Running over a boulder is not a foul per se.
However, it clearly meets the first half of G38c (holding one or more boulders against a field element, in this case the carpet or other floor surface). Whether the referee rules that this was an attempt to shield or guard is dependent on the situation, for example:
- Did you run over a boulder that an opposing robot was trying to pick up?
- How long did you stay on top of the boulder?
As noted above, if you cause a boulder to cross over a defense towards your opponent's tower, or certain berms and lines, these can be fouls.
Also, if you run over the boulder in a direction that it would benefit your alliance even if it did not cross a proscribed defense/berm/line, it could be ruled herding.