The dwellings of freemen and their families were often surrounded by a circular rampart called a "ringfort". There are two main kinds of ringfort. The "ráth" is an earthen ringfort, averaging 30m diameter, with a dry outside ditch. The "cathair" or "caiseal" is a stone ringfort. The ringfort would typically have enclosed the family home, small farm buildings or workshops, and animal pens. Most date to the period 500–1000 CE and there is evidence of large-scale ringfort desertion at the end of the first millennium. The remains of between 30,000 and 40,000 lasted into the 19th century to be mapped by Ordnance Survey Ireland. Another kind of native dwelling was the "crannóg", which were roundhouses built on artificial islands in lakes.