Wall foundation is tufo, highly degraded in places - perhaps by plowing western limit is not original, eastern limit is defined by change in construction to basalt (given 2309 from this point) suggesting separate times of construction.
2261-tufo wall, 2309-basalt wall
E-W tufo wall S of walls 2276 and 2278 and Grey tufo wall foundation. Eastward extension of 2261
SU 2261, SU 2309