A student volunteer from Newcastle University found the piece of carved scrollwork on Monday.
Archaeologist Tony Wilmott, who is leading the excavations with professor Ian Haynes, of Newcastle University, said: “It is early days yet but it’s all looking very promising and we are hoping for good things in the next two months.
“To find something like this on the first was great for the volunteers. We are getting hints that there is a lot of archaeology there.
“We have been very pleased with the dedication and willingness of the students and local volunteers.”
Peter Greggains, chairman of the Senhouse Roman Museum Trust, said: “This is the culmination 10 years of research.
“We have got a terrific team and we were thrilled to get them.”
The 28-strong team of volunteers is working to unravel the mystery surrounding a unique cache of 17 altars discovered there in 1870.
They had been buried in a series of pits to the north and east of the fort but no one knows when, why or by whom.