In deciding to come to the woods, Thoreau says, one attraction he anticipated was watching spring arrive. The ice on Walden Pond is always the last of all the ponds' ice to melt, because that pond is the deepest. Thoreau provides temperature readings of the water in different parts of the different ponds to demonstrate how ice in shallow water typically breaks up more quickly than ice in deep water. An old man told Thoreau that once, while he waited in the bushes with his gun for ducks to play in a part of the pond that had melted, he heard a great rumbling sound that turned out to be the movement of the ice against the shore.
The ice on the pond fascinates Thoreau in its great power and mystery. The changes it undergoes in spring are among the most exciting and spiritually enriching events that happen to him in the two years he spends at Walden.
Thoreau likes to watch the flowing sand and clay by the railroads as the spring thaws, comparing the patterns that form to blood vessels and to leaves, which he says are nature's master pattern. Then he compares the parts of the human body to flowing mineral mater and to leaves. He feels as if he is "in the laboratory of the Artist who made the world" and he sees all of nature in these sand patterns, calling the earth "living poetry."
First Thoreau compares nature to the human body, then he compares the human body to nature. In essence, they are parts of the same whole, and by observing them, Thoreau pays respect to the great mysteries of life, which are always available in the smallest details of natural life.
Thoreau celebrates spring as a glorious influx of light and warmth, and he details the first signs of vegetation, the animals beginning to move around, and the sparrows and other birds returning to the woods. Nature calls on humankind to live blessedly in the present moment, he believes; spring can teach men forgiveness and how to treat each other well. Once, Thoreau saw a hawk in what he calls the most ethereal flight he ever witnessed, having no companion but not at all lonely. Thoreau believes that spring is proof of immortality, and that wild nature can refresh men and bring them back to life. After two years, two months, and two days, he leaves Walden.
Thoreau believes that, paradoxically, nature can be a model both for mankind's solitude—as in the hawk—and for its society—as in the seasonal changes, in which all is forgiven and brought together. Nature is the source of life and in it miracles can be witnessed.