Geograph - photograph every grid square

The Swellies - Pwll Ceris in Welsh - is the most treacherous section of the Menai Strait. A medieval document quoted in the fabulously illustrated book 'The Menai Strait' [2003] (also published in Welsh under the title 'Y Fenai'[2002]) by Gwyn Pari Huws and Terry Beggs (Gwasg Gomer Press) states
'In that arm of the sea that departeth between this island Mon and North Wales is a swelowe that draweth to schippes that seileth and sweloweth hem yn, as doth Scylla and Charybdis - therefore we may nouzt seile by this swalowe but slily at the full see'.