It is easy to guess at the sequence: a hole was dug near a spring to provide a basin of water; perhaps a stone was put over it and some steps were added to make access easier (Carn Euny); a small shelter was put over the top to protect the spring from the weather and rubbish (St Ruan); eventually the well houses became more elaborate and needed a door with a lock (Dupath) to restrict access by ‘undesirables’.
Not all these wells were necessarily holy, many having simply served their local communities, but they have become so through tradition. It is a good question as to why some of these were venerated more than others.
The best are set around with hawthorn or blackthorn.
Many have been ‘re-discovered’ as spiritual sites in the late 20th century and have attracted collections of votive offerings. Madron well, for instance, was once a lonely place in a thorn grove. At times it is now like a ticker-tape reception with obscure offerings attached to the surrounding trees.
There is a map of holy wells here.