The forest begins as a lively, ever-moving place where creatures have come and gone for ages. Then the ground rumbles, thunder roars, and a huge silver "beast" tears through the trees, screaming and breathing fire. The poem never names it, but the imagery of metal, smoke, speed and a shrill scream suggests a train, or some similar machine, crashing into the natural world.
The speaker watches from the animals' side, and the feeling is mostly helpless fear. The creatures don't understand what is happening, can't stop it, and know there is no escape. The plants wither, the water boils, and the animals flee with almost nothing.
The mood shifts from calm to dread to emptiness. The ending is the bleakest part: the beast vanishes as fast as it came, leaving a barren, rotten wood. The last line, "no one to remember, and no one to forget," suggests a loss so complete that even grief has nobody left to carry it.