Previews of Field Trips for
Physical Processes of Planet Earth
at Colby College (Click on any framed image for an enlarged view.)
Geology is a field-based science and is taught that way at Colby.
Students in the introductory course (GE141: Physical Geology) go on four
separate field trips during the semester, which introduce them to various
aspects of the local geology of Maine. The views below are
representative of
some of the features we have seen on these field trips.
This house is one of many over the years that have been left
dangling
in mid-air at Popham Beach, on the Maine coast, after being undercut by
wave erosion during a major series of storms. The house has since been
moved back from the bluff face by 6-8 meters, but is still seen by
students on the all-day trip we
take to the coast in the introductory course in physical geology.
Sometimes it's close to the rear of the active beach, sometimes it's over
a hundred meters inland. Coastal systems ARE dynamic environments!
Other homes, unfortunately, have not been so lucky, and
students can
see only foundations now partly buried in dune sand and overgrown with
beach grasses. Ironically, some new homes are being built on sites not
long ago washed clean by the waves!
Fort Popham, built at the mouth of the combined Kennebec and
Androscoggin
Rivers, is a never-completed Civil-War-era fortification built of cut
granite.
The fort is built on the bedrock outcrop shown here, and though this
field trip
is principally to look at coastal features, the rocks tell an interesting
story
as well and are not ignored.
Currents through the channel (shown at right) are generally swift and
strong,
between river flow (a combined mean annual discharge of some 17,000 cubic
feet
per second) and tidal flow; they can really rip when the tide is going
out or the river flow is high! This is a common fishing area for harbor
seals, and on rare occasions students can watch U. S. Naval vessels pass
by en route to or from the Bath Iron Works shipyards upriver.
Since the Popham Beach field trip is an all-day trip, students get to
eat
lunch on the beach when conditions permit. One year, the waves were
actually
crashing on the rocks beneath the cottage that now lies far in the
background in
this picture, several hundred meters from the current water's edge! The
lunch
stop can also be instructive, since there is almost always some wind and
students
can watch fine sand blowing along the beach. Much of the beach system
here is in
constant transition, with dunes eroding to form beach sands, and the
beach sands
being blown back again into new dunes. Two nearshore islands also
complicate
sediment movement in the Popham-Hunnewell Beach complex.
The heavy
mineral sands at Popham show a variety of complex silicate minerals.
The tight folds in the rocks at the right is a situation many
people
would think impossible until they actually see it for themselves. These
folds in the
rocks of the Waterville Formation are exposed on the bank of the Kennebec
River across the river from Waterville. [If you trace a single bed
closely, you can see the zig-zag fold pattern.]
These fine-grained metamorphic rocks were once mud on the
bottom of
the ocean, and were recrystallized and folded this tightly during uplift
of the
northern Appalachian Mountains in the Acadian Orogeny.
Fossils are present, but very rare, in these rocks; the most significant
is
probably Monograptus colbiensis, described in the 1920s from
specimens
collected along the Kennebec River by the old Colby campus and from a
quarry near
the current campus. These fossils indicate the sediments that became
these rocks
were deposited in the ocean during the Silurian Period of Earth history,
since fossils of Monograptus are index fossils for the
Silurian, meaning they are known almost exclusively from sedimentary
rocks of Silurian age.
Rivers and other streams are major molders of modern
landscapes, as
they erode and transport the materials of the land gradually down to the
sea. Geology students here are observing and discussing stream
discharge, erosion, and
sediment transport on a field trip to the Carrabassett Stream in the
nearby
village of Canaan. On this trip, students also measure and calculate the
amount
of water flowing through the stream at this and another location, and
compare and
contrast the results from the two different sites. An important lesson
is that
the eye is not always as accurate as a mechanical measurement!
The stream is shown here in spring flood; when we go to this
same
site in
the fall, there is less than 1/10 this flow through the channel. As one
might
well guess from the picture, most stream erosion and sediment
transportation is
going to take place at times of relatively high water such as this, when
rocks
tumbling in the bed of the stream can erode into the underlying bedrock
and the
swift current carries loose debris away.
Much of Maine's current landscape is a product of the
repeated
glacial advances of the Pleistocene over the underlying bedrock framework
of
granite and meta-sedimentary rocks.
These are sediments deposited in a glacio-marine delta in the
town of
Belgrade, about a half-hour drive from campus. This delta was created by
a
glacial meltwater stream, where it flowed out from under the melting ice
directly
into the margin of the ocean. Although global sea level then was still
about 45
meters (150 feet) below its modern position, the area had been depressed
far
below its current elevation by the enormous weight of the overlying
glacial ice.
In postglacial time, it has risen some 150 meters (500 feet), elevating
features
such as this along with it. These glaciomarine deltas are important
sources of
industrial sand and gravel in central Maine, while opening a window as
well on
our recent geologic past.
