The Inside Story - Vermont Agency of Natural Resources

Bedrock Map of Vermont – The Inside Story
The Bedrock Geologic Map of Vermont incorporates 30 years of new approaches to geologic mapping
and the technologies that support it. As a fundamental data layer, it is a show piece of the present
understanding of Vermont geology. The map will be the geoscience base for years to come, helping us
to address Vermont’s environmental issues and informing our interaction with and protection of our
natural resources.
A map is the medium by which geologists communicate data and its interpretation. Mapping at the
surface allows geologists to predict what is in the subsurface. Predictive capabilities are important for
understanding issues such water supply, contaminant remediation, natural geochemical hazards, and
rockfalls.
The Vermont Geological Survey at the Agency of Natural Resources, the United States Geological Survey,
and University of Vermont have participated since the early 1980's in a cooperative venture to produce
the new bedrock geological map of Vermont. The map has a 2011 publication date which is 150 years
(sesquicentennial) since the first 1861 geologic map of Vermont and 50 years since the 1961 Centennial
Geologic Map of Vermont. Approximately 60% of the State was mapped at 1:24,000 scale using plate
tectonic concepts not yet developed for the 1961 map. The theory of plate tectonics states that the
Earth's outermost layer is fragmented into a dozen or more large and small plates that are moving
relative to one another as they ride atop hotter, more mobile material (USGS web site). The history of
these movements is recorded in the rocks now exposed at the surface in Vermont.
The 2011 Bedrock Geologic Map of Vermont is by Nicholas M. Ratcliffe, Rolfe S. Stanley, Marjorie H.
Gale, Peter J. Thompson and Gregory J. Walsh with contributions by Norman L. Hatch, Jr., Douglas W.
Rankin, Barry L. Doolan, Jonathan Kim, Charlotte J. Mehrtens, John N. Aleinikoff, and J. Gregory McHone.
Cartography is by Linda Masonic. The map spans the careers of three State Geologists: Charles Ratte
(1976-1991), Diane Conrad (1991-1995), and Laurence Becker (1995 - present).
Citation is:
Ratcliffe, N.M., Stanley, R.S., Gale, M.H., Thompson, P.J., and Walsh, G.J., 2011, Bedrock geologic map of
Vermont: U.S. Geological Survey Scientific Investigations Map 3184, 3 sheets, scale 1:100,000.
Major findings:
1. The geology represents fault-bounded slices of rock. The fault slices are the result of plate tectonic
collisions and show the juxtaposition of rocks from a variety of geologic settings. Mapped faults are
ancient faults and many are only recognizable in map pattern. Very few faults were shown on the
previous maps.
2. Fold structures in Vermont include anticlines (arches) and synclines (troughs); many have been folded
more than once and some are even upside-down! Major folds include the Middlebury Synclinorium,
Green Mountain Anticlinorium, Connecticut Valley Trough, and Bronson Hill Anticlinorium.

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/