During the long period of their formation bog habitats have slowly grown and have kept a hidden history of their development and the changes in the landscape around them over thousands of years.
Yn ystod eu hanes hir mae mawnogydd wedi cronni'n raddol ac wedi cadw cofnod cudd o'u datblygiad a'r newidiadau a fu yn y dirwedd o'u cwmpas dros filoedd o flynyddoedd.