Avalanches
If you mix two types of sand they will behave differently when you shake or pour them, so they can separate into different layers.
Part of this is due to the 'Brazil nut effect', essentially if a mixture of particle sizes are shaken whenever the large particles lift slightly a small one can fall into the gap under it but it doesn't work the other way around.
This effect is one of the ways that geologists can tell which way up a rock was created, as well as causing lots of headaches for muesli manufacturers.