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GLS100 Lab: Investigating the Geology of Forest River Park
GLS100 Physical Geology –Dr. Hanson
This field exercise will shed light on the origin of the topographic features observed on the Salem
topographic map during the previous lab.
Goals: Students will develop a model of landform development based on field observations and data
from the literature.
Objectives:

Interpret sequence of events by applying the laws of relative age dating

Identify features of glacial erosion and make interpretations regarding:

o How they were formed
o The indicated direction of glacial flow
o The type of glacier that formed them
o The time during glaciation that they were formed
Interpret rates and processes of Holocene weathering and erosion
Figure 1. Forest River Park.
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Part I Relative Age-Dating Events
The purpose of this section is to identify events recorded in earth materials, the features that
represent them, and their order of occurrence. We will apply three laws of relative age dating:
1. Law of superposition
2. Law of cross cutting relationships
3. Law of inclusion
Stop 1: Small bedrock outcrop southeast of Parking Lot
Forest River Park is underlain by bedrock with a thin mantle of overburden (dirt). The outcrop seen
here is just one of many exposed throughout the park.
a. Place a small dot where we are located on figure 1.
b. Is the sediment (regolith/overburden) younger or older that the outcrop? (circle one)
c. State the law used: _____________________________________________
d. The area of this small exposure exhibits the following features:
i. Glacial striations/grooves
ii. A light colored felsic intrusion
iii. Overburden/regolith
iv. A dark-colored mafic intrusion
v. An unconformity (buried erosional surface)
vi. Joints (fractures)
Stop I Exercise:
1. In the table below order the features according to their relative age and hypothesize the event
that created each. (1=oldest)
Age
6.
5.
4.
3.
2.
1.
Feature
Event Recorded
2. Determine the orientation of the glacial striations. ___________
3. Using the symbols in figure 2 plot the striations on figure 1. The dot is drawn on the outcrop
where the feature is located.
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3. In the box below draw the outcrop and label the relative age of each feature. Include a north
arrow and scale.
Introduction to local erosion and glaciation
Erosion is the removal of rock and weathered debris. Sediment created by weathering is removed
by streams, waves, wind and glaciers and eventually deposited in the ocean. By removing rocks and
sediment erosion removes evidence of past events. Geologists use the analogy of pages torn from
a book. Each series of rocks tells a story, however when some rocks are missing our ability to
interpret the story is incomplete. For this reason we are missing detailed information regarding
what happened in the area between the time the rocks were formed and glaciation.
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Surface water is the most effective agent of erosion. Over 90% of all sediment transported to the
oceans is done so by streams. In the last 400 million fluvial activity has removed miles of rock and
sediment from this region and the rest of the Appalachians.
More recently (within the last 2 million years) glaciers flowed across the area scouring and shaping
the already deeply eroded landscape. The abundant exposed surfaces are a by-product of
glaciation.
During the Pleistocene (Ice Age), glaciers advanced over New England at least twice. Beyond the
glacial limit, south of Long Island, sandy flat coastal beaches rest on a cover of Late Mesozoic and
Cenozoic coastal plain sediments. In New England glaciation stripped off these younger sediments
and pushed then off onto the continental shelf. Preglacial sediments in Massachusetts are only
found where they have been bulldozed by the ice margin and incorporated in the terminal moraine
making up Martha’s Vineyard and Block Island. Locally glaciation modified the landscape by
scouring basins and valleys in weak rock, modifying the shape of bedrock hills, and depositing
glacial sediment.
NOTES:
________________
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Field Evidence: Making Observations
Geologists can observe trends and patterns on maps. However, to truly understand what is going
on we need to study features in the field. Recall from the previous lab that numerous small
orthogonal valleys, and asymmetrical hills with steep-facing southerly slopes characterize the
Salem area. We are now going collect data (make observation) and develop some hypotheses to
explain the observed topographic pattern.
Stop 2: Forest River Park and the large outcrop near swimming beach.
Study the area around Forest River Park and answer the following questions:
1. Is the bedrock in the area shallow (<10 feet beneath the surface) or deep? How do you know?
2. Is the topography control by the bedrock or the overlying sedimentary deposits? Explain.
3. Locate the outcrop near the shore. Is there any evidence of glaciation here? Why?
4. Note the small NW-SE trending valley cuts this outcrop. Why does this valley exist? Indicate
the orientation of this feature with are heavy black line.
Move on to the next stop.
Stop 3. Large outcrop on southern shore
Introduction:
The Pleistocene Epoch (2 ma – 10 ka) of the Quaternary Period is known as the ice age. During
this time the climate was cool enough for ice sheets to form and descend from Canada into New
England. The glaciers scoured, polished, and quarried bedrock surfaces, and locally deposited
glacial till and other sediments.
Observe the following features:
1. What features do you see at the base of the large outcrop along the shore? Explain why the
rock smooth.
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2. Measure the trend (_____________) of these features and plot on your map.
3. Note the valley separating the two large outcrops. In which direction does it trend? Offer an
explanation for the presence of the valley. Cite any observed evidence in support of your
hypothesis. Mark the location and trend of this feature with a heavy dark line.
4. Tide permitting, you will see a small asymmetrical knob on the low-tide terrace, this is a
roche moutonnee. (See figure 1.) Draw a 2-D side-profile of it. Note the trend of the
striations on its surface. In your illustration label the steep, plucked lee-side slope and the
gentler abraded stoss slope, draw an arrow indicating the direction of ice flow. Explain how
you were able to determine the flow directions.
Field sketch of Roche Moutonnee at Stop 3
Figure 3. Formation of a roche moutonnee beneath a glacier. These small asymmetrical bedrock
knobs are formed by abrasion (up-ice side) and quarrying (down-ice side). Their presence tells is
that 1) the glacier was warm-based, which means that it was riding on a thin layer of meltwater,
and 2) the absolute direction of glacial flow.
5. Locate this feature and use the proper symbol (figure 2) showing the direction of glacial flow.
Questions:
1. Was this feature formed by a warm-based or cold-based glacier? Cite your evidence.
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2. Do you think that this was formed when the glacier was advancing or retreating. Why?
3. From your observation, which process results in the greatest amount of erosion, plucking or
abrasion? Explain.
4. Waves, wind, glaciers, and stream are all agents of erosion. What indication is there that
glaciers were the last major agent of erosion to affect the area?
5. Study the large outcrop leading up to the park. Explain what happens to the striations and
rocks’ surface roughness as you move up the outcrop and why.
6. Cite any evidence that the rocks in the outcrop do not weather at the same rate. Which
weathers fastest the dark or light rock?
Bringing it all together:
7. Recall the direction that the steep side of the roche moutonnee faces? Relate the topopgraphy
observed on the Salem topographic map to the observed field evidence.
8. What is the origin of the orthogonal valleys throughout the area? Why are those that trend
NW-SE typically longer?
Summarize
In one or two clearly written paragraphs summarized, and post on the class wiki, what you learned
about the topography of the area and the geologic processes that shaped it.
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