Gneiss Outcrop in Montana

February 25, 2014

Gneissbarnes (2)

Photographer: Lindsey Barnes
Summary Author: Rod Benson; Rod's Web site

This outcropping of gneiss is located in a cirque called Beehive Basin, about 35 mi (56 km) south-southwest of Bozeman, Montana. Gneiss is a metamorphic rock that forms when sedimentary rock (such as shale) or igneous rock (such as granite) is changed by heat and or pressure and in some cases even by chemically active fluids.

Gneiss makes up most of the lower portion of Earth's continental crust -- very old material referred to as basement rock. In most places, basement rock is covered with younger sedimentary rocks. However, drill down far enough anywhere on a continent and you’ll usually strike gneiss. In fact, the rock that makes up the mountains in southern Montana, including the gneiss in the photo, are among the oldest in the state -- formed during the Archean Eon over 2.5 billion years ago. Here, blocks of crust were forced upward and many of the younger rock formations eroded away, exposing the basement rock. Today this gneiss makes up many of the mountains in and around Montana, including the Beartooths, the Tobacco Roots and a significant portion of the mountains in between. Photo taken on January 18, 2014.