Grinding paste?

Every cyclists who cares for their machine ought to be aware that sand mixed with or or grease to makes grinding paste, and if the sand isn't removed (somehow)  then the gear teeth and chain are likely to become decidedly worn.  I have seen gears looking like the blades of circular saws.

The advice is:-  keep your bikes away from sand if you can.  .

With those bikes shown in the photo the derailleur hanger arm is lying directly on the sand.  Not good!