Rocks Lab 3 - Student Handout
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School
Colorado State University, Fort Collins *
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Course
121
Subject
Geology
Date
Dec 6, 2023
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docx
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Uploaded by PresidentSwan3772
GEOL 121
Name _________________________________
Lab Section ____________
Rocks Lab 3 – Rocks of Colorado and Geologic Maps
Learning Objectives:
1.
Describe, Identify, and Interpret Rocks from Colorado
2.
Find information about the geology of a region from a geologic map
In this week’s lab, you will continue to practice your skills at describing and identifying rocks, this time
using rock samples from the Colorado Front Range. All of the rocks that you will see in lab this week are
rocks that you can see when you are out exploring around Fort Collins and in the foothills. You will also
use a geologic map to gain more detailed information about the rocks of the Front Range.
Part 1: Rocks of the Colorado Front Range
For each unknown rock sample, you should provide:
1)
A detailed description. Your description should be detailed enough that someone reading
your description could easily pick out which rock you are talking about. Your description
should include information such as the rock’s color, texture, grain size, minerals present,
layering, fossils present, variability, abnormalities, etc.
2)
Whether the rock is igneous, sedimentary or metamorphic, and how you know.
3)
An identification of the rock.
4)
A hypothesis of how the rock formed and the evidence supporting your hypothesis.
Rubric for rock descriptions:
0.5 pt.
0.25 pt.
0 pt.
Descriptions contain enough detail that
someone reading the description could
picture the rock in their mind. Features such
as color, shape, texture, irregularities,
patterns, layering, grain size and shape,
minerals present, etc. are described when
appropriate.
Descriptions are given but lack
adequate detail to fully under-
stand what is being seen.
No response given.
Rocks Lab 3
GEOL 121
Sample #
Rock Description
Rock Type (ign, sed, or
meta)
How do you know?
Rock ID
Formation Hypothesis
And Reasoning
Sample 1
Pink black silver in color
Large grain size
Visible black crystallization
Phaneritic texture
Igneous intrusive – large
visible crystals
Granite
I think there was a
volcano where this rock
was formed. The magma
from the volcano likely
cooled within earth and
crystalized into a solid rock
Sample 2
Fine grained
smooth texture
Mud sized clasts
Effervesces
Chemical sedimentary
Because the rock was formed
by tiny clasts compacted
together
Muddy limestone
the water picked up
limestone sediments that
that compacted mud clasts
together and formed this
rock
Sample 3
Sand like texture
Not very dense
Light pink in color
Sedimentary rock
Large grain size along with
sand line texture
sandstone
I think this rock was
formed in a river or
sedimentary basin the
continued buildup and
pressure likely created this
rock
2
Rocks Lab 3
GEOL 121
Sample #
Rock Description
Rock Type (ign, sed, or
meta)
How do you know?
Rock ID
Formation Hypothesis
and Reasoning
Sample 4
Dark in color
Fine grain size
Linear foliation
Small visible crystals
Metamorphic
We know this because the
small crystallization and
banding are arranged in a
linear pattern
Schist
I think that this rock was
previously some type of
mudstone or shale that was
dragged under a tectonic
plate that was then put into
an environment of high
heat and pressure which
caused the formation of
schist overtime
Sample 5
Dark in color
Very smooth texture
Dusty texture
very brittle
Very small grain size
Sedimentary rock
Its very brittle so we know it
is layered together the way it
breaks apart
Shale
I think it was formed in a
mineral rich silt/mud
environment and over time
it was compacted/cemented
into a hard rock
3
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Rocks Lab 3
GEOL 121
Part 2: Geologic Maps
Take a few minutes and examine the map that has been provided.
1) What is this map showing you? What spatial area is this map showing? What do the colors
represent?
The map is showing the age of the rocks that makeup Horsetooth Res. The colors on the
map represent different rock/mineral types and the age of the rocks. The map is showing
the Horsetooth Reservoir Quadrangle.
2) In what area of the map do you generally find igneous and metamorphic rocks? In what area
of the map do you generally find sedimentary rocks?
Igneous rocks can be found near the bodies of water and the metamorphic rocks can be
found at higher elevations. Sedimentary rocks are mostly found by the creeks or smaller
areas of water.
3) Find the rock formation labeled Xbc on the map and in the legend.
What is the name, age, and
composition of the formation? Which of the rocks that you previously described best matches the
description for Xbc?
Boulder Creek Granodiorite. 1660 Ma
Light to dark gray
Medium grained
It best matches limestone shale
4) Find the rock formation labeled Xqs on the map and in the legend.
What is the name, age, and
composition of the formation? Which of the rocks that you previously described best matches the
description for Xqs?
Quartzofeldpathic Gneiss
1710 Ma
Fine grained, medium gray
Micah schist and phylitte interbedded with Quartzofeldpathic metasandstone
It best matches granite
5) Find the rock formation labeled PPf on the map and in the legend.
What is the name, age, and
composition of the formation? Which of the rocks that you previously described best matches the
description for PPf?
Name: Fountain Formation
Age: From the Lower Permian and the Upper and Middle Pennsylvanian period
Composition: reddish brown to purple gray arkosic conglomerate
It best matches schist
4
Rocks Lab 3
GEOL 121
6)
Find the rock formation labeled Kn/Kns on the map and in the legend.
What is the name, age,
and composition of the formation? Which of the rocks that you previously described best
matches the description for Kns?
Smokey Hill Shale Member
Upper Cretaceous Period
Dark gray on a fresh surface but can weather to light gray
It best matches shale
7) Find the rock formation labeled Kpl on the map and in the legend.
What is the name, age, and
composition of the formation? Which of the rocks that you previously described best matches the
description for Kpl?
Pierre Shale
Upper Cretaceous Period
Dark gray/olive gray shale containing limestone and ironstone concentrations
8) Find Horsetooth Reservoir on the map.
a. Which formations form the east and west edges of Horsetooth Reservoir? What is their
composition?
Lions Sandstone and the Morrison formation
Lion sandstone is composed of well sorted quartz sandstone commonly cemented
with quartz cement
Morrison formation is composed of weathered claystone and siltstone and
interbedded with
gray micrite and gray fine to medium grained sandstone
b. Which formations are underneath Horsetooth Reservoir? What is their composition?
Lykins Formation – reddish brown siltstone and fine-grained sandstone
9) Why do you think the reservoir was placed where it was?
Because there is a basin between the two rock elevations and because siltstone is an
aquitard so it helps to contain the water and doesn’t allow it to seep through. It is
essentially a natural waterproofing.
10) What are one or two other potential ways that geologic maps such as this one could be used
by non-geologists to inform their decisions or planning?
They could be used by construction companies to know what kind of ground they are
digging into, building on top of and it would inform them of any major unseen rock
obstructions.
5