Interpretation of reflection seismic data from the Sundre area of southern Alberta has revealed the detailed internal geometry of a ＇＇compound tectonic wedge＇＇ beneath a gently eastward-dipping homocline. The wedge extends for a distance of 8 km into the foreland basin east of the exposed apex of the triangle zone and is separated from the overlying homocline by a smooth, listric upper detachment occurring within the Upper Cretaceous Brazeau Group. Maximum dip of the upper detachment is 20 degrees. The lower detachment of the wedge rides within Lower Cretaceous rocks, varying locally in position between shales of the Alberta Group and coals of the Blairmore Group. It is approximately 1.5 km deeper in the section than the upper detachment at the foreland limit of the wedge. Thickening of the wedge occurred by displacement along a series of frontal ramps which branch from the lower detachment and generally merge with the upper detachment toward the foreland. A backthrust also developed within the wedge, resulting in the formation of a pop-up with a vertical uplift of approximately 300 m. The backthrust to the pop-up branches from the lower detachment and cuts upsection toward the hinterland, steepening to a dip of nearly 60 degrees at its tip point. The pop-up terminates to the south into a fault propagation fold which has only a small displacement.