Simplify the following text for a Grade 9 native English-speaking person.
Original: The Malvern Hills , which run from the south of the county into Herefordshire , are made up mainly of volcanic igneous rock and metamorphic rock , some of which date from before 1200 million years ago .
Simplified: The Malvern Hills , which run from the south of the county into Herefordshire , are made up mainly of volcanic rock , some of which date from before 1200 million years ago .