The Australian continental crust was created in three cycles from the oldest Archaean cratons in the west to the younger orogenic formations in the east (built about 541 million to 252 million years ago).^[143][146] The oldest Australian surface rocks date to the Archaean period. Some in Western Australia are older than 3.7 billion years and others in South Australia are over 3.1 billion years old. The oldest zircon crystals on Earth, dating back 4.4 billion years, have been found in Western Australia. However, about 80 per cent of Australia is covered by sedimentary rocks and regolith that are less than 250 million years old.^[143]