Bones become rocky outcroppings.
Teeth become gravel and boulders.
Flesh becomes the earth.
Blood becomes the sea.
The Norse creation myth imagines the landscape to be made up of parts of a dismembered human(-like) body. We know the bloody corpse thing is a bit freaky, but overall, it's not as strange as it sounds. Come on – haven't you ever noticed how much the jut of your knee resembled a nearby hill, or seen human and animal shapes while cloud-watching on a lazy summer afternoon? But what does it really mean to see bodies in the landscape?
For the Lakota people of western South Dakota, a long ridge of red and yellow rock in the Martian-like landscape of the Badlands are the vertebrae of a water-monster named Unktehi, who laid down there after causing a great flood. The Aboriginal people of Australia believe that all of the boulders, hills, caves, lakes and other distinctive landforms they see are the bodies of ancestral animals and people who turned to stone at the end of the "Dreamtime." For both the Lakota and the Aborigines, bodies in their landscapes explain why it looks the way it does, and imbue it with a sacred power. Seeing the bodies of their ancestors or mythical characters in the landscape also link that specific land to their culture and history. It's a way of connecting themselves to the land they live in and of expressing that feeling of connectedness in the stories they tell about themselves.
Is that what's going on as the body parts of the frost-giant Ymir become the Norse cosmos? Maybe. Although he's a part of their mythology, Ymir the frost-giant probably isn't someone the storyteller would have identified with. He was a wild, ugly monster who had to be destroyed before the Norse world could come into being. So seeing his parts in the landscape might also be a way of expressing a feeling of conquest or ownership over a land that had been "tamed."
Describing the landscape as the body of someone you can conquer and tame is actually a pretty common literary technique to make an unfamiliar or "wild" landscape feel safe and justify taking it. Colonial accounts of the Americas describe it as a "virgin" female body waiting to be possessed by Europeans. Maybe this technique is just a natural extension of the mythological impulse to see the human body in the landscape.