A NEW COSMOGENIC EXPOSURE AGE FOR EMPLACEMENT OF NEW HAMPSHIRE'S MADISON BOULDER, REPUTEDLY THE LARGEST ERRATIC IN NORTH AMERICA

The 2,156 m³, ~6,000-ton Madison Boulder (43.931^oN, -71.168^oW, 184 m a.s.l.) has been the subject of speculative folklore and a geologic curiosity since its undocumented discovery in the late 17^th or early 18^th century by local settlers. The boulder is generally claimed to be the largest known glacial erratic in North America, although bedrock mapping shows it is not an erratic rock type as it rests directly upon locally buried outcrops of its parent Conway Granite. Madison Boulder was clearly plucked from Whitton Ledge, ~2.5 km to the NNW, in agreement with regional ice-sheet flow indicators, before it was transported and embedded by the Late Wisconsinan ice sheet. When this process took place has been a matter of discussion over the years, as bog- and pond-bottom ¹⁴C ages suggest that the ice sheet melted out this area about 14,000 cal ¹⁴C years BP. For example, a bulk sample of the deepest visible organic matter at 9.35-m depth in a sediment core recovered from Big Pea Porridge Pond, about 4 km to the ENE, provided a 13.84-14.15 ka cal ¹⁴C minimum age for deglaciation in the area. A similar bulk organic sample from near the base of a sediment core from Echo Lake, 12 km to the NNE, yielded a 12.68-13.38 ka cal ¹⁴C minimum age. Although both of these organic samples are AMS ages, bulk sediments could include older material deposited from the melting ice, offsetting some of the lag time typical between ice melt out and deposition of the first organic matter in bog and pond basins. Nevertheless, most of these lake- and pond-bottom basal ages appear to be consistent with the ice-marginal reconstructions of Ridge et al. (1999, 2001, 2012), which are based on varve, paleomagnetic, and ¹⁴C chronologies from proglacial lakes, including Glacial Lake Hitchcock. A recently obtained rock sample from the top surface of Madison Boulder provides a ¹⁰Be cosmogenic nuclide exposure age of 13.11 +/- 0.84 ka yr BP, in general accord with other ages for the deglaciation chronology in the area.