The north side of the central Pioneer Valley lacks a distinct boundary although it contains the short Pocumtuck Range which includes Mt. Sugarloaf in South Deerfield. The range has a Deerfield Basalt layer sandwiched between a thick sequence of rift sediments, indicating the continuation north of the Metacomet Ridge.
Just north of Sugarloaf lies Mt. Toby, which is comprised of conglomerate and arkosic sandstone. This landform largely lacks basalt, although according to the U.S. Geological Survey a portion of the Deerfield lava sheets rsts in sequence on the Mt. Toby conglomerate.
