Biogeography, Ecology, and Geology
Due to fires in the early 19th century which burned a majority of the top 300 feet (90 m) around the summit, there is little soil cover on the peak and surroundings upland. Although the mountain has recovered to the degree that its landscape appears natural, Mount Monadnock is an example of a mountain which has an artificial tree line, occurring well below the climatic tree line of similar mountains in the region.
From the base to the summit, floral species diminish with increasingly shallow soil levels. The summit is home to many different specialized sub-alpine species which can retain moisture for long periods of time. Alpine and sub-alpine species include mountain ash, cotton grass, sheep laurel, mountain sandwort, and the mountain cranberry. Krummholz, trees stunted by harsh weather, are found on Mount Monadnock, as are several alpine bogs. Lower elevations on Mount Monadnock are clad in northern hardwood forest species; middle elevations support stands of red spruce. Before the fires, Mount Monadnock was totally covered in a red spruce forest. Since the summit has been barren of soil cover, red spruce have been slowly ascending back towards the top in a process known as ecological succession.
The mountain is largely composed of highly metamorphosed 400-million-year-old schist and quartzite rock primarily associated with the Devonian Littleton Formation, a stratum that extends south into Massachusetts and north into the White Mountains. 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 450 feet (140 m) west of the summit), shown in the 1942 edition of Marland P. Billings' Structural Geology. In addition to impressive folds, the Devonian Littleton Formation also shows large pseudomorphs of sillimanite after andalusite. These sillimanite crystals, normally needle thin, occur as 4-inch (102 mm) long "turkey tracks" on Mount Monadnock and were formed during prograde metamorphism.
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