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The Geology of Haystack Mountain, Castle Hill, Maine
Rocks exposed at the summit of Haystack Mountain are felsic volcanic rocks of the Winterville Formation (Roy, 1987). The Winterville Formation is a widespread unit of sedimentary and volcanic rocks of Late Ordovician age (Roy and Mencher, 1976). These rocks are interpreted to have been deposited by deep-water, submarine sedimentation and volcanism (Churchill-Dickson, 2007). Later mountain-building events and erosion through geologic time have left a complicated pattern of surface exposure.
Regional Bedrock Geology
Central Aroostook County is characterized by gently rolling hills to the east along the border with New Brunswick, Canada, while the topography to the west is more elevated and rugged. This difference in landscape can be attributed in a general way to the underlying geology. The eastern region is underlain by more easily eroded limestone and calcareous shale, while the western region is underlain by more resistant sedimentary and volcanic rocks. The Winterville Formation forms the most northeastern part of the Ordovician volcanic belt in Maine, extending southwest into New Hampshire and probably northeast in the subsurface into New Brunswick where it is covered by younger rocks of Silurian and Devonian age (Roy and Mencher, 1976).
Local Bedrock Geology
Winterville Formation. Across the region, the Winterville Formation contains a variety of volcanic and sedimentary rocks, including basalt, rhyolite, slate, chert, graywacke, and conglomerate. The complicated relationships among these different rocks have not been worked out in detail, though volcanic rocks are probably most abundant followed by dark gray to black slate (Roy, 1987). Fossil graptolites in the slate from several localities show that the formation as a whole spans the Caradocian and Ashgillian stages of the Late Ordovician.
The rock exposed on Haystack Mountain is rhyolite, one of the volcanic rock types of the Winterville. It is a light-colored rock composed predominantly of fine-grained feldspar. In places, it contains coarser mineral grains in a porphyritic texture. Because of its fine-grained crystalline texture, it is difficult to break and resists erosion. This is why Haystack Mountain stands above the surrounding landscape. The age of these felsic volcanic rocks is not known precisely, but fossils from just below the overlying Pyle Mountain Argillite imply that the Winterville Formation in the Castle Hill area dates to the Caradocian stage of the Late Ordovician (Boucot and others, 1964), approximately 450 to 460 million years ago.
Pyle Mountain Argillite. The Pyle Mountain Argillite (Boucot and others, 1964) is a poorly exposed, thin unit of siltstone and slate containing fossil trilobites, brachiopods, ostracodes, and other organisms. It is younger than the Winterville Formation and partly overlies it in the Castle Hill anticline, especially to the east (Figure 3). The Pyle Mountain Argillite contains a Foliomena fauna, a Late Ordovician assemblage characterized by very small brachiopods, typically 2 to 4 mm across (Neuman, 1994). The nature of the sediment and the fossils together imply deposition in a cold, deep-water marine environment. The Pyle Mountain Argillite contains the only reported occurrence of the Foliomena fauna in North America (Neuman, 1994). Roy (1987) proposed the name of the formation be changed to Pyle Mountain Formation.
Boucot, A. J., Field, M. T., Fletcher, R., Forbes, W. H., Naylor, R. S., and Pavlides, L., 1964, Reconnaissance bedrock geology of the Presque Isle quadrangle, Maine: Maine Geological Survey, Bulletin 15 (Quadrangle Mapping Series No. 2), 123 p.
Churchill-Dickson, L., 2007, Maine's Fossil Record, The Paleozoic: Maine Geological Survey, 500 p.
Naylor, R. S., 1980, Bedrock geology of the Presque Isle area, in Roy, D. C. and Naylor, R. S., 1980, The geology of northeastern Maine and neighboring New Brunswick: New England Intercollegiate Geological Conference, 72nd Annual Meeting, October 10-13, 1980, Presque Isle, Maine, p. 169-178.
Neuman, 1994, Late Ordovician (Ashgill) Foliomena fauna brachiopods from northeastern Maine: Journal of Paleontology, v. 68, no. 6, p. 1218-1234.
Roy, D. C., 1987, Geologic map of the Caribou and northern Presque Isle quadrangles, Maine: Maine Geological Survey, Open-File Report 87-2, map and report, 44 p.
Roy, D. C. and Mencher, E., 1976, Ordovician and Silurian stratigraphy of northeastern Aroostook County, Maine, in Page, L. R. (editor), Contributions to the stratigraphy of New England: Geological Society of America, Memoir 148, p. 25-52.
Text and photos by Robert A. Johnston.
Originally published on the web as the October 2008 Site of the Month.
Last updated on November 7, 2008
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