Simplify the following text for a Grade 10 native English-speaking person.
Original: The upper part of the mountain mesa consists of Ordovician quartzitic sandstone , commonly referred to as Table Mountain Sandstone ( TMS ) , which is highly resistant to erosion and forms characteristic steep gray crags .
Simplified: The upper part of the mountain mesa ( table ) consists Silurian\/Ordovician quartzitic sandstone , commonly referred to as Table Mountain Sandstone ( TMS ) . It is highly resistant to weathering and forms steep gray crags .