geology 106 geological mapping techniques suny … · lab 12: applied subsurface mapping in...
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Geology 106 Geological Mapping Techniques
SUNY University at Buffalo
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Why are maps important in geology?
Geology and Reliefof the Conterminous US
- The shape of Earth’s surface , the spatial arrangement of different rock types, and therelative ages of those rock units allow us to reconstruct Earth history, deduce the workings of plate tectonics, and understand surface processes
- Geologic maps are, in a sense, a synthesis of what’s known about the geological history of an area
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Course ObjectiveAt the end of this course students should be able to read and interpret maps commonly used in the Earth and environmental sciences to understand two- and three-dimensional spatial relationships in the surface and subsurface.
- This course will focus mainly on the reading and interpretation of existing maps (w/ paper and pencil)
-We’ll aim for a quantitative description of the surface and subsurface
- My primary goal for you in this course is to be able to move easily between a 2D representation (a map) and the 3D reality it depicts
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Learning how to position ourselves on the globe, and how to represent thatposition on a flat piece of paper, has been the focus of cartographers and geodesists for centuries
2D3D
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The Grand Canyon
N
The topography provides a “window” into the Earth
2D3D
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Geologic Map of the Grand Canyon
N
A geologic map depicts that window in 2D, providing a tool for interpreting the3D arrangement of rock units we can’t observe directly
2D3D
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Course Materials (Bring every day!)
- Graph paper tablet (1/4 inch rule) - Regular pencils and colored pencils (12 color set is fine)- Ruler AND protractor - Calculator (need trig functions) - Tracing paper- Spencer Text (will use some of the maps included with
the text)
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Course Outline*Labs 1-4: Introduction to maps of the Earth’s surface: topography and geomorphologyLabs 5-9: Introduction to geologic maps: connecting surface and subsurface in 3D Lab 10: Interpreting deformation from geologic maps: faults and folds Lab 11: Making measurements in the field**Lab 12: Applied subsurface mapping in petroleum geology
*Subject to change**This lab may happen earlier in the semester
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GLY 106Laboratory 1
Reading a Topographic Map
- Today we’ll practice reading a standard USGS Topographic Quadrangle map of South Buffalo
- Before starting, let’s have a look at some essential map elements
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USGS Topographic Quadrangle Map Legend
(Notice your map is not square….why?)
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For example: On a 1:24000 quad map, scale factor = 24000, so a distance of1cm on the map represents a true distance of 24000cm or 240m.
Note: Your map has been rescaled to fit on an 11’’ x 17’’ sheet of paper, so it’s no longer 1:24000! The scale bar, however, is still accurate.
Map Scale
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Map Projection
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The Datum: A mathematical model of the Earth(Basis for NAD83 and WGS84)
Map Projection
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The Transverse Mercator Projection
A conformal projection, meaning local angles and shapes are preserved.
Tangent Case Secant Case
Secant Case
Yellow zone bounded by two standard meridians and onecentral meridian
Map Projection
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The Transverse Mercator Projection
Each UTM zone = 6 degrees of longitude
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Universal Transverse Mercator (UTM) Zone Coordinate System
CentralMeridian
CentralParallel
(+,+)(-,+)
(-,-) (+,-)
“FalseOrigin”
EastingsN
orthings
Northings/Eastings are reported in meters
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Example: Easting 675000m, Northing 4750000m(Note orange UTM grid: 1000mx1000m)
Universal Transverse Mercator (UTM) Zone Coordinate System
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True North, Magnetic North, and Grid North
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- Earth exhibits a magentic field roughly analogous to a magnetic dipole
- However, the poles of the magnet do not coincide with the geographic poles(i.e., the spin axis)
- At any point on the planet, a compass reading (which measures the magnetic field) will show magnetic north, which in most places is offset from true (geographic) north
True North vs. Magnetic North
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Grid North
GN
True North vs. (UTM) Grid North
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- Contours join lines of equal elevation above a datum (≈ sea level)- Provide a 2D representation of a 3D surface
Topographic Contours
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Landscape Perspective View (3D)
Topographic Contour Map (2D)
Topographic Contours
Topographic Cross-Sections
Topographic Contours
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GLY 106Laboratory 2
Making a Topographic Profile
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Topographic Contours
- Contours join lines of equal elevation above a datum (≈ sea level)- Provide a 2D representation of a 3D surface
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Topographic Contours
- Any arbitrary traverse across a topographic map can be converted to a topographic profile (i.e., a plot of elevation)
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Topographic Contours
Distance
Elevation
Resolution (level of detail) depends on contour interval; scalecan be adjusted to give the desired vertical exaggeration
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780800820840860
Distance(Scale = Map Scale)
Elev
atio
n (ft
)
Traverse From Map
Elevation Profile
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Distance
Elevation
H2
H1
Distance
Elevation
H2
H1
High Vertical Exaggeration
Low Vertical Exaggeration
Note: Both plots convey the same information, but the vertical exaggeration affects the appearance of the plot
Topographic Contours
VE=Vertical Scale/Horizontal Scale
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Topographic Contours
- For this lab, you’ll make three topographic profiles; each parallels a 4km road segment in the SE Buffalo Quadrangle
- You are given specific instructions on the scale and vertical exaggeration for the plot
- You are given specific instructions for the plot labels and interpretation (and don’t forget instructions #6 and #7)
- Take your time!
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GLY 106Laboratory 3
Surface Hydrology and Topography
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Have you noticed a relationship between topography and watercourses (i.e., creeks and rivers)?
Today we’ll explore this in more detail
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Drainage Area Above Point P
P
Drainage Basins (or “Catchment Areas”)
- The “collection area” for water that falls on the Earth’s surface
- Defined above any point on a stream
- Separated by drainage divides
- Not fixed over geologic time
- “Nested”
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Drainage Basins of
W NY
Example of aregional drainage dividecrossing the Ellicottvillearea
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To Cattaraugus Ck…
To Allegheny River…
From the pattern of the streams, we can make out the regional drainage divide
Ellicottville Quad
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To Cattaraugus Ck…
To Allegheny River…
From the pattern of the streams and topography, we can map surface drainage in great detail
A
BC
D
Ellicottville Quad
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A B
Elev
atio
nEl
evat
ion
Distance
Distance
C D
Stream Stream
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Ellicottville Quad
To Cattaraugus Ck…
To Allegheny River…
From the pattern of the streams and topography, we can map surface drainage in great detail
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11
1 11
21
2
33
6
Drainage Area Above Point P
P
Stream Networks
- Drainage areas evolve stream networks
-The networks are commonly characterized by their branching patterns
- Stream Order indices are usedto describe the branching pattern in a network
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To Cattaraugus Ck…
To Allegheny River…
Example of Stream Ordering
11
2 1
3
1
1 1
2 1
3
?
Ellicottville Quad
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Elevation
Distance Downstream
?
Stream Long-Profile
Just a topographic profile that traces the axis of a stream or river
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Rainfall rate (L/T)
Area of Bucket Opening (L2)
Discharge (L3/T)
How to think about question #4
Water level (constant in time)
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GLY 106Laboratory 4
Contouring Spot Heights (known points of elevation)
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Idealized Case
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Actual Case
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814ft777ft
793ft
What if we wanted contours at 785 and 800?
Example: Linear Interpolation(793-777)/(Map Distance)=Elevation change per unit map distance, or slope
So for the 785 contour:
(8/(793-777)) x Map Distance = Map Distance from the 777 measurement
785
800
793ft
777ft
Map distance
Elev.
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814ft777ft
793ft
- By using linear interpolation between known points, you can incrementally constrain the shape of the desired contours
-BUT keep in mind: Some interpretation is still required
- Would you choose A, B, or C for the 785 contour?
785
800
A? B?
C?
What if we wanted contours at 785 and 800?
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Today: You’ll construct the contours specified in the lab tutorial
A few suggestions…
- Work from the outside in
- Maintain accuracy and recognize redundancy
- Remember, there will always be some interpretation
- Keep your contouring objective in mind (see tutorial)
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GLY 106Laboratory 5
Introduction to Geologic Maps
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Geologic Maps- Geologic maps are those that show the spatial distribution of materials exposed at the Earth’ssurface, i.e. rocks or broken up bits of rock (sediment)
- Geologic maps are most commonly overlain on a topographic base, i.e. information aboutsurface geology is combined with information about topography
- Geologic maps subdivide the materials at the Earth’s surface into units based on lithology(rock or sediment composition), age, or other criterion (e.g. depositional environment for sedimentary rocks, metamorphic grade for metamorphic rocks)
- KEY Point: Although some geologic maps contain explicit information on the subsurface (fromwells), most are only an expression of what can be observed at Earth’s surface….yet,much can be inferred about the subsurface from a geologic map on a topographic base
Geologic maps come in two main types:
1) Surficial Geologic Maps show the distribution of loose (uncosolidated, unlithified) material,and are most commonly used where glacial deposits are widespread or to emphasize river courses and deposits2) Bedrock Geologic Maps exclude surficial deposits to show the distribution of bedrock and arethe most common type of geologic map
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Remember Some Fundamental Principles(1) Superposition: Rocks formed from deposition at the Earth’s surface tend to be layered (strata) and widespread , with older rocks beneath younger rocks
(2) Cross-cutting and Deformation: Rocks formed from igneous materialthat intrudes and cross-cuts strata are younger than that strata; if strataare broken (faulted) or highly deformed (folded), than the faulting ordeformation postdates the formation of the strata
(3) Unconformities: Represent significant breaks in deposition, often accompanied by erosion, and represent a geologic time gap (hiatus)
….And Some Conventions(1) Sedimentary rocks are classified and grouped hierarchically (by scale):
Group -> Formation -> Member -> Beds
(2) The first letter defining a map unit typically refers to the geologic period in which the rocks formed
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Unit AUnit BUnit CUnit DOld
Young
Unit AUnit BUnit CUnit DOld
Young
Old
Young
Simple Superposition
Superposition & Cross-Cutting (Intrusion)
Deformation (Tilting)
Unit AUnit BUnit CUnit DOld
Young
Superposition & Cross-Cutting (Erosion)
Unit E
Angular Unconformity, Superposition
Topography
Unit AUnit B
Unit C
₊ ₊ ₊₊ ₊₊ ₊
₊₊
₊₊ ₊₊
₊₊₊ ₊ ₊₊
₊₊
₊ ₊₊ ₊₊ ₊ ₊₊
₊ ₊₊ ₊₊ ₊₊
Noncomformity, Superposition
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So Why Are Geologic Maps So Varied?
- Patterns on surficial geologic maps arise due to surface processes: mainly sediment deposition by glaciers and rivers
-Patterns on bedrock geologic maps are a function of two main factors:
1) Topography2) Shape and orientation of the geologic units
Unit AUnit BUnit CUnit D
Unit AContact A:BContact B:CContact C:D
3D Block Diagram 2D Geologic Map
Old
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“Outcrop”
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3D Block Diagram 2D Geologic Map
Unit A
Unit B Unit C
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Unit A Unit B Unit C
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The Stratigraphic Column
Unit AUnit BUnit CUnit D
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Distance
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- The 2D map pattern of geological surfaces (contacts, or top and base of a rock unit) isstrongly dependent on the 3D orientation of that surface
- The orientation is generally described by two measures: Strike and Dip
Strike and Dip
dip
N
S
Dip Angle: Maximum inclination ofthe surface from the horizontalDip Direction: Direction towards which the surface is inclinedStrike: A line on the surface 90°from the dip direction
Example shown here:
Strike is 0° (or 180°)Dip is 40°W
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Strike and Dip
N
What are the strike and dip shown here?
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Contact C:D
Contact B:C Contact A:B
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Unit D
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Unit D
Topography
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What’s Unit E’s dip direction and strike?
What Units A-D’s dip direction and strike?
What Units A-D’s dip direction and strike?
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Today: Some practice interpreting geologic maps
The Grand Canyon National Park(Spencer Map, Ap2-13)
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Note the following:
PCbr = Brahma Schist PCr = Rama Schist PCgd = Zoraster GranitePCg = Zoraster GranitePcum = Ultramafic intrusives
Vishnu
IgneousIntrusives(ZorasterComplex)
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GLY 106Laboratory 6
Planar Contacts and Topography
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N
- Last week, we introduced the “Strike” and “Dip” definition of a planein 3D space
-Contacts between geologic units are called planar or “homoclinal” if they have a uniform strike & dip
- Many units are more or less planar at large scale (over small areas)
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The Three-Point Problem
If a contact is planar (or can be approximated as such), its strike and dip can be defined from three points of known elevation
450ft
600ft
300ft
Map View 3D Perspective View
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Old
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Structure Contours
We can represent the elevation of any surface on a map using contours
When that surface is a geologic contact and extends below the Earth’s topographic surface, we call the contours defining its elevation structurecontours
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The intersection of structure contours and topographic contours on a map define where a unit ‘outcrops’, or is exposed at the Earth’s surface
Map View
Structure Contours
Topographic Contours
StructureContours on Top of Unit X
Top of Unit X Outcrop
Unit X
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Map View
Structure Contours
Topographic Contours
StructureContours on Top of Unit X
Top of Unit X Outcrop
Unit X
Unit XUnit X
Cross-Section
Depth
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The intersection of structure contours and topographic contours on a map define where a unit ‘outcrops’, or is exposed at the Earth’s surface
Map View
Structure Contours
Topographic Contours
StructureContours on Top of Unit X
Top of Unit X Outcrop
Unit X
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Formation Depth
The last question today asks for the depth of a contact at a particular location; the depth is the vertical distance beneath the ground surface
dip
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GLY 106Laboratory 7
Structure Contours and Outcrop
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Old
Young
Structure Contours
We can represent the elevation of any surface on a map using contours
When that surface is a geologic contact and extends below the Earth’s topographic surface, we call the contours defining its elevation structurecontours
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Structure contours are often extended (interpolated) across regions where they no longer exist because the contact is eroded
Structure Contours
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Even though the river has eroded them away, we can infer the extension of units andcontacts across the canyon
The Grand Canyon
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The Grand Canyon North
Structure contours
Even though the river has eroded them away, we can infer the extension of units andcontacts across the canyon
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True vs. Apparent Thickness
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Unit AUnit BUnit CUnit DOld
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Truethickness
Truethickness vsApparent thicknessdip°
True thickness = Apparent thickness x cos (dip°)
depth
depth
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Old
Young Truethickness vsApparent thicknessdip°
True thickness = Apparent thickness x cos (dip°)
depth
True vs. Apparent Thickness
top top top top topbase basebase base base If a unit dips, its thicknessmeasured by differencing overlying structure contours on its top and base will be an Apparent thickness. The difference is negligible for small dips, but appreciable for large dips
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True vs. Apparent Dip
True dipvs
Apparent dip
True dip refers to the maximum dip, or slope, of a surface, as measured perpendicularto the strike of the surface (and thus perpendicular to its structure contours)
Apparent dip refers to a dip less than the true dip, measured oblique (or parallel) to the strike of the surface
tan (True dip°)= tan (Apparent dip°)/cos (β°)
β° = Angle between Apparent and True dip direction
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True vs. Apparent Dip
A single 2D exposure will most likely yield only an apparent dip
dip°
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NOTE
When reporting strike and dip, the compact and conventional format is:
Strike/Dip Magnitude & Quadrant Direction
So if a unit has a strike of 45° and a dip of 30° towards the NW, we’d write:45°/30°NW
N
WE
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Today’s Lab:
More practice relating structure contours, topography, and outcrop
Practice obtaining true vs. apparent dips and thicknesses
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GLY 106Laboratory 8
Structure Contours, Outcrop, and Cross-sections I
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Recall
Geologic maps typically display two of three types of key information:
(1) Topography, (2) Outcropping Contacts, and (3) Structure Contours
In general, if we have two of the three – structure contours, outcrop pattern, and topographic contours – we can obtain the third
Today we’ll use all three to interpret a map and draw a geologic cross-section
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The intersection of structure contours and topographic contours on a map define where a unit ‘outcrops’, or is exposed at the Earth’s surface
Map View
Structure Contours
Topographic Contours
StructureContours on Top of Unit X
Top of Unit X Outcrop
Unit X
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Map View
Structure Contours
Topographic Contours
StructureContours on Top of Unit X
Top of Unit X Outcrop
Unit X
Unit XUnit X
Cross-Section
Depth
A
B
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A
B
600450
300150
Dept
h
Sample Cross-Section
Vertical Datum
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GLY 106Laboratory 9
Cross-Sections II
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A
B
600450
300150
Dept
h
Sample Cross-Section
Vertical Datum
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Today’s Map: Salem (KY) quad, Spencer Ap-26
- Line of cross-section shown
- Your section will display the unitsabove the base of the Bethel S.S. (Mcb), the shape of which is shownwith the red structure contours
-To construct your cross-section, you should plot points where yourline of section crosses the structurecontours (=28pts)
- I give you the topographic elevations at these points
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3D Block Diagram 2D Geologic Map
Unit A
Unit B Unit C
Unit D U
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Topography
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Contact B:C Contact A:B
Unit A Unit B Unit C
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Unit B Unit C
Unit D
Unit A
Contact A:B
Contact B:C
Contact C:D
Old
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Contact A:E
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Unit A
Unit B
Unit CUnit D
A Quick Primer on Folds and Outcrop Patterns
Unit C
Unit B
Unit A
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GLY 106Laboratory 10
Mapping in GoogleEarth
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Map View
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Perspective View
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Perspective View
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Perspective View
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Perspective View
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Perspective View
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Perspective View
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GLY 106Laboratory 12
Field Lab II and Exam Review
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Today
1. Finish plotting points collected during last week’s lab (see lab tutorial from last week).
2. Exam overview
3. Open review
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Understanding the USGS topographic quad (map scale, map projection, true/magnetic/grid north, datum)
Topographic contours: how they’re defined, the contour interval, reading and interpolating elevations at specified locations
Using topographic contours to interpret geomorphology and construct topographic profiles
Part 1: Topographic Maps
Outline of Topics
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Outline of Topics
The basics: What does a geologic map depict, why do geologic mapslook the way they do?
Strike and dip of a planar surface: how they are defined and measured
Structure contours on geologic contacts and how they differ from topographic contours
The three-point problem: Defining a planar geologic contact based onthree points of known elevation
Part 2: Geologic Maps
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Outline of Topics
The relationship between structure contours, topographic contours, and unit contacts on a geologic map
Distinguishing true thickness from apparent thickness and true dip from apparent dip
Constructing geologic cross-sections from a geologic map on a topographic base
Part 3: Geologic Map Interpretation
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Outline of Topics
The Brunton compass: Magnetic declination, reading bearings, triangulation, measuring dips using the clinometer, measuring strike
Field Lab
Text Readings
Spencer: Chapters 1,2,3,6, 7, 8, 9