Photo copyright - David Napier under the Creative Commons
"Located on the southern fringes of the village, Oldbury's parish church stands boldly on the summit of a tumulus which may be pagan in origin, commanding fine views of the nearby Severn Estuary and downstream, the Severn Bridge motorway crossing opened in 1966."
St.Arild's church is one of those churches that come under the heading of christianised pagan site. Now I cannot verify if there is a tumulus under the church, but given its prominent position on top of a hill overlooking the Severn Estuary and with a well nearby that runs red giving rise to the legend of Saint Arild, there is indeed longevity in the use of the site, probably because of its position as a navigational point. The well is called a chalybeate one, meaning that it seems to run red, though chalybeate is really about iron in the water whereas the reason the water at St.Arild's well runs red is to do freshwater algae called Hildenbrandia rivularis. The following gives the legend of St.Arild. The circularity of the church yard also points to an earlier phrase in its history, probably giving it a pre-christian origin, and roman finds have also been found here.
"John Leland, the sixteenth-century traveller and writer gives us some more information, gathered during his visit to Gloucester Abbey. He tells us that St Arilda, 'martyred at Kington by Thornbury [and] translated to this monastery had done many miracles', and that she was martyred 'by one Muncius, a tyrant who cut off her head because she would not consent to lie with him'. Kington near Thornbury is now in the parish of Oldbury on Severn (which itself was once a chapel of ease to Thornbury church), and here we find the third memorial to St Arilda: her well. A local tradition that the water runs red with her blood is well-founded, as the stones in the well's outflow are stained red."