Unit is named the Post River Formation for the Post River, McGrath 1:250,000 quad, AK. Is divided into five members (ascending): lower siltstone member, mudstone member, upper siltstone member, Graptolite Canyon Member (new), and limestone member, Consists mainly of fissile shale, mudstone, and silty and argillaceous limestone. Basal contact is thrust(?) fault; conformably underlies the Terra Cotta Mountains Sandstone (new). Thickness is 465+m. Age is Early Ordovician to Early Silurian (Tremadocian to Llandoverian) based on contained fossils (graptolites).
Source: GNU records (USGS DDS-6; Menlo GNULEX).
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