Reply to Robert F. Gerath and Brian K. Fowler's Discussion of “Late Wisconsinan Mountain Glaciation in the Northern Presidential Range, New Hampshire,”

Abstract

Gerath and Fowler call into question the quality of the field data upon which the conclusions outlined in the earlier report were based. Fowler's unpublished stone count differs significantly from mine in the assignment of stones to two distinctive formations, the Littleton and the Ammonoosuc. The Devonian Littleton Formation is a sequence of interbedded metapelites and pelitic quartzites metamorphosed in this part of New Hampshire to sillimanite grade (Billings, 1941). The high aluminum content is reflected mineralogically by muscovite, sillimanite, and garnet totalling 15 to 45 modal percent (Billings, 1941: 894, Table 6). By contrast, the Ordovician Ammonoosuc Volcanics consist largely of mafic to felsic metavolcanics. Many of these rocks are totally devoid of muscovite; furthermore, in those lithologies which do bear muscovite (2 to 3%), it can only be detected microscopically (Billings, 1941: 873). Billings (1941: 877) noted that only 3% of the rocks within the outcrop area of the Ammonoosuc Volcanics are muscovite-bearing metasediments. Admittedly, such rocks would be difficult or impossible to distinguish from Littleton in stone counts, álthough three misidentifications per hundred would not significantly alter the results. Moreover, in two careful traverses of Snyder Brook (Bradley, 1981: Fig. 2, south of Loc. 4), where Ammonoosuc Volcanics (and younger granites) are exposed for 1.2 km and outcrop is about 50%, no muscovite-bearing metasediments were discovered. For these reasons, all metasedimentary rocks with megascopic muscovite were confidently assigned to the Littleton Formation in the original stone census. Gerath and Fowler's generalization that the Ammonoosuc Volcanics contain 5% muscovite is erroneous (Billings, 1941: 874, Table 1) and explains the discrepancy of the Durand Lake stone counts.