The Inside Story - Vermont Agency of Natural Resources

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