Valmiki Ramayana Yuddha Kanda Sarga 22 Shloka 35

तेन by the, वज्राशनिसमप्रभः like that of Indra's glow, शरः dart, यत्र there, निपातितः let down, तत् that, पृथिव्याम् region of land, मरुकान्तारम् Marakantaram, विश्रुतंखलु indeed was known as popularly.

By the letting down of the dart that resembled the glow of Indra that part of the land was indeed popularly known as Marakantara (Marwar and Bikaner region of Rajasthan).