The flexure of the Mississippian Unconformity (MU) is used to constrain the stiffness of the lithosphere beneath the Alberta Foreland Basin (AFB). This flexure supports the sedimentological evidence for the absence of a forebulge in the AFB and implies that the peak of the forebulge lies further east of the Alberta Saskatchewan border. It is demonstrated that an eastward stiffening lithosphere is required in order to fit the flexure of the MU. When flexural stiffness is expressed in terms of effective thickness, it varies from about 38 km west of the Rocky Mountains to more than 200 km underneath the North American craton. This variation of stiffness indicates that there is a strong lateral temperature and chemical variation underneath. Eastward stiffening also implies an eastward thickening of the elastic lithosphere. Such a model is in good agreement with recent petrological and geophysical evidences in the west and underneath the craton. ¿ American Geophysical Union 1991