There are some monument categories, which are so scarce in certain periods that all surviving examples, which still retain some archaeological potential, should be preserved.
Mae rhai categorïau o henebion sydd mor brin mewn rhai cyfnodau fel y dylai’r holl enghreifftiau sydd wedi goroesi ac sydd â rhywfaint o botensial archaeolegol o hyd gael eu diogelu.