Densification in highly porous, isothermal firn occurs primarily by Newtonian sliding on grain boundaries, but the coordination number increases with density and restricts further sliding. At a relative density of about 0. 6 the coordination number approaches 6 and sliding ceases to be a primary mechanism of densification; the resulting decrease in densification rate causes the critical point in depth-density profiles. A simple model for densification by boundary sliding yields a good fit to observed profiles. The viscosities so obtained give an activation energy equal to that for grain-boundary diffusion.