and is part of an area of crustal extension that extended from Long Island Sound into Vermont and New Hampshire. The thick Sugarloaf arkose is the basal unit of the Deerfield rift basin and was formed by fluvial and alluvial processes. It is overlain by of lacustrine deposits called the Fall River beds. The Fall River beds are capped by of tholeiitic basalt flows which are most visible as part of the geology of the Pocumtuck Ridge to the north. The basalts are overlain by a black siltstone to conglomerate called the Turners Falls Sandstone and Mt. Toby Conglomerate. Mount Toby, the type locality for the Mt. Toby Conglomerate, can be seen directly across the Connecticut River from Sugarloaf Mountain. 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. Faulting and earthquakes tilted the layers; subsequent erosion and glacial activity exposed the tilted "layers" of sandstone, basalt, and erosion-resistant conglomerate visible today. Although not composed of basalt trap rock, Sugarloaf Mountain is a closely associated with the Metacomet Ridge by virtue of its origin via the same rifting and uplift that created those mountains. The Sugarloaf Formation (or Sugarloaf Arkose), of which Mt. Sugarloaf is partially composed, was deposited mainly by rivers and alluvial fans, with some localized floodplain ponds or swamps. 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. Additionally, the Sugarloaf Formation preserves rare examples of lithified armored mud balls. Recreation A seasonal auto road climbs the summit of South Sugarloaf (open late spring through the fall foliage season), and a small seasonal observation tower is located on the summit. Several steep hiking trails, including the Pocumtuck Ridge Trail, make the mountain a favorite local hiking destination. Most of the mountain has been conserved as part of the Mount Sugarloaf State Reservation. The reservation is easily accessible via Route 116 and is only a 15-minute drive from the University of Massachusetts Amherst campus. There is parking at the base of the mountain and at the top of the paved road that leads from the parking area to the summit of South Sugarloaf. It is free to hike the trails or climb the