Simplify the following text for a Grade 10 native English-speaking person.
Original: Most of Kakadu was under a shallow sea approximately 140 million years ago , with the escarpment wall formed from sea cliffs and Arnhem Land from a flat plateau above the sea .
Simplified: Most of Kakadu was under a shallow sea about 140 million years ago , with the escarpment wall formed from sea cliffs and Arnhem Land from a flat plateau above the sea .