ENVIRONMENTAL CHANGE RECORDED BENEATH MIRROR LAKE, NEW HAMPSHIRE: A MID-HOLOCENE DELTA, LATE-GLACIAL COEVAL SLUMPS, GLACIOFLUVIAL CLINOFORMS, AND MID TO LATE GLACIAL FLOODING INTERPRETED FROM GROUND-PENETRATING RADAR PROFILES

Ground-penetrating radar profiles recorded on Mirror Lake in West Thornton, New Hampshire, reveal episodes of environmental change dating to the late Wisconsin, at least. Investigations based on coring in the 1970s and 1980s concluded that Holocene gyttja–organic sediment–was unusually focused to the deeper basin and most strongly after 4000 years BP because accumulation rates between peripheral and mid basin subsequently, and most significantly, differed. Based on presumed pollen rates, decreased allochthonous or autochthonous input was discarded. The age-dated cores reached a layer of sub Holocene, 11–14 ka BP glacial silt, which generally coincides with recent varve dating for glacial retreat in this area, but went no deeper. However, ground-penetrating radar profiles I recorded over Mirror Lake go much deeper to reveal several episodes of environmental change. A delta is buried within the gyttja, and its age of final deposition approximately corresponds with the 4000 year depth of gyttja. Beneath at least these two stages of gyttja deposition is the silt. Late glacial coeval slumps beneath the silt and recorded along different profiles imply either massive fluvial input or an earthquake. The glaciofluvial clinoforms that comprise the delta lay beneath the glacial silt and may be the source of the slumps. The clinoforms are up to 11 m thick and must be older than 14 ka BP. Deeper still are layered, coarsening-upward diamicton that resides on bedrock. The source is surely the boulder till that surrounds the lake and the stratigraphy of the diamicton suggests catastrophic flow.