The final part that we will discuss today has to do with the End Devonian to the Early Mississippian (~350-340 ma). During this period we find the western edge of NA switching from a passive margin to an active one where deformation due to subduction has thrust the distal sediments of a ‘phase 2’ age up and over the near-shore passive margin sediments. This mountain building event, called the Antler Orogeny and was driven by convergent mountain building whose origin remains a subject of much controversy. This constructed a large mountain range bounded on the east by the leading edge of the thrust (the Robert’s Mountain thrust). The loading of the crust due to the emplacement of the thrust sheet created a flexural depression in the crust (the Antler Foredeep) on the east side of the mountains. This depression or foredeep was then the locus of deposition for sediments eroded off the Antler Orogeny. The age of the distal sediments overthrust in the Antler Orogeny are the same as the near shore passive margin and the fossils within the distal sediment are also of North American affinity. This then explains that this overthurst material was ‘indigenous’ (or contiguous with the footwall rock) rather than ‘exotic’ as the island arcs are. Though the Antler Orogeny was due to the interaction of convergent activity along the western edge of north America, it would not be the last. There are plenty more to come! Image courtesy of Professor Burchfiel.