The Rock is a Transformed substance (Muto Requisite) so it is going to be resisted.
The motive force is also a magical medium so also resisted.
Look at the examples in ARS5 of the magical jet of magical water vs the magical jet of natural water.
When the propelling force is magical, then the propelling force is resisted.
Had this simply launched natural mud at the resisting target rather then sharp rocks then he would become forcelessly covered in mud
A