The Inside Story - Vermont Agency of Natural Resources

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3. There are 51 new age dates on the map. The ages of the bedrock in Vermont range from Mesoproterozoic (1.4 billion years) to Miocene (20 million years). The oldest dated rocks in Vermont are from South Londonderry, VT; the youngest unit on the map is the Brandon lignite, a small deposit of brown coal and clay, which is dated as 20 million years old based on fossil seeds and flora. 4. Rock geochemistry has provided data to discern different ancient plate tectonic environments such as rifted or broken apart continental crust, mid-ocean ridge spreading centers, and volcanic island arcs. 5. Rocks of the ancient continental margin, fragments of oceanic crust, and relict metamorphic rocks from subduction zones are part of the Vermont landscape and provide some important deposits such as marble, talc and slate. 6. Age, geochemistry, structure, and plate tectonic environment impact the interpretations shown in the cross-sections. Cross-sections are projected to a depth of 10 km. Comparison of 1861, 1961 and 2011 maps: The 1861 Geologic Map of Vermont was begun in 1845 by State Geologist Charles Adams, work continued by Zadock Thompson and Augustus Young, and the map was completed by Edward Hitchcock. The map focused on ores, had 24 map units, had no topographic base, and was at a scale of 1:400,000. The Centennial Geologic Map of Vermont by Charles Doll, Wallace Cady, James Thompson and Marland Billings was published 100 years later in 1961 at a scale of 1:250,000. It was the first map to show stratigraphy for the state and assign ages to the formations. It had 137 map units, 2745 polygons. The 2011 Bedrock Geologic Map of Vermont (scale 1:100,000) is more detailed than previous work, contains 486 different rock types and 22,000 polygons. It is the first map published since the development of plate tectonics. The map is available as paper copy, digital images and in GIS format, allowing users to select and highlight units and use the map as a computer-generated data set for evaluating a multitude of issues. Map-making process: The new map incorporates field mapping studies at various scales by more than 60 geologists. Mapping was done by pace and compass on a variety of base maps and there was no use of GPS or digital GIS mapping in 1984. Geologists observe the exposed rock types and structures (folds, faults, fractures) in the field, sample the rock for microscopic, chemical, and geochronological studies, then use this information to develop a three dimensional model of the underlying bedrock and to understand the depositional and deformational history of Vermont. Maps were hand drafted in ink on mylar, reproduced on paper, then hand colored. The entire process from field work to map publication is labor intensive. Mapping is still done through walking the land, but now (2011) incorporates the use of Global Positioning Systems (GPS) and Geographic Information Systems (GIS). For more on making the map and a timeline, please see http://www.anr.state.vt.us/dec/geo/BedrockMapofVTHistory.htm Map Elements: The bedrock geological map shows: the type of intact, continuous, solid bedrock at or near the earth's
surface projected onto a 2 -dimensional surface; rock descriptions; age relationships shown on a correlation of units chart; major structures and tectonic elements on a generalized map; cross-sections which show the interpretation of the map pattern in the subsurface and projected in the air; quadrangle index maps; and references. Geologic tidbits: Of national significance, zircon obtained from the Braintree Complex in Vermont proved to yield exceptionally reproducible results and has become the mineral standard (R33) used by Stanford University, USGS, and other national laboratories. The mineral zircon is used to obtain uranium-lead age dates for igneous rocks. Fossil control is good in the Champlain Valley and Taconics, but fossils are rare elsewhere. However, conodonts (microscopic fossil teeth of an eel-like animal) were reported by Ratcliffe et al., 1999, in rocks from West Bridgewater in the heart of the Green Mountains. Conodonts were later found at Buels Gore and are a species, Leptochirognathus quadratus, which lived only from about 463 to 460 Ma (Thompson et al., 2002b). Therefore, the rocks are much younger than previously thought and the conodonts provide age control for some of the faults in Vermont. Doll Peak, just south of North Jay Peak, is included on the topographic base map of the 2011 map. Doll Peak was named in honor of Charles Doll, editor of the 1961 Centennial Geologic Map of Vermont and State Geologist from 1947 to 1976. The Champlain Thrust is an ancient world-class thrust fault exposed in Burlington at Lone Rock Point. The fault is mapped in western Vermont and continues north into Quebec and south into New York. The fault is thought to have moved westward some 35-50 miles. That would put its root zone somewhere to the east of Montpelier and last motion around 350 million years ago. How to obtain copies of the map: The map is sold by the Vermont Geological Survey and the US Geological Survey. The map is also available for download from the State and Federal governments as pdf files, shapefiles, and kmz files. Vermont Geological Survey 103 South Main St., Logue Cottage Waterbury, VT 05671-2420 Phone: 802-654-8951 E-mail: Marjorie.gale@state.vt.us Web site: http://www.anr.state.vt.us/dec/geo/vgs.htm Or visit our office in the VSAC Building, 10 East Allen St., Winooski, VT. Please call ahead. U.S. Geological Survey Denver, CO 80225, USA Phone: 888-275-8747 http://www.usgs.gov/pubprod/ http://pubs.usgs.gov/sim/3184/
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The Inside Story - Vermont Agency of Natural Resources

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