After excavation, it indeed appears to be a wall foundation, which fills foundation trench SU578. See SU578 for more information.

Linear spread of rubble composted of smallish (13x11.5x5cm is the largest visible stone, smallest are pebble-sized, i.e. 1-3 cm in width) chunks of mostly gray tufo with some yellowish and some orangish tufo. Some sherds of tile and pottery are also visible between the stones. May be a possible wall foundation related to the structure; runs perpendicular to the lines of walls SU541 and SU420 and is also about at the point where we noticed a change in the finds in SU419, with more tile to the southeast and more pottery to the northwest.