A zone of maximum underpressures, i.e., subhydrostatic pore pressures, is found 500 to 1000 m below the present land surface in Upper Cretaceous and Tertiary strata of west central Alberta. Vertical hydraulic gradients in overlying and underlying strata indicate vertical fluid-driving forces convergent into this zone. Lateral gradients indicate fluid flow towards the area in which maximum erosional stripping has occurred since mid-Tertiary. Rock-pore dilation in hydraulically ＇＇tight＇＇ strata due to erosional unloading is the best explanation for these observations.