Silt mantles the upland slopes and ridge tops throughout the unglaciated interior of Alaska, reaching maximum thickness along the north side of the Tanana River Valley. The silt is probably loess deposited during glacial advances by south winds blowing from the glaciated Alaska Range across outwash plains in the Tanana Valley.
In the Fairbanks area, where it is best exposed, the upland silt is 1–80 feet thick on tops of low hills, 50–150 feet above the valley floor. It is 10–100 feet thick on middle slopes of higher hills and thins to a few feet on the higher slopes of ridges 800–2000 feet above the valley. Much silt has been reworked and moved by stream erosion into valley bottoms where it forms a fill 10–300 feet thick.
The upland silt is well sorted. The texture and mineral composition are uniform throughout the Yukon-Tanana upland, whether the silt overlies schist, chert, granite or basalt. The silt stands in sheer cliffs and is massive, showing little or no stratification.
During the past 50 years, fluviatile, marine, estuarine, lacustrine, residual, and eolian hypotheses have been proposed to explain the origin of the upland silt. The fluviatile marine and estuarine hypotheses never had strong support, but the lacustrine concept was popular in the early part of the century.
Most of the upland silt is considered to be wind blown, derived from glacial outwash because: (1) it occurs as a surficial mantle (2) it is lithologically independent of the underlying material, (3) stratification is indistinct or absent, (4) it is associated with sand dunes and ventifacts, (5) it contains fossils of air-breathing land animals, (6) the sorting and texture are similar to upper Mississippi Valley loess and wind-blown dust, (7) the grains are angular and relatively fresh, and (8) loess is being deposited in the region today.