The earthly chill of dusk and slow Upon those under lands the vast And ever climbing shadow grow The night is approaching from the east, that is, the sun is setting in the west. The shadow here implies approaching death. Finally, this stanza sets up the westwardly movement that is to be present throughout the remainder of the poem. Once the ancient capital of Media, Ecbatan is now part of Iran. This is not just spatial distance, but temporal distance as well.
Trees are often symbols of age and time.
Moreover, the mountains changing refers to them darkening as well as being altered by wind and rain over the centuries. Lastly, if we take this verse in conjunction with the previous two, we can see a person in Iran facing west towards Mecca as he prays, facedown beneath the fading sun. So we now have what appears to be a reference to Islam in a poem about time, death, night, and eternity. And through the twilight now the late Few travelers in the westward pass Kermanshah is a district and city in Iran. Here the poem further traces the path of the spreading darkness.