Region 3: The Northern Rocky Mountains

The Northern Rocky Mountains provide only a glimpse of the Rocky Mountains that nearly bisect the country. The Rockies formed as the result of both volcanism and terrane accretion. From the Jurassic to the Cenozoic, oceanic lithosphere was subducted under the continent at a very shallow angle, driving volcanism farther inland than is typical. The subducting plate also brought terranes towards the continent, which accreted during events like the Laramide Orogeny. The coastline of the continent began to grow and move farther from the Rocky Mountains, also pushing the subduction zone farther away. With the lack of nearby subduction, volcanism ceased and the Rockies were left far from the coast, tectonically inactive and gradually eroding.

The mountainous terrain and past volcanism are equally responsible for the soil types present in the region. Inceptisols are common due to the steep slopes and wet conditions found in elevated areas. The rapid erosion and frequent washing away of soils means that soils in many areas are poorly developed. Although the bulk of Inceptisols occur on the East Coast, there are parts of the Western US, like the Rockies, that are both humid and forested enough for them to form. The Northern Rockies are unique to the rest of the Rockies in that they also harbor Andisols, a relict of the region’s volcanic past that are more dominant in the Cascade-Sierra region.