Five thousand joints were examined in gently folded sandstones of the Cretaceous Cardium formation along the Bow River in the Rocky Mountain foothills of central Alberta. Twenty domains, based on homogeneity of bedding and joint orientation, were established. Joint sets from all domains fall into four classes, J1–4, all normal to bedding. J1 parallels the structural trend of the foothills, and occurs in all domains. J3 and J4 form conjugate pairs whose acute bisectrices are approximately normal to J1, and occur in western domains; the dihedral angle between conjugate pairs tends to decrease eastward. J2 is approximately normal to J1; poles to J2 sets have elongate maxima, and the sets replace conjugate J3−J4 sets in eastern domains. No joints are displaced by bedding-plane slip.J1 and J2 sets are interpreted as extension sets, and J3 and J4 sets as conjugate shear sets. Each J2 set represents conjugate shear sets with a dihedral angle too small to allow their separation. J2, J3, and J4 sets are older than J1 sets. By analogy with experimentally produced internal fractures, the easterly decrease in dihedral angle associated with the older sets resulted from an easterly decrease in the magnitude of the causative stress system, the effective minimum principal stress of which was tensile and approximately parallel to fold axes. This stress system was a residual of the orogenic system modified by postorogenic horizontal extension related to uplift. After the older joints had formed, further uplift led to the maximum principal stress becoming normal to bedding and to the development of J1 joints.