Could it be a variation on Glendon? Because the sound for Glendon and Glendan would both be "glen-dun", right?
Glendon is a boy's name, but boys names have been used for girls for a long time. Interesting that's she was Glendan Hall because there is a place called Glendon Hall in Northamptonshire.
You can read about Glendon here:
http://www.behindthename.com/submit/name/glendon