Abstract: Throughout most of its length, the foreland margin of the Cordillera in Alberta is marked by a system of eastdipping under- thrust faults that overlie the more familiar west- dipping foothills imbricate thrust structures .Over 13000km² of the foothills, including many major oil and gas fields, lie concealed beneath the undertbrust zone, which is the erosionai remnant of an upper detachment that formerly covered a much larger area of the eastern Cordillera. The juxtaposition of east- and westdipping beds cropping out along opposite sides of the detachment results in a pseudo-aaticline hundreds of km in length, known as the Front Fold or Triangle Zone. Despite its apparently chaotic subsurface geology, the Triangle Zone can be shown to be compased of a systematic arrangement of stacked thrust sheets, whose thrusts are folded by step faulting and merge with an upper detachment zone that is itself foi ded by the tectonically thickened units beneath it. it is well known that the west- dipping thrust imbrications of the foothills beit and Front Ranges of the Rockies are rooted in a sole fault，decollement, or lower detachment. The geometry of the Triangle Zone proves the existence of an upper detachment, Merging of the upper and lower detachment beneath the axis of the Alberta syncline marks the eastern limit of foothills deformation. Cross sections through foreland margins of other thrust belts suggest that underthrust margins are common and that Triagle Zone , geometry and geological principles may explain some common structural anomalies.