UKC

I recently enjoyed the splendid Zippo E3 5c on Carreg Mianog in the Ogwen valley. In a recent face book article local activist Mike Bailey calls the rock type gritstone and local geologist Mike Hammill calls it 'a coarse sandstone'whereas UKC lists it as rhyolite.
My friends seem somewhat disbelieving that there is a gritstone crag in the Ogwen valley?
Can anyone come up with a definitive answer?

In reply to mark hounslea:
I guess coarse sandstone and gritstone are pretty much the same thing, and I think most of the Ogwen area stuff has just been assumed to be volcanic/ metamorphic - at the time the rocks were formed the area was in a subduction zone on the edge of a continental shelf with volcanoes breaking through the floor of a small shallow ocean - hence the large deposits of metamorphosed sedimentary stuff like shale and slate. I guess the odd pocket of poorly metamorphosed sedimentary stuff can persist.

I'll update the UKC page now, (but willing to be given better information)!

In reply to mark hounslea:

Actually, having looked at http://mapapps.bgs.ac.uk/geologyofbritain/home.html
it looks like this is an outcrop of the (surprisingly large) Cwm Eigiau Formation Sandstones - see http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=CEI

In reply to mark hounslea:

Yes, it is part of a very large sedimentary deposit and has never been metamorphosed. In addition to Carreg Mianog, all of the climbing on Carnedd Y Filiast (Cairn of the Greyhound Bitch) is also on gritstone; Atlantic Slab, Red Slab etc are all formed of gritstone although the UKC crag listing states that the rocktype is rhyolite, which is clearly an error.

The grit is of late Cambrian age, same as in the Rhinogydd. The Snowdon volcanic group of rocks are much much younger, being late Ordovician in age. Between the deposition of the Carnedd y Filiast grits (a beach deposit) and the formation of the Snowdon volcanics, all of what is now Snowdonia is thought to have been above sea level.

Grit deposits can also be found on Tryfan, particularly towards the summit. Notch Arete on its west face is formed of gritstone. Virtually all of the Rhinogydd is also gritstone and a number of its northernmost grit crags can be clearly seen from Bwlch y Moch. So, all in all, the Ogwen-Marchlyn grits and the Rhinog grits combined means that there's actually *a lot* of gritstone in Snowdonia.