Mountains have got Deep Roots or Pegs into the Ground

Have We not made the earth as a wide expanse? And the mountains as pegs?  

Surah An-Naba 78: 6-7

٦ أَلَمْ نَجْعَلِ الْأَرْضَ مِهَادًا

٧ وَالْجِبَالَ أَوْتَادًا