In the poem "The Listeners," what does "iron on stone" mean?

Much of the imagery in "The Listeners," by Walter de la Mare, is of sound, as the title portends.  In  order to understand what is meant by that description, we need to remember that the Traveller in the poem has arrived at this house on horseback and that the path or road to the house must be made of stones, possibly cobblestones.  If we are to be in a time period during which people get...

Much of the imagery in "The Listeners," by Walter de la Mare, is of sound, as the title portends.  In  order to understand what is meant by that description, we need to remember that the Traveller in the poem has arrived at this house on horseback and that the path or road to the house must be made of stones, possibly cobblestones.  If we are to be in a time period during which people get about on horseback in the forest, it makes sense that any paths or roads would be of stone, since paved roads would be completely incongruous.  Now, let's look at the final four lines of the poem:

Ay, they heard his foot upon the stirrup,   

   And the sound of iron on stone,

And how the silence surged softly backward,   

   When the plunging hoofs were gone (33-36). 

Can't you hear this? The Traveller swings up on his horse, and the horse's hooves, which have iron horseshoes on them, are striking the stones, a kind of sound that is quite distinctive. We can hear the sound diminish, as the Traveller and his horse move on, and the silence takes over again. I love the idea of silence surging, in waves. Of course, sound really is waves, but this gives us a way of "seeing" silence.