Geograph - photograph every grid square

Atlantic Slab high up in Cwm Graianog is arguably one of the geological wonders of Snowdonia if not of all of Wales. It has been named for its likeness to the surface of the sea with wave upon wave following each other, albeit 'frozen' in rock and the whole set at an alarming angle to the horizontal.Is there more of a link to the ocean?

Well, yes - but not the Atlantic! The rock is a coarse-grained sandstone known to geologists as the 'Carnedd y Filiast Grit'. Just short of 500 million years old, and so placing it in the Cambrian period, this great slab was laid down horizontally like all the other sedimentary rocks of Snowdonia. These beds of sand accumulated just offshore beneath the waters of the Iapetus Ocean. That ocean is long gone, squeezed out of existence by the continents on either side moving together and finally colliding during the course of the Caledonian Orogeny; a long-drawn out mountain-building period which, amongst other things, caused the intense faulting and folding of Snowdonia's rock strata.