Simplify the following text for a Grade 8 native English-speaking person.
Original: Many of the western islands are hilly and steep , formed mainly of granite , and are peaks of the northernmost extension of the Great Dividing Range now turned into islands when sea levels rose at the end of the last ice age .
Simplified: The hilly and steep granite islands in the west were once the tops of mountains which became islands when the sea levels rose 8,000 years ago .