I wonder if the reference has something to do with the Little Doward Hill Fort shown on this map,

http://maps.nls.uk/view/102342785

Just south-east of the camp is a so-called Arthur’s Cave. This article mentions a possible connexion between Arthur and Arth (bear).

http://www.vortigernstudies.org.uk/artcit/caerdoward.htm

It's not much but it's the best I can find.