The extensive gold-bearing gravel deposits in the Fairbanks district are covered with a muck-silt overburden varying from a few feet to over 200 feet thick. This overburden is economically important as (1) Its thickness is often a limiting factor in determining whether the underlying gold-bearing gravel can be profitably mined, since with large-scale mining methods it must be entirely removed, and (2) The difficulty and cost of exploration have been increased, as the overburden has concealed many of the gold-bearing channels.
The overburden is of two types—muck and silt. The muck, which is frozen, occupies the present stream valleys and lies unconformably upon, or grades into, the silt, which is unfrozen and occurs on the lower hill-slopes. The inorganic material in the two deposits is essentially the same, but the muck contains about 50 per cent ice and vegetable material as well as an abundant Pleistocene fauna.
The origin of the muck and silt has long been obscure because of poor exposures and the complications of sub-Arctic solifluction processes. It now seems certain that the silt is of aeolian origin, while the inorganic material of the muck consists principally of wind-blown particles that have been reworked by creep, sheetwash, and stream action. A large part of the interior of Alaska is unglaciated; the deep muck-silt deposits occur in this unglaciated area and are probably a result of Pleistocene glaciation in nearby localities, as conditions were then ideal for aeolian deposition.