45. Al-Hijr was the capital city of the people of Thamud, and its ruins are found near the modern city of Al-Ula which is to the north west of Al-Madinah, and lies on the route from AI-Madinah to Tabuk. Though the caravans had to pass through the valley of Al-Hijr, the Prophet (peace be upon him) had forbidden the Muslims to stay in the valley on their journey.
When Ibn Batutah reached there in the eighth century of Hijrah on his way to Makkah, he wrote: I have seen the buildings of Thamud hewed into red mountains; the paintings look so bright as if they have been put on only recently, and rotten bones of human beings are found in them even today." See (E.N. 57 of Surah Al-Aaraf).
46. That is, their strong and secure buildings, which they had hewed into the mountains, could not protect them from the blast.