First published use in vicinity of McCaslin syncline in western Marinette Co, WI (Lake Superior region). Type locality not designated. Derivation of name not stated but apparently named from Thunder Mountain, which consists of quartzite striking north-south. Description at field trip Stop 3 at Thunder River Trout Rearing Station as follows: "The main rock here is Thunder Mt. quartzite (presumably the same as McCaslin quartzite. At this location it is relatively fine-grained and somewhat impure--more properly, perhaps, a quartz-rich greywacke. Along the west edge of the north part of the outcrop, even beds of quartzite may be seen dipping westward. These are in contact, a few feet to the east, with a massive grey rock containing conspicuous phenocrysts of hornblende and whitish feldspar...contact with the quartzite appears to be intrusive." The unit called Thunder Mountain quartzite at Stop 3 is mapped as McCaslin quartzite on geologic sketch map (p. 3). Age is not stated.

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