The region was not glaciated during the Pleistocene epoch. Soils Dominant soils of this ecoregion are Histic Pergelic Cryaquepts, Pergelic Cryaquepts, Aquic Cryochrepts, Pergelic Cryochrepts, Typic Cryochrepts, Typic Cryorthents, and Pergelic Cryumbrepts. Many upland soils were formed in silty, micaceous loess and colluvial material. Where mantles of loess are lacking, upland soils formed in stone and gravel weathered from local rock. Lowland soils were formed in silty alluvium and loess derived from the floodplains of large rivers.
Soils are generally shallow, often overlying ice-rich permafrost.
Those soils with permafrost, especially in the eastern portion of the ecoregion, are very susceptible to alteration upon disturbance of the organic mat. Organic mat disturbance, as by wildfire, can result in warmer soil temperatures, lowered permafrost tables, and significant changes in soil physical properties and hydrology. Vegetation Interrelationships among permafrost, surface water, fire, hillslope aspect, and soil characteristics result in a finely-textured, complex pattern of vegetation across the ecoregion. Soil temperatures may differ greatly from air temperatures, so patterns in vegetation may not correspond with expected site conditions. Needleleaf, broadleaf, and mixed forests occur over a variety of sites.