The last chapters in Walden links between the natural world and a spiritual communion. This link has helped develop our nowadays appreciation of ecology and nature. There is also a seasonal structure that symbolizes a spiritual rebirth. This is illustrated in Henry's walk from the moment he arrived at the pond in March till he reached the following spring.
Based on this, the description that best fits the overall structure of Walden would be:
The work describes the effect spending time in nature has on Thoreau, and then explains why it has that effect.
