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Friedrich was facinated with nature and often he would juxtapose nature with man made objects.
This painting is an expression of his love of nature and yet like many of his paintings its main theme is death, Friedrich often seemed to focus on death in his paintings, for example, giant grave by the sea and yet he doesnt depict death he states it by showing tombs.
Yet graveyard in the snow is also about mans preoccupation with death and burial, the open grave in the foreground has two spades carelessly dropped as though the owners of the spades have gone for a tea break. And yet more, could he be warning that the open grave is the final destination for everyone and as such we should see the warning in the painting that we all come to the end and die.
Even the plants in the painting are dead and the snow hints at the coldness of death, but wait, the plants could be just sleeping and will awaken in the spring and the snow will disappear. The workmen will return to pick up their spades and life is returned to the scene.