"these chalky hogs'-backs"
Unpainted pig sculptures being prepared for
GNU Free Documentation LicenseUnpainted pig sculptures being prepared for "Pigs on Parade" in downtown Seattle - Credit: Joe Mabel

Hogs'-backs are hills.

The Hog's-back is a raised part of the North Downs in Surrey. Hardy is using the term in a more general sense here to describe any hills, ridges or escarpments that mimic the narrow, rounded shape of a pig's back.