Once the feature 2851 was removed, it became clear that this was an anthropic cut into the bedrock likely associated with a hut. The 'fills' are deposits that continued under the feature and were excavated out of sequence.
This is a cut running beneath 2851 that is not fully exposed. Possible foundation cut for feature. The relation between this feature and posthole 2904 is not clear yet.
Semicircular cut along edge of 2851, filled by 2899.