Re excavated clay walls
Cob walls have been excavated on a number of British sites and are usually
easy to recognise. The lower parts of the walling may survive to some height
especially from medieval towns where is a great general build-up of strata.  I
imagine the best US examples are the excavated clay slave cabins of S.
Carolina and adjacent areas- these also seem pretty obvious and have the added
advantage in some cases of having post supports within the walls. Much more of
a problem are turf walls which I have both dug and written up from other
people's excavations. The underlying soil may be a few cms higher  than the
floor & exterior and may be marked by lines of stones and a concentration of
gravel. The odd post-hole or hearth surviving also helps. They would be very
easy to miss and the British examples are usually recognised before excavation
by being on earthwork platforms on upland slopes or in deserted villages.
paul courtney, leicester, Uk