Overview of New
England Geology and Topography
The following chart, showing the
New England area, is reproduced from the USGS
site Tapestry
of Time and Terrain. The USGS claims this map
of the U.S. is the most detailed and accurate
portrait of the land surface and the ages of its
underlying rock formations yet displayed in the
same image.
The
USGS map resembles traditional 3-D perspective
drawings of landscapes with the addition of a
fourth dimension, geologic time, which is shown
in color. In mutually enhancing the landscape
and its underlying temporal structure, this
digital tapestry outlines the geologic story of
continental collision and break-up,
mountain-building, river erosion and deposition,
ice-cap glaciation, volcanism, and other events
and processes that have shaped the mainland 48
states over the last 2.6 billion years.
An Overview of Bedrock in the
Narragansett Bay Area
Rhode Island is now largely
covered by sand and gravel. The bedrock
shown in the map below is rarely seen except in
road cuts and, as in the Jamestown area, along
the periphery of the island where relatively
recent glacial gouging cut through the younger
deposits, exposing underlying formations.
 |
Rhode
Island Bedrock Identification Key
| Color |
Description |
Geologic Age |
| Light Green |
Narragansett Pier Plutonic
(Granites) |
Permian
(> 245 million yrs) |
| Shades of Yellow |
Narragansett Bay Stratified
Group |
Pennyslvanian (290 million
yrs) |
| Shades of Dark Green |
Conanicut
Group |
Cambrian
(>439 million yrs) |
| Shades of Rose |
Granites, Gneiss |
Protozoic
(> 570 million yrs) |
| Shades of Purple |
Newport
formations |
Protozoic
(> 570 million yrs) |
A more detailed
key is available
|
| Click on image for larger version Click
here for the original
(959 kb) image,
showing all of Rhode Island, prepared by
the Office of the Rhode Island State
Geologist
|
Underlying all of Rhode
Island is Avalon Terrane, but that does not mean
that the bedrock is uniform throughout the area.
The original volcanic arc was several thousand
miles long and consisted of many individual
islands. Within Rhode Island there are two
subterranes, separated by a arcing shear zone
located close to the western boundary of the
state.
The shear zone extends into
Connecticut near the center of the western state
boundary, passes to the east of Hope Valley and
terminates near Charlestown in the southern part
of the state. Most of the state is to the east of
this shear zone, in the Esmond-Dedham subterrane.
The subterrane to the west of the shear zone is
the Hope Valley subterrane.
The oldest bedrock in the
Jamestown/ Newport area is shown in the lower
center of the map - colored in shades of purple
and rose. These rocks are located in the
Fort Wetherill area of Jamestown and the
southwestern portion of Newport. They date to the
Late Proterozoic period or earlier (more than 570
million years ago) - when Jamestown was part of
the original volcanic arc in the southern
hemisphere, off of Africa.
The large area of pinkish-rose
coloration (west of Narragansett Bay and inland
from it) represents granites, gneiss and
granadiorite of similar Late Proterozoic age.
West and north of the Fort
Wetherill area on Conanicut Island - everything
south of Great Creek (Marsh Meadows) - is
Cambro-Ordovian era bedrock, which is colored in shades
of dark green. These rocks were formed
during the period when Avalonia collided with
Laurentia approximately 425 million years ago.
The large area in shades
of yellow are rocks formed after
Avalonia was united with Laurentia and include
the period when the area was united with Northern
Africa - when Pangaea existed. These rocks were
created from sediment laid down in the
Pennsylvanian period, roughly 290 to 325 million
years ago in a large depression created by the
forces of continental impact.
The light green
strip near Narragansett Pier represents Permian
period granites formed as magma made its way to
the surface near the end of the Alleghenian
Orogeny that occured in conjunction with
formation of the Pangaea supercontinent. They
were formed between 245 and 290 million years
ago.
The small amount of light
blue in the upper left corner is a
portion of a relatively large area (about ten
miles by fifteen miles) of Triassic and Jurassic
bedrock located west of West Warwick. These
rocks, primaily vein quartz, are 146 to 245
million years old.
| The USGS is currently developing
The
North America Tapestry of Time and
Terrain, which will include Canada
and Mexico. Check on the status
at the USGS website. |
 |
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