The mountain is largely composed of 400-million-year-old schist and quartzite rock primarily associated with the Devonian Littleton Formation, which extends south into Massachusetts and north into the White Mountains.Raymo, Chet and Maureen E. Written in Stone: A Geologic History of the Northeastern United States. Globe Pequot, Chester, Connecticut, 1989. At lower elevations on the mountain, and stratigraphically below the Littleton Formation, the bedrock is from the Silurian period, consisting of Rangeley Formation schists, Perry Mountain Formation quartzites, the Francestown Formation granulite, and the Warner Formation granulite. Structurally, the mountain is part of an overturned syncline – called a fold nappe – that was caused by the compressional forces of the Acadian orogeny. Dramatic small- to medium-scale metamorphic folds are visible on many of the rock faces of the mountain, including the famous Billings Fold (a recumbent syncline found about west of the summit), shown in the 1942 edition of Marland P. Billings' Structural Geology.Thompson, Peter J., "Geology of Mount Monadnock, New Hampshire", in West, David P. and Bailey, Richard H., eds., Guidebook for Geological Field Trips in New England, Geological Society of America, United States, 2001 In addition to impressive folds, the Devonian Littleton Formation shows large pseudomorphs of sillimanite after andalusite that occur as "turkey tracks".
The mountain is largely composed of 400-million-year-old schist and quartzite rock primarily associated with the Devonian Littleton Formation, which extends south into Massachusetts and north into the White Mountains.Raymo, Chet and Maureen E. Written in Stone: A Geologic History of the Northeastern United States. Globe Pequot Chester, Connecticut, 1989. At lower elevations on the mountain, and stratigraphically below the Littleton Formation, the bedrock is from the Silurian period, consisting of Rangeley Formation schists, Perry Mountain Formation quartzites, the Francestown Formation granulite, and the Warner Formation granulite. Structurally, the mountain is part of an overturned syncline – called a fold nappe – that was caused by the compressional forces of the Acadian orogeny. Dramatic small- to medium-scale metamorphic folds are visible on many of the rock faces of the mountain, including the famous Billings Fold (a recumbent syncline found about west of the summit), shown in the 1942 edition of Marland P. Billings' Structural Geology.Thompson, Peter J., "Geology of Mount Monadnock, New Hampshire", in West, David P. and Bailey, Richard H., eds., Guidebook for Geological Field Trips in New England, Geological Society of America, United States, 2001 In addition to impressive folds, the Devonian Littleton Formation shows large pseudomorphs of sillimanite after andalusite that occur as "turkey tracks".