Simplify the following text for a Grade 11 native English-speaking person.
Original: In general , the Riverina is an alluvial plain formed by deposition carried from the Great Dividing Range by streams between 30,000 to 15,000 years ago .
Simplified: In general , the Riverina is an alluvial plain formed by soil carried from the Great Dividing Range by streams between 30,000 to 15,000 years ago .