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GLS100 Lab: Investigating the Geology of Forest River Park
GLS100 Physical Geology –Dr. Hanson
This field exercise may take two lab periods. Our ability to cover all stops will depend on the
weather, tides, and time available at the end of the semester.
Goals: This lab gives you the opportunity to apply the knowledge and skills that you obtained
throughout the semester to an actual field setting.
Objectives:

Identify rock types in the field

Interpret sequence of events by applying the laws of relative age dating

Identify structures formed by brittle deformation

Locate and identify intrusive rock bodies

Classify beach sediment and hypothesize potential source with supporting arguments

Map strike and dip of planar features

Calculate a hypothetical rate of erosion and understand its limitations

Identify and interpret features formed by glacial erosion
Part I Interpreting the Rocks
The purpose of this section is to define events -- the features that represent them, and
their order of occurrence. For review, the laws of relative age dating are listed below:
1. Law of original horizontality
2. Law of superposition
3. Law of cross cutting relationships
4. Law of inclusion
5. Law of faunal succession
Stop 1: Small bedrock outcrop southeast of Parking Lot
Forest River Park is underlain by shallow bedrock covered by a discontinuous mantle of
regolith. The outcrop seen here is just one of many exposed throughout the park.
a. Evaluate the rocks: Look for the presence of sedimentary layering (stratification) and
metamorphic foliation.
Characteristics: (cross off all that do not apply) clastic (composed of detrital grains),
crystalline, foliated, non-foliated, stratified
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b. What minerals do you see? ( Circle all that apply) ferromagnesium silicates, nonferromagnesium silicates, calcite, halite, clay
c. Based on the data above identify the rock category:
(circle one) igneous, sedimentary, or metamorphic
d. Is the sediment (regolith) younger or older that the outcrop? (circle one)
e. State the law used: _____________________________________________
f. The area of this small exposure exhibits the following features:
i. glacial striations/grooves
ii. a felsic intrusion
iii. overburden/regolith
iv. a mafic intrusion
v. an unconformity
vi. joints
Age
6.
5.
4.
3.
2.
1.
Feature
Event Recorded
In the box below draw the outcrop and label the relative age (1=oldest) of each feature.
Include a north arrow and scale.
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g. State the law you used to relative age date these events.
______________________________, _____________________________,
___________________________________
h. Are there any events that may overlap? Explain.
Stop 2: Shore exposure (photo on next page)
a. Which statement best describes these rocks (circle one)
i. they were deposited as layers of sediment
ii. their mineral grains aligned (foliated)
iii. they exhibit multiple episodes of intrusion and fracturing
b. On figure 1 label the following features and determine their relative ages:
i.
prophryritic basalt dike with round mafic phenocrysts (d) ______
ii.
prophryritic basalt dike with feldspar phenocrysts (b)______
iii.
grey gabbro (e) ______
iv.
syenite (granitic) intrusions (c) ______
v.
fault (a) ______
d. List the two laws used to determine the relative age of these features.
___________________________ and ________________________________
e. Which two laws clearly don’t apply?
___________________________ and ________________________________
f. In the table below summarize the geologic history interpreted from this exposure.
Age
5.
4.
3.
2.
1.
Feature
Event Recorded
g. Draw a north arrow and scale on the photo. For the two dikes and the fault draw the
strike and dip symbols indicating their orientation.
h. Identify the fault: (circle one) normal fault, reverse fault, thrust fault, strike-slip fault
i. Draw arrows on the photo indicating the relative motion along the fault.
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NOTES:
Figure 1. Outcrop at stop 1.
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Erosion and Unconformities (complete on your own)
Erosion, which is the removal or rock and weathered debris, tears pages from the rock
record. Surface water is the most effective agent of erosion and is responsible for
removing miles of rock and sediment from this region since the formation of the
Appalachians. More recently (last 150,000 years) glaciers flowed across the area
scouring and shaping the deeply eroded landscape. The abundant exposed surfaces and
are a by-product of glaciation.
Assume that the rocks exposed here at Forest River Park are approximately 400 million
years old and were formed 2 miles (~10,560 feet) beneath the surface, as indicated by the
mineralogy and texture of the large intrusions. The local sediments overlying the rock
consists of glacial till, deposited around 12,000 years ago, beach deposits, and landfill,
dumped within the last 100 years. The contact between this sediment and the underlying
rock is a buried erosional surface, or unconformity. Other than the Mesozoic dikes that
cut these rocks there is no record of the geologic events occurring between the time the
rock was formed and the overlying sediment was deposited.
a. Approximately how many years of geologic history has been removed by erosion?
b. If glaciers removed only 100 feet of rock and sediment, how much was eroded from
the landscape prior to glaciation? ___________________feet.
Calculation:
c. Assuming that this area was exposed to erosion over the last 400 million years,
calculate the rate of erosion? _______ in/yr.
Calculations:
The assumption in c is easy to make, but probably not entirely correct. Mountains and rift
basins were created and destroyed when Pangea assembled and rifted apart--between
the present and the time these rocks were emplaced. Sea level also rose and fell several
times, producing episodes of deposition followed by erosion. These events are recorded in
the sediments of the continental shelf and in isolated rift basins preserved in other areas,
but not here.
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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 pushed then off onto the continental shelf.
Preglacial sediments in Massachusetts are found only in the terminal moraine composing
Martha’s Vineyard and Block Island. Locally glaciation modified the landscape by
scouring basins and valleys in weaker rock, and shaping the hills of solid rock and glacial
debris.
e. Briefly state why most of the geologic record for the last 400 million years is missing
from this area.
NOTES:
________________
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Part II – Sedimentary Environments
Stop 3: Small pocket beach between rocky headlands
a. Describe the beach sediment:
i. Sorting--berm (circle one): non-sorted, poorly sorted, well sorted
i. Sorting--beachface (circle one): non-sorted, poorly sorted, well sorted
ii. Dominant grain size--berm: (circle one): clay and silt, sand, gravel
iii. Dominant grain size--beachface: (circle one): clay and silt, sand, gravel
iv. Angularity--berm (circle one): angular, moderately well rounded, extremely well
rounded
v. List all material composing the beach sediment:
b. Would you classify this sediment as mature or immature? (circle one and explain)
c. Name the rock formed if the sediment on the berm were lithified.
Circle one: sandstone, breccia, conglomerate, shale
d. If after 10 million years the rock formed from this sediment were exposed, what
features would best indicate its age.
e. Origin of beach sediment? There are four possible source of sediment on this
beach: It was a) carried from offshore, b) produced by erosion of the headlands,
c) dumped by the town to create a swimming beach, and/or d) derived from
erosion lf fill dumped behind the beach.
f. Develop a hypothesis explaining the source of gravel in the beach.
List and explain all evidence supporting your hypothesis.
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g. Explain why you would not expect to see the same sediment a half mile off shore.
h. Sketch a map view of the beach and its headland and explain why this beach is called a
pocket beach.
NOTES:
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Part III – Relative age dating and glacial flow indicators
Stop 4. Large outcrop on southern shore
a. This site contains two outcrops are cut by a south-dipping dike. Is the valley between
them a fault? (circle one and explain) Yes/No
b. Can the dike be the same age at the basalt dikes seen at stop 1? (circle one and
explain) Yes/No
Signs of Glaciation
The Pleistocene Epoch (1.8 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 descend form Canada and
cover New England. The glaciers scoured, polished, and quarried bedrock surfaces, and
locally deposited glacial till, a poorly-sorted sediment containing boulders and pebbles in
a matrix of sand and clay. Glacial striations, grooves, polished pavements, and
streamlined asymmetrical bedrock ridges and knobs (fig. 2) are all signs of glacial
scouring.
Figure 2. 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.
a. Looking along the base of the large outcrops, identify and describe any signs of
glaciation and describe how these features were created.
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b. Tide permitting, observe and draw a profile view of the roche moutonnee located at
the low tide level.
Roche Moutonnee at Stop 4
e. On your sketch draw an arrow indicating the direction of ice flow. Include a north arrow and
scale on your sketch map.
f. Referring to figure 2 explain how this feature was formed.
Part IV: Describing The Landscape.
Make observations of the local landscape and describe it. Is it flat? Hilly? Is the bedrock
deep or shallow? How does the topography compare to what you observed while
completing the topographic map lab? Is there a general relationship between the
topography exhibited on the scale of the outcrop with that exhibited by the topographic
map. What structures do you think control the locations of valleys? Why do of the hills
on the topographic map exhibit steeper southerly slopes and a NW-SE orientations.
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Part V: Summary
In one to two well thought out and legible paragraphs summarize what you
learned about the geology of Salem. I do not want a stop-by-stop description of
the park, but a general understanding of the local geologic history as evidences
by the features you observed. Include formation of bedrock, its structure,
exposure by erosion and modification by glaciation.
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Brief Geologic History of the North Shore
The rocks exposed at Forest River Park are just a piece of the puzzle. Rarely is the geologic
history of an area interpreted from a single location, but from data obtained regionally. The
scenario below describes the prevailing tectonic hypothesis interpreted from studies of the
rocks in and around the North Shore and beyond.
Paleozoic and Mesozoic History
Most rocks underlying Salem, MA are over 300 million years old, and reflect a long history of
volcanism and faulting related to both collision and rifting. The ancient Precambrian rocks that
underlie portions of Boston and the North Shore are part of the Avalonian Terrane, a microcontinent that collided with North America during the Paleozoic. These ancient rocks include
the 600+ ma Dedham granodiorite and Lynn Volcanics exposed locally on Marblehead Neck
and in Lynn. Avalon (figs. 1 and 2) was a micro-continent that geologists believe was a volcanic
arc either attached to or located near Proto-Africa (Gondwana). Avalon drifted towards ProtoNorth America (Laurentia) while the ocean basin, the Iapetus Ocean, subducted between the
two continental masses. Collisions with North America occurred during the Ordovician Taconic
orogeny and again during Sirluro-Devonian Acadian orogeny. Avalon is split by a SiluroDevonian bimodal intrusive belt suggesting that backarc rifting was contemporaneous with
closure of the Iapetus to the west (figs. 1 and 2).
Figure 1. Tectonic zones of the North
Shore. Avalon is the microcontinent
that collided with North America
during the Acadian Orogeny. The
Paleozoic Arc Complex is the remains
of the subduction zone. The backarc
rift zone includes a broad belt of
bimodal intrusive rocks.
The sediments of the Iapetus Ocean were deformed, metamorphosed and folded into a huge
mountainous belt known as the Appalachian Mountains. The belt of metamorphic schists and
gneisses that comprises central Massachusetts are the deeply eroded remains of the deformed
oceanic sediments that were thrust westward and accreted to the North American Margin.
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During the Permian Period Gondwana collided with southeastern North America and then
moved slightly northward along strike-slip faults. Gondwana underthrust the Laurentia further
uplifting the Appalachians. This third event, known as the Alleghenian orogeny produced the
fold belts and mountains of the southern Appalachians and metamorphosed rocks in southern
New England. These three orogenic events were part of a series of worldwide collisions that
culminated in the formation of the Pangea supercontinent during the Permian Period. Pangea
remained intact for approximately 100 million years, when it started to rift apart.
Figure 2. Tectonic cross section of Acadian subduction and backarc spreading during the
Acadian Orogeny.
The last tectonic pulse to shape the area occurred during the Late Jurassic and Cretaceous
periods (about 150-80 ma) when Pangea started to rift apart and the modern passive margin
of the Atlantic Ocean began to evolve. Erosion continued, interrupted periodically by brief
periods of deposition related sea level fluctuations and glaciation.
The Cenozoic and the Present Landscape
The landscape of the North Shore is the product of differential weathering and erosion of a
brittly deformed igneous terrain followed by glacial scouring and deposition. Irregular basins
and straight valleys, developed along faults and fracture zones. Knobby highlands and ridges
are of rock less fractured. Forest River Park is characterized by this knock-and-lochan
topography, a galic term referring to a glacially scoured region of irregular knobs and basins.
The visible outcrops are bedrock highs poking through an irregular blanket of glacial sediment
(and man-made fill) covering the bedrock surface.
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NOTES:
_________________
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