Sorting and Orientation

After death, skeletal remains are moved by water or wind action and orientated relative to their hydrodynamic properties. Water currents transport lighter and smaller elements such as vertebrae, ribs, and foot bones at faster rates than limb bones, cranial bones and teeth.  Lighter bones move more quickly which causes vertical and horizontal sorting effects.  

Most bones are readily transported in water currents with elongate elements orienting either parallel or normal to the current direction.  Elongate bones with one end larger than the other generally orient with the larger one upstream, while equidemensional bones tend to roll transverse to the current.

This shows an allignment of shells.  The larger
end points upstream. The red arrow indicates the
direction of current flow.