The Ison Branch Formation, at the base of the Clarkston slice in the Atlanta thrust sheet, is here named. It consists of metamorphosed, finely laminated, graphitic, calcareous and pyritic felsic tuff which weathers to a nearly white spongy saprolite. Structurally overlies the Stonewall Formation in the Stonewall slice of the Atlanta thrust sheet, and underlies the Barrow Hill Formation in the Clarkston slice with probable gradational contact. Thickness is a few hundred m. Age is Late Proterozoic to Early Ordovician(?).

Source: GNU records (USGS DDS-6; Reston GNULEX).

Rocks mapped as unnamed fine-grained felsic gneiss in Six Mile thrust sheet are equivalent, at least in part, to Ison Branch Formation of Higgins and others (1988).

Source: GNU records (USGS DDS-6; Reston GNULEX).

Age of Ison Branch Formation of the allochthonous assemblage is changed to Late Proterozoic(?) to Middle Ordovician(?) based on a more conservative theory that the allochthonous rocks are 1) probably not older than Late Proterozoic because they are not part of Middle Proterozoic (Grenvillian) basement and 2) they could be as young, but not younger than, the time marking the beginning of closure of the ocean they originated in, as set by the slope reversal marked by deposition of Middle Ordovician Rockmart Slate upon unconformity at top of Upper Cambrian to Middle(?) Ordovician Knox Group and Middle Ordovician Lenoir Limestone. Report includes geologic map and correlation chart.

Source: GNU records (USGS DDS-6; Reston GNULEX).