Ice-lobe Retreat, Glacial Lake Levels, and the North American Varve Chronology in the Deerfield Basin of Western Massachusetts

From Hadley, MA north to Turners Falls, MA, there is a broad lowland known to geologists as the Deerfield Rift Basin. The basin is formed by bedrock lithology and structure within the lower stratigraphic section of the Mesozoic Hartford Rift Basin. Sedimentary bedrock lies beneath the lowland while many of the higher ridges are capped by basaltic lava flows. The Deerfield Basin is topographically separated from the southern part of the Connecticut Valley Lowland (Hartford Basin) by the eastward bending Holyoke Range (fig. 1). The modern Connecticut River flows through a narrow gap in this ridge. In most places the preglacial bedrock surface in the basin is covered by thick glacial deposits, largely glacial lake sediments. The postglacial Connecticut River flows over these deposits in most places but has cut down to bedrock through the Turners Falls area and at North Sunderland. The modern river has been diverted from its preglacial course by the deposition of glacial lake deposits, and the paleo-river channel in bedrock is not beneath the modern Connecticut River in most places. Figure 1 shows the position of the paleo-river channel where it lies at altitudes below 25 m in the north and at about sea level as it exits the basin through the Holyoke Gorge. The bedrock valley cut into less resistant sedimentary rock has been glacially scoured and over-deepened in some places to below -50 m in altitude.