Geologic units in Galax City, Virginia

Additional scientific data in this geographic area

Ashe Formation - Biotite gneiss (Proterozoic Z) at surface, covers 80 % of this area

Medium- to light-gray, massive, conglomeratic biotite schist and gneiss, with feldspar, quartz, and granitic clasts; grades upwards into medium- to fine-grained, salt-and-pepper-textured two-mica plagioclase gneiss with very-light-gray mica schist interbeds. Quartzite, impure marble, calcareous gneiss and amphibolite occur locally. Some dark-gray to black, pyrite-bearing mica schist occurs at tops of thick, fining-upwards graded sequences. Mineralogy: (1) quartz + plagioclase + potassium feldspar + biotite + muscovite + chlorite + epidote + ilmenite; (2) quartz + plagioclase + biotite + muscovite + epidote-allanite + garnet + titanite + ilmenite; (3) quartz + calcite + plagioclase + biotite + muscovite + epidote + ilmenite + titanite; chlorite occurs as a secondary mineral. Unit is unconformable on Grenville basement and cut by Late Precambrian mafic and felsic dikes.

Ashe Formation - Mica schist or phyllite (Proterozoic Z) at surface, covers 12 % of this area

Medium- to dark-gray, medium- to fine-grained mica schist, phyllite, and slate. Mineralogy: (1) quartz + muscovite + magnetite; (2) quartz + muscovite + chlorite + stilpnomelane; (3) quartz + plagioclase + biotite + garnet + magnetite; (4) quartz + biotite + stau ro lite + garnet + magnetite; (5) quartz + biotite + kyanite + garnet + staurolite; chlorite occurs as a secondary mineral. Geophysical signature: isolated magnetic peaks resulting from concentrations of magnetite in the wider belts of mica schist. Although these rocks have been grouped as a single unit following Espenshade and others (1975), mapping in Floyd County (Dietrich, 1959) suggests that the unit includes rocks stratigraphically at the base of the Ashe as well as rocks stratigraphically above the Ashe, coeval in part with the Alligator Back Formation.

Ashe Formation - Amphibolite (Proterozoic Z) at surface, covers 8 % of this area

Dark-greenish-gray to black, coarse to fine-grained amphibolite, hornblende gneiss, and schist, with interlayered biotite-muscovite gneiss and mica schist. Coarse garnetiferous amphibolite, pink and white marble, and pyrite-chalcopyrite-calcite veins are common near the top of the Ashe. Mineralogy: (1) quartz + actinolite + epidote + chlorite; (2) quartz + hornblende + plagioclase + epidote + garnet + magnetite. Geophysical signature: amphibolite, and hornblende gneiss and schist give positive linear magnetic anomalies. Relict amygdaloidal textures and hyaloclastic (pillow) structures indicate massive to thick-bedded amphibolite and hornblende gneiss were derived from basaltic flows or shallow sills. Some thin-bedded hornblende gneiss and schist units that commonly contain interbedded micaceous and feldspathic layers may be derived from volcaniclastic sedimentary rocks.