For decades, continental shields and cratons have largely been assumed as rigid blocks resisting deformation and providing a backstop against which active tectonic process terminate. These stable islands of ancient material suffer little internal deformation, and are potentially altered only by extreme events, such as the passing of mantle plumes or onset of rifting. However, as additional insights and geoscientific techniques have peered at and modelled these regions in more detail, it has become clear that they have undergone much more extensive internal deformation and alteration than once supposed. In this work, we examine the leading edge of the North American continent, specifically that which abuts immediately against the Canadian Cordillera, and presumably provided a rigid backstop isolating hundreds of millions of years of tectonic activity to be confined with the Cordillera and offshore plate boundaries. Through examination of tomographic models along the extent of the tectonic transition from Canadian Cordillera in the west and stable shield and craton to the east, we demonstrate that this ``stable’’ buttress is in fact, likely not a stable feature after all. Using recent results from geodynamic modelling, along with our observations from seismic tomography, we highlight that the interaction between the mantle lithosphere and asthenosphere of the cordillera and the craton varies substantially along the strike of the boundary. Furthermore, the present observed state of this boundary at some locations necessitates that its geometry is evolving through time, on time-scales of only 10s of millions of years.