, causing valleys to burn and killing all vegetation. Then the basalt lava rapidly cooled, which caused it to shrink and crack into even polygonal shaped rocks (blocks). This in turn caused column-type joints to form beneath the earth. Fifty-eight million years ago, another series of volcanic eruptions produced a lava flow with a different chemical composition. When this lava cooled, it did not form columns as definite and staunch; it cloaked the well-defined, durable column structures beneath the surface. At the end of the Ice Age, 15,000 years ago, the frozen ocean chiseled its way past the high basalt elevations, eroding the shoreline and unveiling the cliff of columns, which resulted in the Giant’s Causeway.
 
 
Source :- http://www.unbelievableinfo.com/2013/11/the-giants-causeway.html

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