Named for Goose Creek, Park Co, MT in the Northern Rocky Mountain region. No type locality designated. Is the oldest rock in report area. Consists of gneissic gray granite that locally carries inclusions of biotite or hornblende schist. Is extensively exposed in northern part of geologic map. Its metamorphic texture ranges from strongly banded biotite gneiss to granite distinguished by granulated feldspar. Commonly consists of 65 percent gray feldspar (orthoclase and oligoclase), 20 percent quartz, and about 15 percent biotite. Medium- to coarse-grained textures predominate; fine-grained facies rare. Narrow masses of hornblende and chlorite schist cut the unit; these probably represent early basic intrusive rocks rendered schistose by the same event that gave gneissic structure to the granite. Cut by Cooke granite (new). Of Archean age.

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