23 The New Bedrock Geologic Map of Vermont: New Answers, New Problems, and New Uses of Bedrock Geologic Data (Posters)

Sunday, 14 March 2010
1:30 PM - 4:15 PM
International ABCDF

C. H. HITCHCOCK'S 1877-78 GEOLOGIC MAPS OF NEW HAMPSHIRE, VERMONT AND WESTERN MAINE: HIS FIRST RELIEF MAP RESTORED

Paper number: 23-3
Program: Symposia Sessions
Time: 1:30 PM – 4:15 PM

Authors

Abstract

Charles H. Hitchcock’s first relief map at one inch to the mile and five times vertical of north-central New England has been restored to represent his 1878 interpretation of the geology of New Hampshire, Vermont and western Maine. His ten-year effort to map New Hampshire and to compile his and his father’s geology in the neighboring states was a monumental undertaking and magnificent success. Except for alluvial materials, our efforts portray all his geology and use a color palette that provides easy comparison with modern maps of the region. The transfer to the relief map from his 1878 NH Atlas and 1877 VT-NH compilation, joined at the RMC, generated a number of “ahah” moments that emphasize Hitchcock’s detailed observations, many with implications for interpretations that followed. These include: widespread “bands of quartz” in SE and SW NH now recognized as Mesozoic brittle faults; aligned soapstone quarries within “ferruginous slates and schists” of S-central NH correspond to the Concord Tectonic Zone (Lyons et al., 1982); discontinuous quartzite and conglomerate lenses within the “Coos Group” outline tight folds now define the nappes of western NH (Thompson et al., 1968); and even an isolated patch of rocks at Fall Mountain, NH, is now interpreted as a klippe. In contrast his Laurentian belt through VT has become restricted to the Green Mountain and Lincoln Mountain massifs and his Huronian, including the distribution of ultramafic rocks in north-central VT (some much-extrapolated, perhaps because of his responsibility to the New Hampshire survey), has undergone significant modification.

Hitchcock’s 1870 telegram to Dartmouth College colleagues stating “No longer call New Hampshire Azoic [Precambrian]. Silurian fossils discovered to-day” (Naslund, 1985) set the stage for Billings (1934, 1937) and others to unlock the stratigraphic framework of New Hampshire at increasing metamorphic grade. In addition a growing interest in magmatic complexes increased the number of map units from ~20 stratigraphic/20 magmatic units in 1878, ~40/40 in 1955, to ~80/80 in 1997. A similar progression exists for ME and VT. However, even with rock unit name changes, stratigraphic reassignment and refined geochronology, his basic map patterns and the contrasts he recognized in geologic style between NH and VT are well preserved.