Blow, let us hear the purple glens replying (11)
The echoes are coming from the valley below, and the reddish light of the setting sun makes the trees and rivers in the valley appear almost "purple."
O love, they die in yon rich sky, They faint on hill or field or river (13-14)
The speaker says that the echoes "die," or fade away, in one of several outdoor, natural spaces. Why do you think he lists the different natural places that the echoes reverberate and then fade? Why not just say that the echoes fade in the sky and leave it at that? What is the effect of this long list? One possible effect is that it forces the reader to imagine each of those places in turn, which helps to re-establish the beautiful view that the sound of the bugle had temporarily distracted us from.