SU 5121 is a small reddish patch of soil which has been altered by the presence of a fire (see SU 5119 and SU 5120). The layer itself is therefore equal to the surrounding soil SU 5104. The presence of a fire suggests that the top of this layer and of layer SU 5104 (which is on the same level) may represent an ancient surface. The fire may also have affected the much rockier layer beneath this (SU as yet unassigned) since it appears reddish as well.