The Shiants, or at least most of them, are about fifty eight and a half million years old. They were formed from a series of hot, intrusive magmas, giant plugs of molten rock rising from deep within the earth's mantle, which squeezed between much older fossil-bearing rocks above them. Although Frederick Walker's paper is now out of date (he has no understanding of the multiple intrusions which later geologists discovered) it is a fascinating description of the whole cooling mechanism. The SNH Geological Conservation Review presents the modern view in relatively non-technical language, and contains some excellent photographs of the islands and their rocks.

The GEOLOGY  of  the SHIANT ISLES (HEBRIDES) By FREDERICK WALKER, M.A., D.Sc (1929)

Geological Map of the Shiant Isles by Frederick Walker

Shiant Isles SSSI - Geological Conservation Review
by Kathryn Goodenough
Scottish Natural Heritage - September 1999
Reproduced by permission of Scottish National Heritage (3.23 Mb PDF file)
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