The Sarek National Park lies within the Scandinavian Mountains, a mountain chain whose origin is still a matter of debate. The rocks of the Scandinavian Mountains were put in place by the Caledonian orogeny, forming a belt of deformed and displaced rocks now known as the Scandinavian Caledonides. Caledonian rocks overlie rocks of the much older Svecokarelian and Sveconorwegian provinces. The Caledonian rocks actually form large nappes () that have been thrusted over the older rocks. Much of the Caledonian rocks have been eroded since they were put in place meaning that they were once thicker and more contiguous. It is also implyed from the erosion that the nappes of Caledonian rock reached once further east than they do today. The erosion has left remaining massifs of Caledonian rocks and windows of Precambrian rock. The Caledonian orogeny resulted from the collision of the Laurentia and Baltica plates, 450 to 250 million years ago, with the disappearance of the Iapetus Ocean by subduction. This happened just before the formation of the chain and was caused by the appearance of a rift, which finally led to the creation of the Atlantic Ocean. The chain, once split open, continued to erode until it formed a peneplain.