Slope gradients are almost always greater than 5 on hillslopes, and exceed 25 on some mountains. The southern portion of the ecoregion is underlain by granitic batholiths intrusive into moderately metamorphosed, highly deformed Paleozoic and Mesozoic volcanic and sedimentary rocks. Large active volcanoes occur in this area. The central and eastern portions of the ecoregion are part of a broad syncline having Cretaceous rocks in the center and Paleozoic and Precambrian rocks on the flanks. The ecoregion was heavily glaciated during the Pleistocene epoch, and extensive systems of valley glaciers still radiate from the higher mountains.
Rock glaciers are common.
Gelifluction features are well developed. Permafrost is discontinuous in this ecoregion, however, its full extent is unknown. Streams are swift and braided, and most are headwatered in glaciers. Many large lakes occupy the glaciated valleys within the southern portion of the ecoregion. Soils Much of the ecoregion consists of rocky slopes, icefields, and glaciers.