According to Native American myth, Sugarloaf Mountain is the carcass of a human-eating giant beaver who lived in a lake now occupied by the Connecticut River.

A now extinct species of giant beaver as big as black bears once inhabited North America at about the same time (see Pocumtuck Range for details).

[2] Sugarloaf Mountain, geologically contiguous with the Pocumtuck Ridge to the north, is composed of Triassic arkose sandstone and conglomerate.

The rift basin formed in Triassic-Jurassic time and is part of an area of crustal extension that extended from Long Island Sound into Vermont and New Hampshire.

The 2,000 metres (6,600 ft) thick Sugarloaf arkose is the basal unit of the Deerfield rift basin and was formed by fluvial and alluvial processes.

The Fall River beds are capped by 80 m (260 ft) of tholeiitic basalt flows which are most visible as part of the geology of the Pocumtuck Ridge to the north.

About 200 million years ago, as the continent of North America began rifting apart from Africa and Eurasia, a series of erosion and deposition episodes interspersed with basalt lava flows created this layer cake.

[8][9] The formation is known in other locations to preserve Triassic fossils such coprolites, plant remains such as Clathropteris and Equisitites , and fishes such as Semionotus, Redfieldius, and Ptycholepis.

[12] North Sugarloaf Mountain is only accessible by foot; a small cave is located just beneath the summit ledges.