Rocks Rolling Down A Hill?

So i need to develop a group of rocks that roll down a hill and they have to be in a line that cycles an infinite amount of times almost like the rocks from zelda. Anyone have any ideas on how to get this done? There has to be multiple of them in the same level so i cant use a spline that detects for one of them i think, if theres a better way to make it then i would love advice!