Pipeline to the Ice Age
|In 1998 geologists had the rare opportunity of examining a continuous
trench excavation across southwestern Maine. This happened when the Portland
Natural Gas Transmission System laid a pipeline from Westbrook through
Cumberland and Oxford Counties to northern New Hampshire and Quebec (Figure 1).
|Heavy construction equipment and miles of pipe were assembled in several
staging areas like the one shown in Figure 2. The project required
elaborate environmental safeguards and machinery that could cross swamps
and bore under rivers.
|During spring and early summer, pipeline workers from around the country
began digging sections of trench through the rugged terrain of the Oxford
Hills region (Figure 3), braving rain, mud, and the worst mosquitoes
in years. Staff from the Maine Geological Survey walked parts of the pipeline
as work progressed, photographing and recording geologic features such
as those described here. This account focuses on the "surficial geology"
along the route, consisting of sand, gravel, and other earth materials
left by the last glacial ice sheet about 25,000-12,000 years ago.
|The information obtained from the pipeline trench improves the detail
of the Survey's geologic mapping and helps us to understand Maine's Ice
Age past. The materials exposed in the trench had to be recorded quickly,
though, during the short time each section was open. Large excavators dug
the trench to a typical depth of 6 to 8 feet. Figure 4 shows a
fresh opening in glacial till. This stony material was released directly
from melting glacial ice. Many large boulders had to be removed from the
trench, as seen in the background to the left of the excavator. The pool
of water in the foreground resulted from digging below the water table.
Long stretches of the trench could not be examined closely because they
immediately filled with water. This necessitated that several sections
of pipe be welded together on top of the ground and then lowered into the
trench all at once!
|In some areas the trench had to be blasted through hundreds of feet
of hard ledge, requiring the use of explosives. Many pipeline workers contracted
mild cases of "rock fever," a contagious enthusiasm for picking up rocks,
as they blasted through the granite veins in Oxford County's famous mineral
belt (Figure 5). They sometimes asked me to identify odd rocks they
had thrown into their trucks, or wanted to know where they could find a
nice tourmaline gem to bring home for a Christmas present.
|Geological examination of the pipeline was timed to fall between digging
the trench and installing the connected sections of pipe. Figure 6 shows this stage of construction where the pipeline crossed the top
of a sandy gravel terrace along the Androscoggin River valley in Gilead.
The gravel was left by streams from melting glacial ice, as they discharged
their sediment load into a lake that once existed in this part of the valley
("glacial Lake Bethel"). This feature is called a delta.
|A nearby section of the trench climbed a gully wall cut into the delta
by a modern brook. Here, the deeper inner portion of the delta was revealed.
Climbing up the trench showed flat sand beds deposited on the old lake
floor, overlain by more steeply inclined beds deposited as the delta built
out into the lake (Figure 7).
|One lesson learned from the pipeline exposures is that the glacial
deposits are often much more complex than we would expect from looking
at the ground surface. Figure 8 shows a good example in Albany
township. Walking across this area before the pipeline was built, boulders
on the ground surface might have suggested the presence of glacial till,
but few other details could have been seen in the wooded terrain. However,
the trench shows abrupt transitions from bedrock (foreground) to glacial
sand midway up the hill, and then to glacial till on the hilltop. This
is just the opposite of the "normal" case, where bedrock outcrops on hills
but is buried by glacial sediments in low areas!
|Our tour stops at the New Hampshire border, where the pipeline trench
crossed a ridge segment belonging to the Androscoggin Moraine system (Figure 9). This is an arcuate series of large till ridges deposited where
a tongue of glacial ice plowed down the Androscoggin Valley from the White
Mountains. The trench grazed the southern tip of one of these ridges, showing
outwash sand overlying morainal till.
We thank the Portland Natural Gas Transmission System for granting permission
to examine the pipeline trench during construction, and the many pipeline
workers who were helpful and courteous throughout this study.
Text and photos by Woodrow Thompson.
Originally published on the web as the January 1999 Site of the Month.
Last updated on October 6, 2005