Altar slab

Hearing there was going to be a pilgrimage in a few days, we decided to clean months of gull-droppings off the altar slab.
There are 5 incised crosses (two of them here) representing the 5 wounds of the Crucifixion (I think). The slab itself was found in bits in one of the transepts, but has been set-up in the position of the high altar at the east end of the ruined church.