Stac Biorach, Coastal rock formation in St Kilda archipelago, Outer Hebrides, Scotland.
Stac Biorach is a roughly 73-meter tall triangular sea stack that rises from the waters between the islands of Hirta and Soay in the St Kilda archipelago. Its narrow column displays steep, smooth faces that have been shaped by waves and weather over thousands of years.
The first documented recreational climbing expedition took place in 1883, led by Richard Barrington with two St Kilda residents. This ascent marked the start of the formation's modern climbing history.
Young men from the St Kilda community had to prove their climbing skills on this rock formation before they could marry. The ritual showed who possessed the abilities needed for survival on these remote islands.
Smooth rock surfaces and powerful ocean currents make reaching the formation difficult and risky. Visitors should seek local advice and attempt an approach only during calm sea conditions.
The name 'Thumb Stack' comes from the fact that climbers can only find finger holds the size of a thumb when ascending this column. The extremely narrow handholds make every climb a test of skill and strength.
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