They are underlain by a very complex geologic matrix of metamorphic, igneous, and sedimentary rocks. There are no glaciers. Low mountains along the Yukon River near Sam Creek.
Vegetation is mostly brush and scrubby spruce, with some aspen and birch.
Forests of white spruce, paper birch, and quaking aspen cover most lower slopes in the south and southfacing slopes in the north. Black spruce forest vegetation grows at higher elevations, on all northfacing slopes in the south, on all but steep southfacing slopes in the north, and on lower slopes with impeded soil drainage throughout the area. Above the black spruce forest, the vegetation is alpine meadow characterized by sedges on poorly drained sites and by low-growing shrubs on drier sites. The entire area is underlain by discontinuous permafrost. Upland furbearers, such as marten, mink, and shorttail and least weasels, are common.