I dont know anything about these forts..but I have observed at a couple of such places in NE scotland that the sandstone type vitreous gunk mentioned earlier can be found sticking lava rocks together. This lava is of the ropey type (Pahoehoe). What I thought was interesting was that there was very little other Pahoehoe on the sites.....methinks because its not the toughest of lava being full of holes and so it got eroded. I have no idea if the rate of erosion of this lava would be a way to backdate the vitrification.

Woodhouse.