Late Pleistocene and Recent geology of the Housatonic River region in northwestern Connecticut

Abstract

An investigation of Late Pleistocene and Recent surficial deposits in western Connecticut and adjacent areas was undertaken, to determine characteristics of Wisconsin glaciation and the history and chronology of deglaciation in part of the finely dissected New England Uplands. The study area lies along the midreach of the Housatonic River in western Connecticut, and has local relief exceeding 1,200 feet. Surface morphology and internal characteristics of glacial and glaciofluvial erosional and depositional features were examined and mapped in detail in the Kent and Ellsworth, Connecticut, USGS 7? minute quadrangles, and by reconnaissance in the surrounding quadrangles. This study contributes to the expanding detailed knowledge of glaciation and geomorphology in western New England and eastern New York state. Ice along the lateral east margin of the southward-waxing, Wisconsin-age, Hudson-Champlain Valley ice lobe successively overran ridges trending northeast-to-southwest. Late Wisconsin ice flow was consistently toward the southeast in the study area. Glacial erosion on the upland surfaces was weak, and several early or pre-Wisconsin meltwater channels persist, which evidence little late Wisconsin glacial or glaciofluvial modification. Deeply weathered rock has been locally preserved beneath unweathered till. Till deposits are generally thin, averaging from 10 to 15 feet in thickness, but till deposits exceeding 200 feet in thickness have been observed. Direct evidence for two or more cycles of till deposition is lacking, although multiple glaciations can be inferred from drainage derangement of the Housatonic River and from anomalies in configuration of old, upland melt-water channels which were re-occupied and eroded by melt water during subsequent deglaciations. The orientation of ridges and the local terrain relief exerted minor control on ice flow during waxing phases of glaciation. Local relief and ridges which were oriented transverse to ice flow became the dominant control factors for ice flow during late phases of deglaciation and ultimately initiated marginal stagnation zones. Late Wisconsin deglaciation evolved in three stages. First, the active ice margin receded rapidly northwestward across, and almost transverse to, the upland ridge crests in response to factors of both backwasting and downwasting. Second, local terrain relief restricted active ice flow, initiated stagnation, diverted melt-water flow and controlled deposition of small active ice-marginal deposits on the northwest slopes of ridges. Third, melting and thinning of stagnant ice tongues in valleys with ice surfaces which were low gradient and southward-sloping caused rapid northward recession of the stagnant ice margin. Sequences of related outwash deposits have been correlated with inferred ice-marginal, recessional positions. In this region, the zone of stagnant ice distal to active ice ranged from 6 to 15 miles in average width. Lacustrine sediments accumulated as stagnant ice blocks melted in isolated basins and other depressions where through-flowing melt-water drainage was restricted or absent. The paucity of ice-contact and outwash deposits in the isolated basins indicates that little entrained debris was present in the stagnant ice. Prograding outwash along the Housatonic River and other major drainage routes infilled glacially overdeepened underlying lacustrine sediments sand and gravel. rock basins and buried beneath upward-coarsening Upland bogs, which have developed postglacially, contain as much as 22 feet of organic material mixed with silt and clay. An age of 12,750 ? 230 years B.P. was determined for materials immediately above three feet of older organic-rich clay layers. This dated material correlates with the upper part of the pollen T zone reported elsewhere in Connecticut and New York.