Todays vast amount of knowledge including Geology defines some functions as to why Allah(God) created mountains and hills on Earth, these functions were foretold by the Holy Quran over 1400 years ago;
In the name of Allah most gracious most merciful
And We have placed in the Earth firm hills lest it quake with them .
Allah indeed speaks the truth
REFERENCE: Chapter (The Prophets) part of verse "31"