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Some Geological Features at Smalls Falls
Smalls Falls is an exceptional series of falls and plunge pools on the Sandy River just upstream from its confluence with Chandler Mill Stream in Township E. The Maine Department of Transportation maintains a rest stop with picnic tables at this location on Route 4 just a few miles north of Madrid. In addition to providing travelers with a scenic respite, Smalls Falls exposes some interesting geology.
General geology of the Smalls Falls area
With its potential to unravel some of the mysteries of the Northern Appalachians, along with a reasonable potential for mineral deposits, the bedrock geology of the area around Smalls Falls has been the subject of considerable inquiry for several decades. The wealth of mapping has revealed a geological history spanning nearly 500 million years. Most of the bedrock units at Smalls Falls and nearby are either Late Silurian (423-417 million years) or Early Devonian (417-391 million years) in age. The units include a variety of sedimentary rock types that have been metamorphosed to varying degrees by a regionally pervasive metamorphic event that is locally punctuated by metamorphism related to abundant intrusive igneous rocks, predominantly granite. Originally horizontal, the metamorphosed sedimentary layering in the rock is now turned up on edge and runs northeast-southwest across the landscape.
Figure 1 is a portion of a regional geologic map by Moench and others (1995) that shows the general distribution of rock units in the area. Rocks of Silurian age are shown in various shades of green with the older rocks generally toward the northwest. The unit labeled Ssf is the Smalls Falls Formation which will be the focus of this discussion. Sp is the Perry Mountain formation, a mix of thick quartzite and schist. Units beginning with Sr are members of the Rangeley Formation. Various Devonian units, younger than the Smalls Falls Formation, are shown in blue shades and have labels beginning with D. The pink blobs on this map are younger Devonian intrusions of granite and related rocks. Figure 2 shows the generalized distribution of the Smalls Falls Formation throughout western and central Maine.
Smalls Falls Formation
The falls at this rest area are quite picturesque and are underlain entirely with the Smalls Falls Formation. This is primarily a black schist with thin layers of light brown quartzite. The schist contains abundant pyrrhotite (an iron sulfide mineral), along with lesser amounts of other sulfide minerals, that result in the distinctive rusty weathering of the schist. Graphite (just good old carbon) is abundant in this rock as well and gives it the dark black color on fresh surfaces. Guidotti and Van Baalen (2001) chemically analyzed some samples of the Smalls Falls Formation and found concentrations of metals such as lead, zinc, chromium, vanadium, nickel, arsenic, and others. An environmental consequence of the abundance of metallic sulfide minerals is that ground water in this formation can be highly acidic and contain toxic metals.
Guidotti, C.V. and Van Baalen, M.R., 2001, Geological, geochemical, and environmental aspects of metamorphosed black shales in central Maine, in West, D.P. Jr., and Bailey, R.H. (editors), Guidebook for fieldtrips in New England: Geological Society of America, 2001 Annual Meeting, p. F.1-F.26.
Moench, R.H., Boone, G.M., Bothner, W.A., Boudette, E.L., Hatch, N.L., Jr., Hussey, A.M., II, and Marvinney, R.G., 1995, Geologic map of the Sherbrooke-Lewiston area, Maine, New Hampshire, and Vermont, United States, and Quebec, Canada: U.S. Geological Survey, Map I-1898-D, 1:250,000.
Text and photos by R. Marvinney.
Originally published on the web as the July 2005 Site of the Month.
Last updated on October 6, 2005
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