Q.very low ground cover for rock garden
My rock garden is a small area in the front of my house which faces the west, so shade all morning till noon then sun till it sets. I was wanting a green under surface to my rocks on top so it has to be very low lying and hearty and comes back every year after a cold winter.
Creeping Thyme would be one of my top choices, it stays very low.
Creeping Phlox is also a very good ground cover.
Check local garden centers, as they will have plants hardy to your zone.