I don't know, I have a Colte-Hoare map which shows it differently and has the spring marked as Swallowhead.
It seems to be a bit vague as no one seems to have had a good look at it.
It maybe a somewhat modern fearure as MJB suggested to do with landscaping at the Manor house.
The Girt Bank thing is also interesting, it is marked on a painting in Trowbridge and shows a large built up bank on the side of the road with Spring pump a short distance away from where the top of the mound is now.
What happened to the Bank I wonder?
Was it cleared and dumped to form Silbaby?
Whatever, it is a bit of a mystery in that little bit of land.
PeteG
BTW I like you website, did you use Vista Pro for the reconstruction pic?

Another map from the Red Lion shows a house next to the mound with a well below it.
http://peteglastonbury.ic24.net/SilMap.jpg
and a similar layout of the river.
The problem is that that there are blind springs which pop up and flood this whole field below Silbaby so any map maker in the past would have a problem plotting its real course.
PeteG