Download GeoLogic Mapping C - North Carolina Science Olympiad

2015 COACHES INSTITUTE
GeoLogicMappingC
Presented By: Kathryn Tesh
Changes in rules this year:
The change is in bold. This is the second year for this event, and the event designers have added some
complexities to the material.
Helpful Resources:
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http://www.sciencenc.com The North Carolina Science Olympiad home page.
http://www.sciencenc.com/events.php NCSO Division B/C Event resources page.
http://www.sciencenc.com/event-help/geologicmapping.php NCSO GeoLogic resource page.
http://ngmdb.usgs.gov/ngmdb/ngmdb_home.html USGS National GeoLogic Map Database
http://portal.ncdenr.org/web/lr/geological_home North Carolina Geologic Survey
http://education.usgs.gov/secondary.html USGS education resources, including topo maps, plate tectonics,
and geologic maps
https://www.youtube.com/watch?v=kCvqSsgyCS8 How to set up a stereonet (1 of 7, follow them all)
www.geos.ed.ac.uk/homes/mattal/Maps.pdf A nice explanation of how to do geologic cross-sections
http://store.usgs.gov/b2c_usgs/b2c/display/(xcm=r3standardpitrex_prd&layout=6_1_61_50_2&uiarea=2&ctyp
e=areaDetails&carea=0000000149)/.do USGS maps of geologic hazards
http://pubs.usgs.gov/gip/TopographicMapSymbols/topomapsymbols.pdf Downloadable chart of topographic
map symbols.
http://commons.wvc.edu/rdawes/CourseInfo/glossary.html#strt Glossary of terms.
http://www.nationalatlas.gov/articles/geology/a_geohazards.html Geologic hazard discussion.
http://www.dnr.wa.gov/RESEARCHSCIENCE/TOPICS/GEOLOGICHAZARDSMAPPING/PAGES/GEOLO
GIC_HAZARDS.ASPX Overview of geologic hazards in Washington state.
http://www.rosemonteis.us/files/technical-reports/011902.pdf Actual geologic hazard study for open-pit copper
mine in Arizona.
http://www.townofscotia.com/Peir/AppendixF-GeoHazard.pdf Geologic hazard study for town of Scotia, CA
http://www.oas.org/dsd/publications/Unit/oea66e/ch11.htm Overview lesson in geologic hazards and
assessment of risk.
http://geohazards.cr.usgs.gov/ USGS Geologic Hazard center. Although several types of hazard are included,
the world map of recent earthquakes is interesting.
http://www2.lbl.gov/Community/CAG/docManager/1000000031/WDM_July%208_Geotech.pdf Discussion of
geologic hazard mitigation at Lawrence Berkley National Laboratory in earthquake-prone California.
http://137.229.113.30/webpubs/dggs/sr/text/sr035.pdf Geologic hazard mitigation in Alaska.
http://www.mysciencebox.org/erosion A lesson in erosion, actually creating erosion in dirt mixtures.
http://en.wikipedia.org/wiki/Erosion Wikipedia page on erosion, with pictures.
http://www.geolsoc.org.uk/ks3/gsl/education/resources/rockcycle/page3598.html Types of intrusions. This
geological society page also has links to other topics.
If you have a large printer you can download the topographic maps free from USGS and print
them yourself.
Topographic Maps As Downloaded from USGS
Geologic Maps As Downloaded from USGS
Topographic Map Notes:
From the upper right corner, the map name and series:
From the lower right corner:
From the lower center, the scale and contour interval:
From the lower center, the location of the map within the state and the adjoining quadrangle maps:
From the lower left-center, the local magnetic declination, in this case 3.0° West:
True North is indicated by the star. Ignore Grid North (GN) for this study. Magnetic North (MN) is shown
with its distance in degrees and direction east or west from True North. If Magnetic North is east of True
North, the local declination is positive. If it is west of True North, the local declination is negative.
Magnetic North varies over time, so an old map (over 15 years) may not be accurate.
Compasses provide Magnetic North. Some compasses have an adjustment for declination. If your
compass has not been adjusted, find the local declination indicator on the map. If you need to walk a
line relative to True North, you will add or subtract the number of degrees indicated from the azimuth
relative to True North. If the local declination is positive, then subtract the declination amount from the
bearing. If the local declination is negative, then add the declination amount to the bearing.
For example, you refer to a topographic map and see that you wish to walk across a field at an azimuth
of 45°. The local declination is 10° east. Subtract 10° from 45° and you see if you use a compass
reading of 35° you will stay on track.
To get current local magnetic declination information, go to
http://www.ngdc.noaa.gov/geomag-web/#declination and enter the coordinates. Use either decimal
degrees or degrees, minutes, seconds (2 integers and one number separated by a space).
The topographic map used in the testing area is likely one of the 7.5’ series. Remember that there are
60 seconds (60”) in one minute and 60 minutes (60’) in one degree (1°). The 7.5’ (“7-and-a-half
minute”) map is so named because it covers an area of 7.5’ north to south and 7.5’ east to west.
Symbol Sheet from USGS
Students may take one of these into the testing area; they should be familiar with it. They will be
expected to be able to identify features on maps.
They are available from USGS free when you order maps, or you can download a PDF file and print it
for the students. The print quality of the downloaded PDF file is poor. I went online and got slightly
better copies.
Horizontal Control
A horizontal control point, third order or better, with a permanent benchmark.
A horizontal control point, third order or better, with a permanent benchmark that also includes
elevation.
Checked spot elevation. Typically indicated by an X on a USGS map, Spot Elevation indicates a
point's elevation relative to sea level. Spot elevations are often used to designate the highest point on a
mountain or hill.
Coincident with section corner.
Unmonumented (Provisional Edition maps only). Indicates that no permanent marker has been
placed.
Vertical Control
Vertical control point, third order or better, with tablet. A tablet is similar to a benchmark in that it is
placed to permanently mark an elevation and horizontal position that has been surveyed as accurately
as possible. Unlike a benchmark, which is marked by a brass plate, the control point shown here is
marked by a tablet: a concrete marker with numbers etched onto the top (like a gravestone).
Vertical control point, third order or better, recoverable mark. A recoverable mark is a naturally
occurring landmark. The term "recoverable" means that it can be readily located on the terrain: for
example, a property boundary monument, a railroad intersection, the corner of a large building, etc.
When a landmark occurs naturally, no tablet or benchmark is necessary to mark the position.
Benchmark found at corner section.
Spot elevation.
Boundary Monuments
Boundary monument with tablet.
Boundary monument without tablet.
Boundary with number and elevation, and without tablet or benchmark.
Other Control Symbols
Aerial photograph roll and frame number (Provisional Edition maps only).
U.S. location or mineral monument. Fixed reference points supplementing regular public land survey
monuments. The survey or mineral claim is tied to the location monument, which is later tied to the
public land surveys; most of them are marked stones with rock cairns nearby.
Topographic
Index Contour Interval. These contour lines have a number written on them. This number
indicates the elevation of all points along the line. The unit of measurement (feet, meters)
depends on the map.
Intermediate Contour Interval. These lines have no numbers. They appear between index
contour lines (see above).
Supplementary Contour Interval. These dotted lines are placed in areas where elevation
change is minimal. If there is a lot of space between Index and Intermediate Contours (as
happens where the land is relatively flat), these lines are added to indicate that there are
elevation measurements, even if they are few and far between.
Depression. A depression is a decline in elevation inside inside a rise in elevation. For
example: a crater, whose sides rise but whose center is lower.
Cut; Fill. Cuts and fills occur where a roadway has been blasted through land, drastically
lowering (cut) or raising (fill) the elevation in a distinct and small area.
Bathymetric
Index Contour Interval.
Intermediate Contour Interval.
Primary Contour Interval.
Index Primary Contour Interval..
Supplementary Contour Interval. These dotted lines are placed in areas where elevation
change is minimal. If there is a lot of space between Index and Intermetiate Contours (as
happens where the land is relatively flat), these lines are added to indicate that there are
elevation measurements, even if they are few and far between.
Boundaries
National boundary.
State or territorial boundary.
County (or equivalent) boundary.
Civil township (or equivalent) boundary.
Incorporated City (or equivalent) boundary.
Park, reservation, or monument.
Small park.
U.S. Public Land Survey System
Township or range line.
Township or range line, location doubtful. When a location is doubtful, it is shown by a
dashed or dotted line. "Doubtful" means that there was insufficient evidence to ascertain
whether the National Map Accuracy Standards are being met. The Location Doubtful
designation appears most often in areas surveyed by agencies other than the Bureau of
Land Management (BLM). Landowners and courts may not accept surveys that have not
been approved by the BLM.
Section line (a line at the extremity of a section boundary).
Section line, location doubtful.
Found section corner; Found closing corner. A found corner is a corner whose original
or restored monument or mark is recovered, or whose position is definitely established by
one or more witness corners or monuments.
Witness corner; Meander corner. A witness corner "stands in" when the true corner
cannot be monumented or occupied because of difficult terrain or obstruction of the corner
sight. The witness corner must be on the section line and is usually 5 or 10 chains away
from the true corner. A meander corner is the intersection of a township, range, or section
line and the banks of a navigable stream or meandering body of water.
Other Land Surveys
Township or range line.
Section line (a line at the extremity of a section boundary)
Land grant or mining claim; Monument.
Fence line.
Surface Features
Levee.
Sandy or muddy area; dunes; shifting sand.
Intricate surface area. Areas that are too irregular to contour except in a
very generalized manner, such as lava beds, sand dunes, and open strip
mines. If patterns are used, no attempt has been made to represent the
topography in detail.
Gravel beach; glacial mine.
Tailings pond. The discarded materials from ore treatment processes. In
some cases, the mill wastes are in the form of finely divided particles
suspended in water and disposed of in tailings ponds.
Mines and Caves
Quarry or open pit mine.
Gravel, sand, clay or borrow pit.
Mine tunnel or cave entrance.
Prospect; Mine shaft.
Mine dump.
Mill tailings. The discarded materials from ore treatment processes. In
some cases, the mill wastes are in the form of finely divided particles
suspended in water and disposed of in tailings ponds.
Vegetation
Woodlands. An area of normally dry land containing tree cover or brush
that is potential tree cover. The growth must be at least 6 feet tall and dense
enough to afford cover for troops.
Scrub. Area covered with low-growing or stunted perennial vegetation, such
as cactus, mesquite or sagebrush, common to arid regions and usually not
mixed with trees.
Orchard. A planting of evenly spaced trees or tall bushes that bear fruit or
nuts. Plantings of citrus and nut trees, commonly called groves, are included
in this type.
Vineyard. A planting of grapevines, usually supported and arranged in
evenly spaced rows. Other kinds of cultivated climbing plants, such as berry
vines and hops, are displayed as vineyards for mapping purposes.
Mangrove. Area of dense, almost impenetrable growth of tropical maritime
trees with aerial roots. Mangrove thrives where the movement of tidewater is
minimal, in shallow bays and deltas, and along riverbanks.
Glaciers and Permanent Snowfields
Contours. Contour lines are the principal means used to show the shape
and elevation of the land surface. Each line represents an area of equal
elevation. The unit of measure for elevation and the contour interval is
specified at the bottom of the USGS topographic maps.
Form lines. Lines used to convey a basic idea of the terrain's shape. Unlike
contour lines, form lines are not measured against a true vertical datum.
Marine Shoreline
TOPOGRAPHIC MAPS
Approximate mean high water line. The average limit of dry land during
periods of highest water level (e.g., high tide). USGS topographic maps set
the shoreline at the mean high water line.
Indefinite or unsurveyed.
TOPOGRAPHIC-BATHYMETRIC MAPS
Mean high water line.
Apparent (edge of vegetation). Representation of the vegetative border is
considered approximate because this line cannot be accurately identified on
the ground, due to intricate growth patterns and change over time.
Coastal Features
Foreshore flat. The foreshore is the lower portion of a beach, extending
landward from the mean lower-low water line to just beyond the mean
higher high-water line: the normal effective limit of waves. Storm waves may
extend beyond the foreshore.
Rock or coral reef.
Rock bare or awash.
Group of rocks bare or awash.
Exposed wreck.
Depth curve, Sounding. A depth curve is a line connecting points of equal
water depth. These depths are measured down from the normal surface
elevation of the specific body of water. They should not be confused with
underwater contours, which are measured from mean sea level.
Breakwater, pier, jetty, or wharf.
Seawall.
Bathymetric Features
Area exposed at mean low tide; Sounding datum. The sounding datum is
the plane to which water depth measurements are referred: the normal water
elevation of the surface of the body of water in question. A sounding datum
is used in the formation of depth curves.
Channel.
Offshore oil or gas.
Sunken rock.
Rivers, Lakes and Canals
Intermittent stream. When referring to a body of water, the term
"intermittent" means that the body of water contains water only part of the
year. Natural factors such as variations in snowmelt, or human control for
industrial or irrigation purposes, can affect water level and flow.
Intermittent river.
Disappearing stream.
Perennial stream. Containing water throughout the year except during
periods of severe drought.
Perennial river.
Small falls; small rapids.
Large falls; large rapids.
Masonry dam.
Dam with lock.
Dam carrying road.
Perennial lake; intermittent lake or pond.
Dry lake.
Narrow wash. The term "wash" refers to the sweep or flow of a body of
water.
Wide wash.
Canal, flume, or aqueduct with lock.
Elevated acqueduct, flume or conduit.
Aqueduct tunnel.
Well or spring; spring or seep.
Submerged Areas
Marsh or swamp.
Submerged marsh or swamp.
Wooded marsh or swamp.
Submerged, wooded marsh or swamp.
Rice field.
Land subject to inundation.
Buildings and Related Features
Aside from roads, buildings are the most common manmade features shown on maps. As it is impractical
to accurately depict every structure in a given area, buildings are shown in various ways. Some
structures are omitted; some are shown larger than actual size. In heavily developed areas, only
landmark buildings are denoted. For purposes of representation on published maps, buildings are
divided into two categories, Class 1 and Class 2, depending on their intended use.
Class 1 buildings are intended to house human activities. Included in this grouping are schools,
churches, residences, shops, hotels etc. They are depicted by a solid or crosshatch symbol. Class 2
buildings are not intended primarily for housing human activities, and include structures such as
greenhouses, barns, storage garages and sheds. These are represented on a map by an open outline,
or single-hatch symbol.
Building.
School; Church.
Built-up area.
Racetrack.
Airport.
Landing strip.
Well (other than water); Windmill.
Tanks.
Covered reservoir.
Gaging station.
Landmark object (feature as labeled).
Campground; Picnic area.
Cemetery: small; large.
Roads and Related Features
Primary highway (hard surface). A primary highway is an all-weather, hard
surface road. Primary highways include Federal Interstate routes, primary
State routes, expressways, turnpikes, freeways, parkways, throughways,
and other roads of equivalent importance.
Secondary highway (hard surface). An all-weather, hard surface road. This
class includes secondary State routes, primary county routes, and
comparable through roads connecting principal towns and cities.
Light-duty road (improved surface). Roads with lighter, mostly local, traffic.
Light duty roads include paved roads not included in a higher class, and
loose-surface roads that qualify as all-weather roads.
Unimproved road (unsurfaced). Unsurfaced roads are used for mostly local
traffic, passable in fair or dry weather.
Trail. Trail classification includes foot trails, bridle trails, ski trails, historic
trails, and old railroad grades.
Dual Highway. Road with opposing traffic lanes.
Dual Highway with median strip.
Road under construction.
Underpass; Overpass.
Bridge.
Drawbridge.
Tunnel.
Railroads and Related Features
Standard gauge single track; Station. Gauge refers to the distance between
the two rails of track. The standard gauge for mainline railroads in the United
States is 4 feet 8-1/2 inches.
Standard gauge multiple track.
Abandoned. Rails and related structures remain in place.
Railroad under construction.
Narrow gauge single track. On narrow gauge railroads, the distance
between the two rails of track is smaller than the standard gauge of 4 feet 81/2 inches.
Narrow gauge multiple track.
Railroad in street.
Railroads in juxtaposition. (Two or more ines of track built on the same
bank of earth.)
Roundhouse and turntable.
Transmission Lines and Pipelines
Power transmission line: pole, tower.
Telephone line.
Aboveground oil or gas pipeline.
Underground oil or gas pipeline.
Geologic Map Notes:
Like other maps, a Geologic map shows where things are in relation to other things. However, the
“things” are subsurface features such as rock layers, folds, strata and faults. Geologic maps are often
overlaid on topographic maps.
Geologic maps are used to plan placement of buildings, spillways, roads, dams and any other
significant structure. They are also used in managing natural resources such as groundwater and in the
assessment of geohazards. They also help understand the geologic history of an area.
Color is used to indicate the geologic age of the rocks on the maps. Although not ‘cast in stone’, the
USGS has developed a color standard. Here are some of the colors recommended:
There are recommended patterns to indicate surficial deposits. For example:
There are line patterns for sedimentary rocks:
These colors and patterns are combined.
In addition to color and pattern, areas are often identified with letters to
indicate the period the rock was formed . This sample is from the Raleigh
West Geologic map above. The key indicates that Qal is a Sedimentary rock
called Alluvium, a tan to light gray rock. There is a lot of description of
Alluvium in the map key.
Remember that these are suggestions. Each map will have its own key. Use that key! The symbols are
shown stacked from oldest at the bottom to youngest at the top.
Geologic Time Chart:
General Map Notes:
Distance values between features (both English and metric units)
Use a small strip of paper. Lay it on the map between the features being measured. Mark the starting
point on the strip of paper and then mark the endpoint on the strip of paper. Be sure not to make any
marks on the map itself! Also be sure the paper does not slip as you measure the distance.
Align the strip of paper along the map scale. If the distance is less than 1 mile (or 1 kilometer,
depending on the map scale), align the right-hand mark at the “0” and find the distance to the left, by
determining where the left-hand mark falls. If the distance is more than 1 mile (or 1 kilometer), align the
right-hand mark at the “1 mile” (or “1 kilometer”) mark and read the distance at the left-hand mark. This
distance is 1 mile + 0.4 mile, or 1.4 miles.
Step 1: This mark must be placed directly beneath the “1 MILE” mark.
Marked scrap paper
Step 2: Mileage is read at this mark.
Read the feet (meter) scale the same way, lining up the right-hand mark with one of the thousands lines
so that the left-hand mark is between 0 and 1000, and then read the distance.
Step 1: When this mark is placed directly beneath the “3000 feet,
Marked scrap paper
Step 2: This mark extends to the left of “0” about 300 feet, for a total distance of 3000 + 300 or 3300 feet.
Some people prefer to use an engineering scale (like a ruler but with more divisions) or other
measuring device instead of scrap paper. The principal is unchanged.
TYPES OF MAP PROJECTIONS
Azimuthal: In standard presentation, azimuthal projections map meridians as straight lines and
parallels as complete, concentric circles. They are radially symmetrical. In any presentation (or
aspect), they preserve directions from the center point. This means great circles through the
central point are represented by straight lines on the map.
Cylindrical: In standard presentation, these map regularly-spaced meridians to equally spaced vertical
lines, and parallels to horizontal lines.
Conic: In standard presentation, conic (or conical) projections map meridians as straight lines, and
parallels as arcs of circles.
Other: Typically calculated from formula, and not based on a particular projection
Polyhedral maps: Polyhedral maps can be folded up into a polyhedral approximation to the sphere,
using particular projection to map each face with low distortion.
Pseudoazimuthal: In standard presentation, pseudoazimuthal projections map the equator and central
meridian to perpendicular, intersecting straight lines. They map parallels to complex curves
bowing away from the equator, and meridians to complex curves bowing in toward the central
meridian. Listed here after pseudocylindrical as generally similar to them in shape and purpose.
Pseudoconical: In standard presentation, pseudoconical projections represent the central meridian as
a straight line, other meridians as complex curves, and parallels as circular arcs.
Pseudocylindrical: In standard presentation, these map the central meridian and parallels as straight
lines. Other meridians are curves (or possibly straight from pole to equator), regularly spaced
along parallels.
Retroazimuthal: Direction to a fixed location B (by the shortest route) corresponds to the direction on
the map from A to B.
Properties
Conformal: Preserves angles locally, implying that locally shapes are not distorted.
Compromise: Neither conformal or equal-area, but a balance intended to reduce overall distortion.
Equal Area: Areas are conserved.
Equidistant: All distances from one (or two) points are correct. Other equidistant properties are
mentioned in the notes.
Gnomonic: All great circles are straight lines.
Common Map Projection types
Mercator – Cylindrical and conformal
Lambert Equal-Area – Cylindrical and Equal-area
Natural Earth – Pseudocylindrical and Compromisd
*Profiles
An elevation profile is a cross section of the earth’s surface along a straight line between two endpoints.
This allows you to see the rise and fall of the terrain along the line.
To draw a contour, each intersection of the line A-B (along which you are making the profile) and a
contour line is plotted. You can add other features than contours, such as roads and streams.
Take a strip of paper and lay it on the map between points A and B. Mark those points on the paper.
Without moving it, mark each feature to be included. These will certainly be contour lines and may be
roads and streams or anything else to be plotted. Then lay the strip of paper at the base of the graph
for the profile. At each location with a contour line mark, put a dot at the correct elevation on the graph.
Connect the dots with a smooth line. Add other features as desired.
Other topic notes:
Plate tectonics is the scientific theory that describes the movement of the earth’s lithosphere. The
lithosphere is the rigid outermost shell of a planet. On earth, this is the crust and upper mantle. It is
broken into tectonic plates. Earth has 7 or 8 large plates and several smaller ones. These plates move
relative to each other, averaging from 0 to 100 mm annually.
The plate boundaries, where different plates meet, are areas of intense lithographic activities such as
earthquakes, mountain and ocean trench building and volcanoes. We call these boundaries ‘faults’.
The most famous plate boundary is the “Ring
of Fire” in the Pacific Ocean. This area has a
high incidence of earthquakes and volcanoes.
Note that in the United States California,
Washington, and Oregon are included in this
area. These areas are at high geohazard risk.
Earthquakes cause damage and they trigger
tsunamis and landslides. Volcanoes in these
areas put heavily-populated areas at risk as
well.
Strike and dip are terms used to describe the orientation of the inclined rock layer
(a plane). It is a way to represent a 3-dimensional structure on a 2-dimensional
map. Strike refers to the compass bearing of the line between the inclined rock
layer and a horizontal plane while dip refers to the downhill angle of the slope.
These are indicated as a ‘strike-dip’ symbol, where the longer line represents the
strike and the shorter line the dip. In this example, the dip is 76°. A dip of 0° indicates a horizontal
structure, while a dip of 90° indicates a vertical slope. The right-hand rule says that the dip is inclined to
the right. This may mean the strike azimuth could be in the southern quadrant.
A stereronet projection is a way of figuring out structures from strike and dip. See the video
referenced above for an explanation of how to do one.
Measurement of strike and dip in the field is done with a pocket transit.
Trend and plunge are used to describe the orientation of a line. Trend is
the direction (compass bearing) while plunge is the inclination below
horizontal. The trend points in the direction of the plunge.
Line trends east, plunges 7°
7
The basic structure of the earth is believed to be a solid
inner core, a liquid outer core, and a plastic mantle
covered by a silicate crust.
There is an inner core composed primarily of nickel and
iron which behaves primarily as a solid. At its
temperature of 2000° these metals are normally molten,
but the intense pressure keeps them apparently solid.
Next is the outer core which includes nickel and iron plus
trace amounts of other lighter elements and is believed
to be liquid. The mantle is sufficiently plastic to flow at
very slow rates. This causes movement of the tectonic
plates. Finally, the crust is the solid part on which we live.
There are geologic principles that describe the layering of rocks and help determine the historical
sequence.
The Law of Original Horizontality says that sedimentary rocks are always deposited in horizontal
layers. If they are not horizontal it is because they have been disturbed by subsequent events.
The Law of Superposition says that in any sequence (layering) of rocks that has not been overturned,
the oldest rocks will be found on the bottom and the youngest on the top.
A Disconformity is an unconformity where the layers of rock below the disconformity are parallel to
those above it.
The Law of Faunal Succession points out that flora and fauna are deposited in rock layers and can be
used to help establish the age of strata.
The history of the earth is divided into eons, eras, periods and epochs. See the Geologic Time Chart
above (with the key discussion for geologic maps).
The strata so neatly laid down under the Law of Original Horizontality can be distorted many ways.
Ductile rocks fold and brittle rocks fault.
Folds can be anticlines, synclines or monoclines. A fold is exactly what it sounds like. There is an axial
plane and the strata are bent on either side of it. The bent areas of strata on either side of the hinge
are called limbs. The highest point of the fold is called the crest and the lowest is called the trough.
Folds may be symmetrical (limbs are equal length) or asymmetrical (limbs are unequal length).
Forms folded down on the sides (concave) are called antiforms. Forms folded up on the sides are
called synforms. Anticlines have the older rocks in the center and synclines have the younger rocks
in the center. A monocline has only one limb.
Faults are breaks or fractures in the strata where movement of the rocks on either side of the break
has occurred. The rock above the fault is called the hanging wall. The rock below the fault is called the
foot wall. In a normal fault, the hanging wall moves down relative to the foot wall. In a reverse fault,
the hanging wall moves up relative to the foot wall.
A strike-slip fault moves laterally rather than up or down. These may be either right lateral faults or
left lateral faults, depending on the direction of movement relative to your position as you look across
the fault.
When stress builds up and a fault slips, we call this an earthquake. Earthquakes are the geologic
hazard that kills the most people.
A Geologic Cross-Section is a view of the side of a slice of earth. It is important because it shows how
the rocks are layered and shows faults and other structures below the surface. They are constructed
based on geology at the surface combined with an understanding of the physical behavior of rocks.
First, determine the line on the geologic map and label the ends A and A’ or as appropriate. Using this
line, construct a topographic cross-section. Be sure that the horizontal and vertical scales are the same
(usually meters). This is called VE=1.00, and shows no vertical enhancement. Label major features on
the profile for reference (such as rivers, faults, towns).
Next, align a piece of paper across the map from A to A’, as was done to determine elevations, but on
this one mark every geologic contact that crosses the line. This is just as you did for elevations for the
topographic profile. Use the stereonet to determine the apparent dip of the contact relative to the line A
to A’. Draw this inclined contact on the profile, being sure to keep the dip direction correct. Label
opposite sides of this contact line with the appropriate code for the strata.
Third, align this piece of paper with the profile drawn in the first step and transfer the contact to the
profile. Only draw very short lines at this point. Label each side of the contact as above.
Finally, finish lightly sketching in the cross-section. It is not uncommon to change this several times so
use light pencil lines. If there are major faults sketch them in first. See the example below:
A stratigraphic cross-section is different from a geologic cross-section, as you can see. It shows the
vertical variation of rocks in a particular area. Typically, the oldest rocks are on the bottom and the
youngest on the top. However, it can show sequences which have become quite complex through
intrusion, folding, erosion or faulting.
Geohazards Notes:
Geohazards are geological states that can lead to damage or risk to people or property. A few
examples are:
Sinkholes
Faults (earthquakes)
Landslides
Debris flows
Flooding
Volcanoes
Tsunamis (from earthquakes or landslides)
Sinkholes are depressions in the ground caused by collapse of a lower layer. They are common where
the ground below the surface layer is limestone or other rock that can be dissolved by groundwater,
leaving a void into which the surface layer falls. They are common in karst areas but can also be
caused by man’s activities as, for example, when an abandoned mine collapses and creates a sinkhole
above it.
Landslides can occur on land or in water. They occur when a slope becomes unstable. They can be
caused by such things as saturation from heavy rainfall or snowfall, loss of vegetation which stabilizes
the slope, groundwater pressure, earthquake, erosion, or volcanic action.
Debris flow is a type of landslide in which a slurry of rock or mud flows downhill, often picking up
structures or vehicles. It can aggravate flooding along its path.
Flooding is an overflow of water in an area that is normally dry.
Aquifer Notes:
Aquifers are under ground water sources. A layer of water-permeable rock holds water, which may be
accessed by means of a well. The aquifer may be contained by non-water-permeable rock. If it is
exposed at the surface it may be a lake or stream. Aquifers are usually saturated with water.
Blank equal-area stereonet:
Use this with a pin, tracing paper, and a pencil.