Simplify the following text for a Grade 8 native English-speaking person.
Original: It is built of unstable volcanic and has dumped clay and rock several meters deep into the Perberton Valley at least three times during the last 7,300 years .
Simplified: In the past , Meager has dumped clay and rock several meters deep into the Pemberton Valley at least three times during the last 7,300 years .