array9

You might have thought that the random action of waves on a beach would have produced spherical pebbles.  But this is not the case, indeed almost all are distinctly not spherical.

Probably the best way of thinking about it is in terms of potential energy and packing efficiency.  If you take a box of squashy balls, you can squish down on them and lower the centre of gravity.  You get them to pack just slightly better.  That’s what nature likes.

Now this does not explain the shape that they actually adopt, as squashing would produce some foam bubble shape.

Maybe they don't start off being highly symmetric, but have certain long and short axes.  And the erosion is likely to be mostly around the short axes, and these will roll more readily.  So you might get the long axes remaining, with the short axes having higher wear rates.  This produces better packing than spheres.  The more dimensions the worse the packing, I guess.  Packing balls is worse than packing rolls.

martin