The rocks are largely volcanic, the stratification being complex.
Its roof is a single flat stratum of limestone; its walls are well marked by lines of stratification; dripstone also partly covers the walls, fills a deep fissure at the end of the cave, and spreads over the floor, where it mingles with an ancient bed of ashes, forming an ash-breccia (mostly firm and solid) that encloses fragments of sandstone, flint spalls, flint implements, charcoal and bones.
St Kilda is probably the core of a Tertiary volcano, but, besides volcanic rocks, contains hills of sandstone in which the stratification is distinct.
- Diagram of the stratification of temperature and the vertical components of currents in high southern latitudes.
2, which are driven across the stratification, or generally anywhere not in coal, are known as " stone drifts."