The earliest versions of the structures are all thought to have been lost, and only their later replacements (mostly in use between the 8th and 12th centuries) survive. These survivors vary in size and arrangement, and include examples constructed in unhewn or roughly worked stones, stone-revetted turf and timber-laced rubble. Beneath some sites remnants of clay daub have been found (Megaw 1978:298), while traces of wall plaster have also been discovered. Keeills may have fallen out of use following the arrival of Viking settlers on the Island, but were then re-established, on the same sites in some cases, once the Vikings had converted to Christianity.