Upon the floor of light, and time, Unmurmuring, of polyp made, We rest; we are, as light withdraws, Twin atolls on a shelf of shade. (5-8)
Where are we? After describing the coordinates of this place, after sketching in the scene, the speaker locates the two lovers on "the floor of light, and time," existing as both real and abstract, both natural and imagined. And then it gets kind of weird. Of all the animals to compare yourself to, who would ever imagine the coral, an animal so unlike humans, it's really more like a geological (or geographical) creation? Here humanity is not active creator, but is instead passively created.
Passion and slaughter, ruth, decay Descend, minutely whispering down, Silted down swaying streams, to lay Foundation for our voicelessness (21-24)
In case you were imagining this was some kind of paradise, the speaker reminds you that this moment was brought to you courtesy of a whole host of bad stuff: passion, slaughter, ruth, and decay. Like the lovers made from little grains and polyps to form atolls, so too did all the "silt" filter and whisper down to create a foundation for this calm and quiet now. Earlier, "The storm of noon above us rolled,/ Of light and fury (17-18). You can picture the whole furious force of human strife being tumbled in the waves, ground down to such particles, until with "unrippling" and "unrocking," things finally settle down.
Our feet once wrought the hollow street With echo when the lamps were dead At windows, once our headlight glare Disturbed the doe that, leaping, fled. (29-32)
They have real lives, these two, really they do. It may seem like these two have been lying here silently doing and saying nothing forever, but they do regular stuff too—walking around, driving in cars. But the streets are hollow and they scared the doe off. It's not such an easy mix. Humanity remains separate and apart from nature, even in this memory.