Ultima 6 Clue to clean shrines with the runes

I'm trying to figure out how i can free the shrines from the gargoyles

1 British's quest to free the shrines

after fighting the gargoyles, i talked with British and he send me to a quest to free the shrines. He told me to ask Geoffrey for further details.

2 Geoffrey and the Guards

Geoffrey told me that a group of guards went to free the shrine of compassion. they failed and i should talk to a wounded warrior in Cove, his name would be Gertan.

3 Gertan and the force field

Gertan in Cove told me they failed because the gargoyles use foul magic, a force field prevents them from freeing the shrines.

Question

how do i know that i have to use the Runes and the Mantra to clean the shrines? - no one told me to do that so far! as well, no one told me that the mayor knows who has the rune.