I'm not familiar with John Rock / Elizabeth Snodgrass.
So far I only have data for Joseph Dennis. I speculate, but haven't verified that Edwin Dennis and Sarah (Rock?) are Joseph's parents. I speculate that this Joseph Dennis is the same Joseph Dennis as seen on Edwin Dennis 1870 census of Giles County VA. I haven't yet located Edwin / Sarah in 1850 or 1860 census.
I have speculated that Thomas Rock / Sarah (Parsons?) may be Edwin's wife Sarah's parents, but again, no evidence of such. There is a Minnie Parsons 64 years old, living 2 doors down from Edwin and Sarah in 1870. So I do find that there are Parsons family in the area. There is a Thomas and Sarah Rock living in Montgomery Co VA in 1850.