Simplify the following text for a Grade 7 native English-speaking person.
Original: The altitude of Geneva is 373.6 m. That is the altitude of the largest of the Pierres du Niton , two large rocks in the lake , are used as the reference point for all surveying in Switzerland .
Simplified: The altitude of Geneva is 373.6 m , and corresponds to the altitude of the largest of the Pierres du Niton , two large rocks emerging from the lake which date from the last ice age .