Many were felled during the first half of the 20th Century, then cleared for agriculture, replanted with conifers or left to regrow unmanaged from stumps or seed.
Dros y blynyddoedd mae llawer wedi cael eu draenio neu eu gorchuddio gan goedwigoedd ac mae'r safleoedd sy'n weddill wedi'u hynysu gan dir amaeth.