Bow at belt went my love riding Riding the mountain down into the silver dawn.
Just like in lines 11-12, we don't quite know that "my love" is the one who shot the arrow. After all, Cummings' speaker doesn't come right out and say, "my love shot an arrow at a deer." Nope, subtlety's more his style. The arrow (20) and its bow (21) are even separated by a stanza break, just to keep things more opaque.
Getting back to our scenery, we should mention that the hunt is moving from the plains into the mountains. If we were into thinking metaphor (which we are, of course) we might even say that we're headed into some rocky terrain. Get it?