High-quality seismic reflection data reveal the geometry of a blind, thinly tapered wedge of allochthonous rocks inserted into autochthonous foreland basin strata for >8 km east of the previously recognized deformation front (triangle zone) of the Canadian Rocky Mountain foothills west of Calgary, Alberta. Upper and lower detachment surfaces have been identified as boundaries between continuous and discontinuous reflection patterns over the length of the wedge. Coherent reflections above the upper and below the lower detachment show that strata outside the wedge are essentially undeformed. The upper detachment is parallel to bedding for at least 7 km, with a dip that decreases gradually from west to east. At its distal limit, the upper detachment lies 350 m above the lower detachment and does not merge with it. The internal reflection geometry of the wedge changes with position. We interpret the thickest part of the wedge to be dominated by thrust slices up to 500 m thick, whereas the toe of the wedge is folded and faulted internally.