Introduction to Earth Science

Chapter 3 - Basic Geologic Principles & Maps

Earth science is founded on basic principles that are useful for making observations about the world around us. This chapter presents a mix of information that is essential (fundamental) to all following chapters. This chapter is an introduction to the rock cycle — a graphic and conceptual model to illustrate common rocks and earth materials and the processes that form or change them, over time.

In addition, basic geologic principles can be applied to resolving the order of events leading to the formation of rocks and landscape features. This section presents many basic concepts that are universal to all physical sciences. Earth scientist use geochronology — the study of the age of rocks using both absolute dating methods and relative dating methods.

The second half of this chapter focuses on maps and the display of geographic and geologic information in two-dimensional and three-dimensional formats. Geologic maps are essential components to resolving the geologic story (and history) of a region, and serve many useful purposes for society including uses in urban planning, agriculture, health and public safety, mineral resource management, water resources, and much more.
Click on thumbnail images for a larger view.
Angular unconformity exposed along a beach cliff in Encinitas, California
Fig. 3-1. Layered rocks in a sea cliff in Encinitas, CA showing an angular unconformity.

Basic geologic principles for interpreting landscape forming processes

The surface of the Earth is shaped by processes moving materials underground and by processes occurring on the surface under the influence of atmospheric, hydrologic, and oceanographic processes. The logical place to start discussion is to examine the material below our feet. Perhaps most fundamental to understanding the geologic origin of landscapes and earth materials is the vastness of geologic time. It took centuries for the scientific community to compile information about the age of different regions in the planet through the study of rocks, the fossils they contain, and the chronology of events in Earth's history (such as the formation of mountain ranges, changes in coastlines, and the evolution of life). One of the products of this collective investigation over time is the geologic time scale (Figure 3-2). The time scale links measures of time (on the scale of periods ranging from thousands, millions, to billions of years) to established names for those time periods used in publications (reports and maps). The geologic time scale has been continuous refined as new data is collected. (See section 1.10 in Chapter 1 for a discussion on geologic time.)

What is bedrock?

Bedrock is the solid rock the occurs beneath soil or alluvium (unconsolidated sediments) that coved the surface of the land in most locations. In some places the bedrock is exposed as rocky outcrops scattered across the landscape, particularly in mountainous areas or along stream canyons.

Why do landscapes in different areas have unique characteristics? Its all about the bedrock and its history!
Landscapes change from one region to the next because of the composition and character of bedrock changes from place to place.

Geologists unravel the geologic history by studying the geometry of outcrops in an area. Subtle characteristics of undisturbed landscapes often reveal the location of faults, ore deposits, sources of groundwater, and many other features of geologic interest.

Geologic Time Scale Fig. 3-2. Geologic Time Scale
Review of Some Basic "Rock" Concepts

The Rock Cycle

Charles Lyell (1797-1875) compiled the first geology textbook entitled "Principles of Geology" in which he promoted concepts of the "rock cycle" (Figure 3-3). The rock cycle illustrates the series of events in which a rock of one type is converted to one or more other types, and then possibly back to the original type. The "rock cycle" is a graphic and conceptual model to illustrate common rocks and earth materials and the processes that form or change them, over time. Figure 3-4 also illustrates the rock cycle, but includes more detail of both the rocks and earth materials, and selected geologic features associated with geologic processes occurring over time. Be sure to examine the arrows on the diagrams! Pathways to rock origins may go several ways.

There are 4 classes of rocks and earth materials: igneous rocks, sediments (which are not rocks), sedimentary rocks, and metamorphic rocks.

Igneous rocks are rocks formed from the cooling and crystallization of molten materials. The word igneous applies to any rock or mineral that solidified from molten or partly molten material (referring to magma underground or lava on the surface). The word igneous also applies to the processes related to the formation of such rocks. Igneous rocks includes intrusive rocks (rocks that cooled below the surface) and volcanic rocks formed on the Earth's surface by volcanism. Igneous rocks also form from melting associated with extraterrestrial impacts. Examples of igneous rocks include granite, gabbro, and basalt. Rocks of igneous origin are discussed in Chapter 9.

Sediments are solid fragments of inorganic or organic material that come from the weathering of rock and soil erosion, and are carried and deposited by wind, water, or ice. Sediments may be made up of rock fragments ranging is size from boulders down to microscopic particles, and may contain organic matter. Examples of sediment include gravel, sand, silt, clay, mud (mix of sand, silt, and clay), soil, lime mud, and ooze. Sediments are not rocks, but they may become rocks through heating, compaction, and cementation. Sediments are discussed in Chapter 10.

Sedimentary rocks
are rocks that have formed over time through the deposition and solidification of sediment, especially sediment transported by water (rivers, lakes, and oceans), ice ( glaciers), and wind. Sedimentary rocks are often deposited in layers, and frequently contain fossils. Sedimentary rocks are often deposited in layers, and frequently contain fossils. Examples of sedimentary rocks include shale, sandstone, conglomerate, limestone, and chert. Sedimentary rocks are discussed in Chapter 11.

Metamorphic rocks are rocks that was once one form of rock but has changed to another under the influence of heat, pressure, or fluids without passing through a liquid phase (melting). Examples of metamorphic rocks include slate, schist, gneiss, marble, quartzite, and serpentinite. Metamorphic rocks are discussed in Chapter 12.

These rock cycle diagrams illustrate how earth materials form and change over time. Both products (rocks and sediments) and processes (such as melting, cooling, erosion, and deposition) are illustrated. The passage of geologic time is an essential component, although some processes are much faster than others. Note that all these types of processes are taking place simultaneously, but at different locations on and within the planet.
The Rock Cycle
Fig. 3-3. The Rock Cycle: processes are in purple; products are in black and blue.
Rock Cycle Illustrated
Fig. 3-4. Rock Cycle Illustrated. This version of the rock cycle shows more detail in graphic form. It is good to compare the two diagrams.
Rocks and Geology of the San Francisco Bay Region is a 64 page report (Adobe .pdf file) that contains basic information about regional geology and the Earth materials that are found in the Central Coast region of California. This guide provides a general tour of the "rock cycle" as it applies to a this region where nearly all rock types are exposed in close proximity.
igneous rocks -  rocks solidified from molten or partly molten material (referring to magma underground or lava on the surface). The word igneous applies to processes related to the formation of such rocks. Examples of igneous rocks include basalt, granite, and gabbro. sediment—solid fragments of inorganic or organic material that comes from the weathering and erosion of rocks and soil, and transported by wind, water, or moving ice.  Examples of sediment include gravel, sand, silt, clay and “mud” (a mix of sand, silt, and clay) and marine sediments, reef rubble, lime mud, and ooze. sedimentary rocks memetamorphic rocks—rock that was once one form but changed to another under the influence of heat, pressure or fluids without passing through a liquid phase (melting). Examples of metamorphic rocks include slate, schist, gneiss, marble, quartzite, and serpentinite.
Fig. 3-5. Igneous rocks Fig. 3-6. Sediments Fig. 3-7. Sedimentary rocks Fig. 3-8. Metamorphic rocks

Minerals formed from molten rock (magma or lava) derived from the Earth’s mantle are composed mostly of iron-and-magnesium-rick silicate minerals (forming rocks of mafic & ultramafic composition).

Fig. 3-9. Igneous rocks from the cooling of molten material (magma, lava) and are rich in silicate minerals.

Molten material derived from deep in the mantle is typically enriched in iron- and magnesium-rich silicate minerals (called mafic or ultramafic). In contrast, magma and lava associated with continental crustal regions are enriched in felsic silicate minerals rich in silicon and aluminum (mostly feldspars).

wMinerals formed at high temperatures and pressures may not be stable in the surface environment. Water is a “universal solvent” and will break down minerals through weathering processes. Fig. 3-10. Weathering of rocks and minerals: water (H2O) occurs in rocks within the earth and is a primary chemical agent on the earth surface. Water is called the "universal solvent," dissolving and transporting material in solution, altering the chemical composition of mineral, and transporting sediment (erosion and deposition). Not all rocks and minerals behave similarly when subjected to weathering and erosion. Hard and durable minerals like quartz tend to resist weathering and erosion, and therefore can be carried long distances carried as sediments by flowing water or wind.
Quartz has a hardness of 7 on the Mohs Hardness Scale.  It is one of the hardest and most abundant minerals on the Earth’s surface. It survives, as grains of sand, whereas other silicates typically decay and change into clay minerals. Fig. 3-11. Durability of quartz: because quartz is an extremely durable mineral (with a Mohs hardness of 7.0) and because is is an extremely abundant mineral in the Earth's crust, quartz is concentrated by erosional processes in the form of "sand" found on beaches and in desert dune fields.
Most sedimentary rocks exposed on the Earth’s surface are made of particles of the minerals: quartz (as sand and silt), clay minerals, and calcite grains. Iron minerals produce nearly all color in rocks. Fig. 3-12. Minerals in sedimentary rocks: most sedimentary rocks are enriched in the minerals quartz, calcite, and some iron and clay minerals. Minerals with high hardness and low solubility are transported by erosion and deposited in sedimentary basins. Typically soft minerals that are highly soluble are dissolved and carried by surface and groundwater where most contributes to the saltiness of seawater. However, dissolved components in water can precipitate to form mineral cements, including like calcite (CaCO3), iron-rich minerals (hematite and limonite), and silica (quartz). The iron-mineral content in rocks are mostly responsible for the earth-tone colors they display.
Under increasing heat and pressure, minerals stable on or near the surface will change into other minerals under increasing heat and pressure with burial. Fig. 3-13. Metamorphic processes cause changes in the mineral composition in rocks: changes in heat, pressure, and exposure to fluids, over time, will change the mineral composition of earth materials, such as converting sediments into sedimentary rocks, changing sedimentary and igneous rocks into metamorphic rocks. Conversely, exposing rocks to fluids—at or near the surface—degrade rocks of many kind into sediments.


Layered versus non-layered rocks

Large bodies of rocks typically display recognizable characteristics, most notable are characteristics of rock "layers" called strata. Strata are layered rocks that are typically formed when an accumulation of rock material is deposited is a setting, then as time passes more layers accumulate on top. If cut and exposed by erosion or faulting, the layers appear as a series of "beds." (A single bed is called a stratum). Geologists who study layers in rocks will assign them names and will described them by their composition and their geologic ages (if possible). A rock formation is a rock unit that is distinctive enough in appearance that a geologist can distinguish it from other surrounding rock layers. A named rock formation must also be thick enough and extensive enough to plot on a geologic map. Some rocks are layered (or stratified), others are not. Sedimentary rocks and volcanic deposits (lava flows and air-fall ash deposits are examples of commonly stratified rocks. Examples of rocks that are unstratified are igneous rocks (like granite) that cooled and crystallized deep underground and did not develop layers. Also rocks that have experienced extensive metamorphism and remelting may no longer preserve "stratification." (As always, in geology, and science in general, there are sometimes exceptions to these rules!)
Layered ("stratified") rocks
Unlayered ("unstratified") rocks
Flat-lying strata at the Del Mar Dog Beach in San Diego County, CA Stacked lava flows in the Grand Coulee, Washington City of Rocks, Idaho Unstratified igneous and metamorphic rocks exposed along the Colorado River in the Inner Gorge of the Grand Canyon, Arizona
Fig. 3-14. Stratified layers of sedimentary rocks exposed along the sea cliff at Del Mar Dog Beach, San Diego County, CA Fig. 3-15. Stratified layers of volcanic rocks (stacked layers of lava flows and ash beds), Grand Coulee, Washington Fig. 3-16. Unstratified intrusive igneous rocks (mostly granite) exposed by erosion at the City of Rocks National Preserve, Idaho Fig. 3-17. Unstratified igneous and metamorphic rocks exposed along the Colorado River in the Grand Canyon, Arizona


Basic Geologic Principles

In 1669, Nicolas Steno first proposed several basic geologic principles that were later embellished by notable Scottish geologists, James Hutton and Charles Lyell. These basic principles are easily observed in geologic outcrops, but have value for any number of scientific and technical applications beyond geology. Figure 3-13 illustrates the three "laws" that are used in resolving the age of rocks and the order in which they formed or geologic events occurred. The three laws are as follows:

Law of Original Horizontality—this law states that most sediments, when originally formed, were laid down horizontally. However, many layered rocks are no longer horizontal.

Law of Superposition—this law states that in any undisturbed sequence of rocks deposited in layers, the youngest layer is on top and the oldest on bottom, each layer being younger than the one beneath it and older than the one above it.

Law of Cross-Cutting Relationships—this law states that a body of igneous rock (an intrusion), a fault, or other geologic feature must be younger than any rock across which it cuts through.
Basic geologic principles
Fig. 3-18.
Basic geologic principles illustrated.
Sedimentary rock formations display features that reveal information about the environments where they formed.

The term sedimentary facies is used to describe or interpret the origin of sedimentary deposits that reflect environmental conditions that existed at the time the sediments were deposited. Examples include: offshore mud facies, reef facies, beach sand facies, terrestrial swamp facies, etc. Facies reflect the character of a rock expressed by its formation, composition, and fossil content associated with the physical conditions at the time that they formed (Figure 3-19).

Sedimentary facies preserved in rock formation reveal much information about earth history including how climates and geography have changed. They can reveal information about the timing of the formation of nearby mountain ranges, the movement of faults, eruption of volcanoes, and the shifting of shorelines over time.
In places where sediments are deposited, evidence of the physical conditions are preserved as sedimentary facies. Sedimentary facies are sedimentary deposits that reflect environmental conditions at the time of deposition of sediments. Examples include offshore mud facies, reef facies, beach sand facies, terrestrial facies, etc.  Facies reflect the character of a rock expressed by its formation, composition, and fossil content.
Fig. 3-19. Sedimentary facies reveal information about past environmental conditions.

Unconformities: Gaps in the "geologic record"

Following on the Law of Original Horizontality and Law of Superposition, both Hutton and Lyell recognized "erosional boundaries" preserved between rock layers representing "gaps in the geologic record." They named these gaps unconformities (examples in Figure 3-20). An unconformity is a surface between successive strata that represents a missing interval in the geologic record of time. Unconformities show evidence that the Earth's surface may have been exposed to erosion for periods of time. Unconformities are produced either by an interruption in deposition or by the erosion of depositionally continuous strata followed by renewed deposition. Note that unconformities may have been forming (due to erosion) in one place, where nearby or elsewhere sediments may have continued to be deposited and preserved.

Several types of unconformity boundaries are recognized:
  • nonconformity—an unconformity between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock
  • angular unconformity—an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers
  • disconformity—an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

  • conformable boundary—an arrangement where layers of sedimentary strata are parallel, but there is little apparent erosion and the boundary between two rock layer surfaces resemble a simple bedding plane. There is typically little evidence to support a significant passage of time occurred at a conformable boundary.
Types of unconformities
Fig. 3-20.
Types of unconformities (boundaries between layered rocks representing "gaps" in the geologic record in a locality).
Examples of unconformities and conformable boundaries in the Grand Canyon of Arizona
nonconformity in the Grand Canyon disconformities in the Grand Canyon Angular unconformity in the Grand Canyon Conformable contacts in the Grand Canyon
Fig. 3-21. Nonconformity in the Grand Canyon (known as the "Great Unconformity") Fig. 3-22. Disconformities between sedimentary formations in the Grand Canyon Fig. 3-23. Angular unconformity between sedimentary rocks of different ages Fig. 3-24. Conformable or gradational contact between sedimentary layers


How do unconformities form?

Unconformities are caused by relative changes in sea level over time. Wave erosion wears away materials exposed along coastlines, scouring surfaces smooth. On scales of thousands to millions of years, shorelines may move across entire regions. Erosion strips away materials exposed to waves and currents. New (younger) material can be deposited on the scoured surface. Shallow seas may flood in and then withdrawal repeatedly. Long-lasting transgressions can erode away entire mountain ranges with enough time.

A transgression occurs when a shoreline migrates landward as sea level (or lake level) rises.

A regression occurs when a shoreline migrates seaward as sea level (or lake level) falls.

Sea level change may be caused by region uplift or global changes in sea level, such at the formation or melting of continental glaciers. Whatever the cause of sea level change, when sea level falls, sediments are eroded from exposed land. When sea level rises, sediments are typically deposited in quiet water settings, such as on shallow continental shelves or in low, swampy areas on coastal plains.

Some unconformities represent great gaps in time. For example, the Great Unconformity in the lower Grand Canyon illustrates where a great mountain range existed in the region during Precambrian before erosion completely stripped the landscape away back down, eventually allowing seas to flood over the region again in Cambrian time. The "gap in the geologic record" in some locations along that unconformity represents billions of years (see Figure 3-21).
Formation of unconformities
Figure 3-25. Unconformities can form by the rise and fall of sea level. Erosion strips away materials exposed to waves and currents. The rise in sea level causes a transgression which creates space underwater for sediments to be deposited. New (younger) material is deposited on the scoured surface.

How are the ages of rocks determined?

Geochronology is the branch of earth sciences concerned with determining the age of earth materials and events through geologic time.

How do geoscientists determine the age of rocks or fossils? How do they figure out how long ago and in what order did geologic processes or events take place? For instance how do they know how often a volcano erupts or how often earthquakes take place? Geologists now have many ways to determine the age of materials using relative dating and absolute dating methods.

Relative Dating

Relative dating is the science of determining the relative order of past events, without necessarily determining their absolute age (see below). Relative dating involved the study of fossils and the correlation or comparison of fossils of similar ages but from different regions where their age is known. Microfossils derived from sediments and cores from wells help in the subsurface exploration for oil and gas.

When studying an area where layered rocks are exposed, the Law of Original Horizontality dictates that the sedimentary layers were originally deposited as flat layers. The Law of Superposition dictates that if a series of rock layers are exposed, the oldest are on the bottom of the stack. That is true, unless the sequence of rock layers have been disturbed by some later geologic even. It that case, the Law of Cross-Cutting Relationships dictates that the rocks will be older than the forces that later changed them. Examples of forces that change landscapes include movement of faults, or the tectonic folding of rock layers, or an intrusion of igneous material, such as the formation of a volcano.

Example! Use these rules to interpret this general cross section of a landscape (Figure 3-26). The diagram could represent a road cut along a highway or a wall on the side of a canyon. The Law of Original Horizontality and Law of Superposition suggest that layer C (layers of shale) was deposited before layer B (beds of limestone), which was deposited before layer A (beds of sandstone). The Law of Cross-Cutting Relationships dictates that next thing to happen was that feature D (an igneous intrusion) cut across the sedimentary layers. After that a fault (feature E) broke through all the older materials. The final thing to happen was erosion of the landscape down to partially expose some of the features on the surface. Note also that the boundaries between layers C and B, and A and B may represent unconformities (possible gaps in time). Relative dating only can be used to sort the exposed visible features in the order that they formed. The exact age of each of the rock units is unknown until it can be confirmed by other means, such as by fossils or absolute dating (discussed below).
Cross section
Fig. 3-26. Applying basic geologic principles
: Laws of original horizontality, superposition, and cross-cutting relationships explain the order of this diagram with the order of formation: rock units C, B, A, D, then E (followed by erosion).
The Waterpocket fold in Capitol Reef National Park UtahFig. 3-27. Rock layers like these in Utah record information about 100 million years of Earth history exposed in the region. These rocks were originally deposited horizontally but then were later folded upward: oldest rocks on the right; youngest on exposed on the mesa top on the left.

Try Relative Dating (unsorting the visually available clues) in map view.

Basic geologic principles are used to interpret the geologic history of an area.

These basic geologic principles have many applications to interpreting the order of events on many scales, ranging from very large (like the Grand Canyon or the geologic history of parts of the Rocky Mountains), to small scale, like interpreting the order of footprints along a lakeshore (as illustrated in this exercise).

Geologists use basic geologic principles to study geology map (as illustrated below), and use them to unravel the complex history of an area, such as planning tunnels in large underground mining operations, or for exploring for oil and gas, and deep-water resources.

Try interpreting the order of events in the diagram involving animal trackways. Notice that some tracks overly (are superimposed) on other tracks. Sometimes things aren't as clear as they seem, but inferences can be made.
Relative dating of tracks
Fig. 3-28. Relative dating exercise #1. Tracks left in the mud along a river bank include a
bear, bird, deer, dog, bobcat, and human (and blood). What happened, and in what order?
Fig. 3-29. Relative dating exercise #2. Tracks left in the mud along a river bank include a
bear, bird, deer, bobcat, duck, human. Can you figure out the chronology of events in this nature scene?

Examples of how the basic geologic principles can be applied to relative dating

Volcanic ash beds offset by minor faults along Interstate 40 near Kingman, Arizona. Folded layers of sedimentary rocks exposed near the San Andreas Fault in Box Canyon near Mecca, California. A range-front fault in the Santa Lucia Range in Arroy A basalt igneous dike cuts through older volcanic rocks in Lake Mead National Recreation Area, Nevada.Recreation
Fig. 3-30. Minor faults cut though layers of volcanic ash beds and sedimentary rocks, exposed along I-40 near Kingman, AZ. Fig. 3-31. Folded layers of sedimentary rocks exposed near the San Andreas Fault in Box Canyon near Mecca, CA. Fig. 3-32. A fault cuts trough sedimentary rocks along Arroyo Seco Canyon in the Santa Lucia Range in Monterey County, CA. Fig. 3-33. A basalt igneous intrusion cuts through older volcanic rocks in Lake Mead National Recreation Area, NV.

In Figure 3-30, the Laws of Original Horizontality and Superposition show a series of sedimentary layers and volcanic ash bed deposited from oldest (on the bottom) to a lava flow (on the top of the cliff). The Law of Cross-Cutting Relationships shows that the layers were offset by faulting after they were deposited as layers.
In Figure 3-31. the Laws of Original Horizontality and Superposition indicate that this series of sedimentary layers were deposited horizontally before tectonic forces pushed up these layers located near to the San Andreas Fault. Therefore, the San Andreas Fault must be younger than the sedimentary layers.
In Figure 3-32, the Law of Cross-Cutting Relationships suggests that the fault in the picture is younger than the sedimentary rock formation on either side of the fault. However, the age of the two rock formation on either side would have to be determined by some other means (such as by the fossils they may contain).
In Figure 3-33, the Law of Cross-Cutting Relationships suggest that the dark, basalt igneous dike is younger than the pink volcanic rocks on either side, however, the exact age of the volcanic rocks would have to be determined by other means.

Cross Sections - interpretations of vertical views of geologic features below the surface.

A geologic cross section is an interpretation of a vertical section through the Earth's surface, most usefully a profile, for which evidence was obtained by geologic and geophysical techniques or from a geologic map.

Cross sections are important tools for relative dating!

A geological cross-section is a graphic representation of the intersection of the geological features in the subsurface with a vertical plane. Where the vertical plane intersects the surface is typically shown as a line on a map. Like geologic maps, cross sections show different types of rocks, their structure, and the geometric relationship between them are represented. Note that geologic cross sections are made by using available mapable features found on the surface or interpreted from data about the subsurface. Natural cross sectional views are sometimes possible along canyon high walls or along steep mountain range front. How most subsurface data derive or imaged through geophysical methods, such as by seismic data (by earthquakes or manmade explosions), by measurements of gravity, magnetism, electrical resistivity, or information derived from wells such as core sample, radiation measurements, or other geophysical methods.

Relative Dating Using Cross Sections

Cross section
Cross section of the South Bay region, California
Fig. 3-34. Cross section of Little Rocky Mountains, Montana Fig. 3-35. Cross section of the South Bay region, California

Figure 3-34 is a cross section of the Little Rocky Mountains region of Montana, used for evaluating locations of possible oil and gas deposits in the region. In this cross section the oldest rocks are the ancient igneous and metamorphic rocks (pale green at the base of the drawing). An unconformity on top of the pale green shows that erosion stripped away ancient mountain ranges before seas finally advanced across the region and started depositing a thick series of layers of sedimentary rock formations, originally horizontal, with the youngest on top. Next, igneous activity pushed magma upward creating two large intrusive igneous bodies (these cut across and deformed the older sedimentary layers. The gravity slide faults in the youngest sedimentary beds on the right are perhaps the youngest features in the cross section.
Figure 3-35 shows a cross section through the Santa Cruz Mountains, Santa Clara Valley, and Diablo Range in the vicinity of San Jose California. The cross section shows a complex series of faults that separate great blocks of rocks of mixed ages and composition in the region. In this case, relative dating at least shows that the faults are younger than the rocks they cut through. However, other means would be necessary to explicitly date the age of the rocks and the (younger) faults.
Geologic Principles Illustrated
Wind River Basin seismic shop Geophysical cross section
Fig. 3-36. Example of a cross section through the Wind River Mountains, Wind River Basin, and Absaroka Mountains of Wyoming. Can you interpret the chronology of geologic events illustrated in this cross section? Fig. 3-37. Seabed exploration produces cross-sectional seismic profiles, raw data that are converted to cross-section diagrams. Modern systems produce views that are in three dimensions. Fig. 3-38. Geologists study cross sections created by geophysical exploration methods. This is an
example of a seismic profile showing the location of exploratory wells.

Cross Section - Relative Dating Interpretation Exercise
Cross Section Quiz
Fig. 3-39. Geologic cross section
Using the laws of original horizontality, superposition, and cross-cutting relationships interpret the order of the formation of features illustrated in this hypothetical cross section (Figure 3-39).

Note that a legend on the bottom of the cross section indicates different kinds of rocks illustrated in the cross section.
Letters on the cross section indicate geologic features including sedimentary rock layers, igneous intrusions, faults, and unconformities (lines representing periods of erosion and non-deposition).
Create a list (letters A to P) of the "events" in order as they occurred, from oldest to youngest.

The answers to this exercise is posted at the end of this chapter (below).

Absolute Dating Methods

Absolute dating is a general term applied to a range of techniques that provide estimates of the age of objects, materials, or sites in real calendar years either directly or through a process of calibration with material of known age. There are many methods of absolute dating rocks or other ancient materials. The methods of absolute dating used depends on whether suitable sample are available for testing.

One variety of absolute dating methods involves the study of the decay of radioactive isotopes. The original isotope is the "parent" which through radioactive decay becomes the "daughter" isotope. Commonly referenced studies of absolute dating utilize the radioactive decay of 238U (unstable uranium isotope) into 206PB (stable lead isotope); or 40K (unstable potassium isotope) into 40Ar (stable argon isotope). Note that here are many other absolute dating methods. Perhaps most important is radiocarbon dating which utilizes the decay of 14C (unstable carbon isotope) into 14N (stable nitrogen isotope). Dating of materials that contain naturally-occurring radionuclides (radioactive isotopes) is possible because the rate of decay of the radionuclides are known. The radiation decay "clock" starts the moment a mineral in a rock forms (or for 14C when an organism dies).

Figure 3-40
shows information about absolute dating methods. Each method uses different "parent/daughter isotopes." Each method has a suitable age dating range and materials that can be dated (such as minerals, shells, organic mater, etc.)
A "half-life" is when only half of a parent radionuclide remains. The next half-life is when only a quarter of the original parent radionuclide remains, and so on. Age determinations can be determined by comparing the percentage of the radionuclides in a new "fresh" sample with the percentage in the old sample material being tested.

Radiocarbon Dating

14C (isotope carbon -14) is a unstable radioactive isotope (radionuclide). Radiocarbon dating involves using ratios of the isotopes of radioactive isotope14C to to stable isotopes 12C and 13C derived from buried or isolated organic or carbonate materials. Figure 3-41 illustrates the science behind radiocarbon dating. The "half life" of 14C [unstable isotope carbon-14] is about 5,730 years. Radiocarbon dating has extensively used in archeological investigation and the study of climate change over the last several hundred thousand years, and precision methods now available make radiocarbon dating highly reliable. Radiocarbon dating is highly effective for extracting ages of organic materials (bone, tissues, wood, etc.) that have been isolated by burial and is effective for dating materials materials from ancient archeological sites to date human activities going back for many thousands of years.

Absolute datingFig. 3-40. Absolute dating methods. Different isotopes are used to study different materials and geologic time ranges.
Radiocarbon Dating methodFig. 3-41. The science behind the radiocarbon absolute dating method.

Understanding Maps

Maps are perhaps the most important tools for evaluating landscapes for a host of issues involving land use and natural resources. Maps have been used by humans back into prehistoric times. However, the evolution of maps in the modern digital world has changed map making—enhancing their use in nearly all aspects of modern science, technology, and culture. Modern maps are created with "geographic information systems (GIS)"—computer-based map-generating programs can combine geographic (spatial) information with many kinds of databases (medical, commercial, civic infrastructure, biological, satellite data, etc.). Whether printed on paper or in a digital format, geologic maps are essential to understanding local and regional geology, including natural resources (minerals, water, soils) and natural hazards (earthquake faults, volcanic hazards, flooding and landslide hazards).

What is a map?

A map is a diagrammatic representation of the earth's surface or part of it, showing the geographical distributions, positions, etc., of natural or artificial features such as roads, towns, relief, rainfall, etc. Maps are a scaled, 2 dimensional representation of the surface of an area or region. Some maps attempt to portray 3rd-dimensional landscape features, such as mountains or canyons. Maps may represent the surface of the land or regions in and around lakes and oceans, the seafloor, or features known or inferred to occur underground. Mapping is also used in astronomy (planet and moons, regions of space, etc.)

All "good" maps include:

title A title should include concise information related to geographic information and theme (focus) of the map's content.
scale Map scale refers to the relationship (or ratio) between distance on a map and the corresponding distance on the ground.(relative to both miles and kilometers, example: 1 inch = 2000 feet)
orientation Maps should include a north arrow and corner coordinates information (longitude & latitude)
legend explanation of symbols used on map, including colors, lines, icons, and symbols.
selected reference locations or features for orientation (such as cities, towns, highways, state boundaries, coastlines, mountain peaks, rivers, etc.)
source information authors, publisher, associated publications (text or guidebook), complete bibliographic information.
base map information (What was the base source of geographic information of a map, such as a USGS topographic map or satellite image? What is the map projection?
date published What year was the map released (or revised). Is the data new or old data?
written text Is there a publication (pamphlet or guide book) associated with this map?


Essential concepts associated with common topographic maps (relief, topography, bathymetry)

Topography refers to the arrangement of the natural and artificial physical features of an area. A topographic map is a graphical representation of a landscape showing selected natural and artificial landscape feature including topographic relief.

is the variation in elevation on the surface of the Earth (topography). Areas of "high relief" have much elevation changes over distance, such as mountainous areas and canyons. "Low relief" occurs where elevation changes are minimal, such as on coastal plains.

Bathymetry refers to the measurement or mapping of water depth (sea bed) beneath oceans, lakes, or other bodies of water. A map of bathymetry of the ocean floor or large lake is called a chart.

A topographic map is a map showing relief and man-made features of a portion of a land surface distinguished by portrayal of position, relation, size, shape, and elevation of the features (see example in Figure 3-42). Topographic maps have contours, which are lines that represent the location of equal elevations, typically measured in feet or meters above standard mean sea level.

Both Topography (relief) and bathymetry are graphically illustrated with contour lines or shaded relief. A shaded relief map uses gray-scale shading giving the appearance of relief similar to "sun shading" highlighting a natural landscape.
Chittenden 7.5 minute quadrangle
Fig. 3-42. Example of a USGS topographic map:
Chittenden, CA 7.5 minute quadrangle

What are Latitude and Longitude?

Locations on the Earth's surface are defined using latitude and longitude coordinate system.

Latitude is a measure of the angular distance of a place north or south of the earth's equator, usually expressed in degrees and minutes. Lines of latitude are called parallels. Latitude lines parallel the Equator. Each degree of latitude is approximately 69 miles (111 kilometers) apart.

is a measure of the angular distance of a place east or west of the Prime Meridian usually expressed in degrees and minutes. In order to make an accurate map of the stars for use in ship navigation, in 1884, a location indicating the precise location of 0° East-West was designated in the cross hairs of a telescope in the Royal Observatory (now located on the grounds of the National Maritime Museum) in Greenwich England. This line marks the reference location of the Prime Meridian now used in all global mapping (including GPS location systems).

A meridian is a circle of constant longitude passing through a given place on the earth's surface and the terrestrial poles. Longitude lines (of equal spacing measured in degrees) are widely spaced at the equator but converge at point at the North and South Poles. The Prime Meridian is designated 0° (zero degrees). Meridian lines east of the Prime Meridian are designated positive values (0° to 180° east); whereas meridian lines west of the Prime Meridian are designated negative values (-0° to -180°). At 180° east or west is the International Date Line. A degree of longitude is widest at the equator at 69.172 miles (111.321) and gradually shrinks to zero at the poles. At 40° north or south the distance between a degree of longitude is 53 miles (85 km).

Defining locations with a latitude-longitude coordinate system
—any location on the planet surface can be defined by a number in degrees, minutes, and seconds north or south of the Equator and east or west of the Prime Meridian. (Compare to hours, minutes, seconds on a clock!)

Example: Location of the Statue of Liberty in New York Harbor

The standard coordinates of the are:
Latitude: 40°68′92"N
Longitude: 74°04′ 45"W.

Described in decimal degrees the coordinates of the Statue of Liberty are: Latitude:40.689758°

Find the latitude and longitude of any named location or landscape feature on the GeoNames website.

Global Positioning System (GPS)—a space-based global navigation satellite system that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth when and where there is an unobstructed line of sight to four or more GPS satellites.

quadrangle—a standardized area used in mapping to designated an area on the Earth's surface. In the United States, the area shown on one of the standard 7.5 minute quadrangle map sheets (published by the U.S. Geological Survey)

Standard USGS topographic maps are 7.5 minute quadrangles with scale of 1:24,000 . The standard USGS 7.5 minute quadrangle map is approximately 17 miles (27 km) north to south and within the lower 48 states range from 11 to 15 miles (17 to 24 km) east to west. In a 1° by 1° quadrangle area, there are sixty-four 7.5 minute quadrangles.

Map Projections

The earth is round (a sphere like a globe) but maps are flat. As a result, maps that show large regions are distorted. Map projections are attempts to portray a portion of the earth on a flat surface. The flattening of a map always causes some distortions of distance, direction, scale, and area. Large scale maps (such as a map of a continent or a world show much distortion, however, maps on small scales (such as a map of a town or neighborhood) have relatively little distortion. Learn more about map projections at the U.S. Geological Survey Map Projections website: Projections/projections.html

Map of world, Mercator Projection Map of North America Lambert Projection Globe view of Earth from space
Fig. 3-53. Map of world showing with Mercator Projection - notice distortion in high latitudes because longitude lines are not converging Fig. 3-53. Map of North America with Lambert Conic Projection - on this scale distortion of America is minimal, but look at South America. Fig. 3-54. A global view is the only way to have perfect map projection!
Global projection Mercator Map
Fig. 3-43. Longitude and Latitude projected on a globe. Fig. 3-44. Map of the world showing latitude and longitude in a Mercator (flat) projection.
California Topographic Map Index California 7.5 minute quadrangle index
Fig. 3-45. California Map Index for USGS 30 x 60 minute quadrangle designations. (Equals 0.5º latitude and 1.0º longitude on each side of quadrangle map.) Fig. 3-46. Portion of the California USGS Topographic Map Index showing map 7.5 minute quadrangle designations.
Devils Tower satellite image from Google Devils Tower Quadrangle
Fig. 3-47. Google Maps satellite view of Devils Tower, Wyoming Fig. 3-48. Part of Devils Tower 7.5 Minute Quadrangle Map, Wyoming showing contour lines, green for wooded areas, elevation markers, etc.

Topographic Map scale 1:24,000
Fig. 3-49. Scale bar on a typical 1:24,000 topographic map Gavilan Colege Quadrangle 2012
Gavilan Topographic Quadrangle
Fig. 3-50. Portion of the Chittenden Quadrangle (CA) topographic map (black and white topo map with contours for elevation) Fig. 3-51. Portion of the Chittenden Quadrangle topographic map (2012 version) showing mapping on a satellite image.
San Luis Rey, California 7.5 minute quadrangle
Fig. 3-55.
San Luis Rey, CA 7.5 minute quadrangle (large .pdf file)
Topographic map of a portion of Washington DC
Fig. 3-56. Southwest corner of the Washington DC 7.5 minute topographic quadrangle map showing locations where most of the government building and monuments are located.

The Public Land Survey System

Historically, as the United States expanded its territories in the 18th to early 20th centuries, land was surveyed (mapped) and subdivided using the Public Land Survey System. This method of surveying involved measuring a grid of lines east-to-west and north-to-south starting with a designated initial point (such as a mountain peak like Mt. Diablo in central California). From an established initial point, a north-to-south was mapped (called the Principal Meridian). From the initial point, a baseline was mapped east-to-west following a line of latitude.

North-south lines are called township lines and east-west lines are called range lines. These lines of a grid are parallel mapped at intervals of at 1 mile apart. Flying over the country today it is easy to see the historical significance of the township and range lines because they are the locations of roads and property boundaries throughout the landscape we see today.

On a limited regional scale, township and range lines work fairly well, but with increasing distances the rectangular pattern begins to fail due to distortion caused by the curvature of the earth. To solve the distortion problem, new grids of township and range lines were set up throughout the country over time. The Public Land Survey System is used for describing locations in where the land grid has been establish. Throughout much of the United States, roads (commonly dirt tracks) follow section lines east and west, and north and south. These lines are considered public access routes in most states, and are a means of finding and locating specific pieces of property or locations on public lands.
BLM Map showing Public Land Survey grids BLM Map showing Public Land Survey System grid in parts of the American West
Fig. 3-57. Principal Meridians and Base Lines of region that used the Public Land Survey System (map provided from US BLM) Fig. 3-58. Detail view of a public land survey of Nebraska Territory showing the initial point, principal meridian, and baseline.
Initial Point marker for the 1855 land survey of Nebraska Territory Public Land Survey System
Fig. 3-59. Google image of the initial point for the Public Land Survey of Nebraska Territory started in 1855. Fig. 3-60. Locations in areas mapped with the Public Land Survey System are designated by township, range, and section.

Selected Map and Digital Map Data Sources

Google Maps provides world-wide map information of basic geography (roads, towns, parks, water, landscape features) or on satellite imagery.

Google Earth is an example interactive geographic information system (requires software download). Many kinds of information linked to a database containing geographic reference data can be imported into Google Earth.

ESRI is a corporation supporting the most widely used map editing (GIS) software in education, industry, and government.

USGS Topographic Maps and information about maps can be gotten from:

USGS Map Locator & Down Loader
(a source for "free" digital topographic maps on several scales for most of the United States)

USGS National Map Viewer
(free online access to digital topographic maps)

US Geological Survey Topographic Map Symbols
is at:

Types of Maps (American Geological Institute, Earth Comm website provides links to many map sources)

See the USGS Maps, Imagery, and Publications (a website to download free digital maps and imagery)

Learn about all kinds of maps for California (Humboldt State University Library website)

See Historic USGS topographic quadrangle maps of the Monterey Bay region (UC Berkeley)
Other common types of maps and digital map data that show topography and integrated into geographic information systems.

Digital elevation model (DEM)—A digital elevation model (DEM) is a digital representation of ground surface topography or terrain. It is also widely known as a digital terrain model (DTM) commonly used as a base map in a geographic information system (GIS). A DEM can be represented as a raster (a grid of squares) or as a triangular irregular network. A DEM is used for the generation of contours, shaded relief, 3-D terrain models and elevation profiles. DEMs are used to make shaded relief maps using GIS software.

Shaded relief map—A map of an area whose relief is made to appear three-dimensions using gray-scale shading based on a hypothetical sun angle, typical of late afternoon (north and east facing slopes appear darker than south and west facing slopes).

Satellite image map
—a map generated from raw satellite imagery data that has been rectified to match a grid associated with a standardize map grid, such as a USGS 7.5 minute quadrangle, a "digital orthoquad", or DOQ. provides satellite image maps along with standardized road maps on their maps search website.
Mount St. Helens Austin Texas DEM Mount St. Helens DEM shaded relief Devils Tower satellite image
Fig. 3-61. Photograph of Mount St. Helens, Washington. Standard photographs are useful in describing landscapes, but they are less important for analyzing data compared with maps. Fig. 3-62. Example of "raw" DEM data: Digital elevation model of Mount St. Helens, Washington. Note that a DEM shows relief only in a vertical orientation in 256 shades of gray. It is only raster elevation data and does not show geographic names, roads, etc. Fig. 3-63. Shaded relief model made with DEM data for Mount St. Helens, Washington. This image was created with a geographic information system by re projecting the raw DEM data from the previous image. Fig. 3-64. Satellite imagery data (digital othoquad [DOQ]) of Mount St. Helens, Washington. DOQs are photographs (images) taken from airplanes or satellites. DOQs are commonly used as base maps in creating other kinds of maps or map layers in a GIS database.

GIS Careers: Many of the top employers are looking for people with geospatial data processing experience. There are many colleges and universities that offer programs that now offer degrees in programs related to map science and geospatial data processing. Students which geographic information systems training typically have a skill set that is desireable to potential employers. Learn about interesting careers and some of nation's leading programs on this website: Geospatial Science and Geographic Information Systems Master’s Degrees

Geomorphology and and the concept of physiographic provinces.

The links between the traditional studies of Geography and Geology intersect in the science of Geomorphology.

Geomorphology is the study of the earth's surface including classification, description, nature, origin, and development of landforms and their relationships to underlying structures and the history of geologic changes as recorded by these surface features.

Primary studies of landscapes involve generating map of many kinds, including natural landscape features (such as rivers, mountains, shorelines), man-made features (cities, roads, dams, etc.), ecology and land use (natural and agricultural) and much more. Regions of the North American landscape have been subdivided in to areas sharing similar physical characteristics, such as topography (relief), geologic history, ecology, and climate. These subdivisions on a map are called Physiographic Provinces.

A physiographic province is a geographic region with a specific geomorphology and often specific subsurface rock type, age, or structural elements. See more information on the Physiographic Provinces of the United States web page.

For more detailed discussion on physiographic provinces see:Regional Geology of North America.

Shaded Relief Map of the United States Physiographic provinces of the United States Physiographic Provinces Map (blank) Physiographic provinces of California
Fig. 3-68.Physiographic Provinces on a geologic map
Fig. 3-65. Shaded Relief Map of the United States
(USGS Map i-2206)
Fig. 3-66. Physiographic province of the United States Fig. 3-67. Blank physiographic Provinces Map
Physiographic Provinces
Tapestry of Time and Terrain (a USGS map website) compares landscapes with underlying geology with physiographic provinces of the United States.

Gail P. Thelin and Richard J. Pike, 1991, Landforms of the Conterminous United States - A Digital Shaded-Relief Portrayal: U.S. Geological Survey
Miscellaneous Investigations Series Map I-2206,

Fig. 3-69. Physiographic Provinces on a geologic map  


What is a Geologic Map?

A geologic map is a special-purpose map made to show geological features. Types and ages of rock units are shown by color or symbols to indicate where they are exposed at or near the surface. A geologic map records the distribution, nature, and age relationships of rock units and the occurrence of structural features (such as the location of faults). Geologic features are illustrated as colors, lines and symbols.

Geologic maps depict the land as if all soil and vegetation were stripped away. Geologic maps show "bedrock" geologic materials and features, and may display shallow surface sediments (alluvium, landslide deposits, floodplain deposits, sand dunes, etc.)
Geologic map of the Cincinnati Ohio region
Geologic maps are used to interpret the geologic history of a region. Geologic maps are used by paleontologists to find areas that are likely to contain fossils, and by geologists and engineers to define the location of faults, economic mineral resources, to find potential underground water resources, used in civic planning, and more. Fig. 3-70. General geologic map of the Cincinnati Arch region, Ohio, Kentucky, Indiana

Essential concepts to understand geologic maps
(common geologic information concepts needed to interpret geologic maps)

geologic time scale
rock types (see rock cycle)
Law of Superposition
Law of Original Horizontality
Law of Cross-Cutting Relationships
relative and absolute dating
strata (beds, rock formations)
surficial deposits (stream deposits, alluvium, landslides, etc.)
strike & dip
Common geologic map symbols Basic Geologic Principles Unconformities
Fig. 3-71. Common symbols representing geologic features on geologic maps. Fig. 3-72. Basic geologic principles: laws of original horizontality, superposition, and cross-cutting relationships Fig. 3-73. Unconformities: types include nonconformities, disconformities, and angular unconformities.
Comparison of a topographic map and a geologic map
San Juan Bautista 15 minute topographic map Geology on topography Geologic map
Fig. 3-74. Topographic Map
San Juan Bautista, CA
15 minute quadrangle
Fig. 3-75. Topographic map with geology mapping Fig. 3-76. Geologic Map
San Juan Bautista, CA
15 minute quadrangle
Modified from: Dibblee, Thomas, W, 1979, Preliminary Geologic Map of the San Juan Bautista Quadrangle: U.S. Geological Survey Open-File Report 79-375, scale 1:24,000.

Interpreting the Earth structure with maps and geologic illustrations

Geologic Maps, cross sections, and block diagrams
are used to show the structure of the Earth in three dimensions. Geologic maps show the arrangement of geologic features and materials exposed on or near the surface. Cross sections show interpretations of geologic features, structures, and materials in vertical profiles, usually perpendicular to the surface.

Examples below are from the Grand Canyon, Arizona area. (Map modified from Billingsley, G. H., 2000, Geologic map of the Grand Canyon 30' x 60' quadrangle, Coconino and Mohave Counties, northwestern Arizona: U.S. Geological Survey Geologic Investigations Series I-2688, map scale 1:100,000.

Grand Canyon photo
Fig. 3-77. Grand Canyon (photo view is looking north from the South Rim)
Grand Canyon cross section
Grand Canyon geologic map Geologic Map Legend
Fig. 3-78. Grand Canyon Geologic Map. Line D - D' is the location of the cross section shown below.

Grand Canyon cross section
Fig. 3-79. Block Diagram of the Grand Canyon Fig. 3-80. Geologic Cross Section of the Grand Canyon Fig. 3-81. Geologic map legend for the Grand Canyon


Finding Geologic Map Information

To find a geologic map of portions of the United State it is usually essential to find the name of the topographic map name of a particular region. The USGS typically names small scale geologic maps with the name of the 7.5 minute topographic map quadrangle (1:24,000 scale). Larger scale (regional maps) are named after the 1:100,000 maps (1° x 2° quadrangle) or larger 1:250,000 scale. State geologic maps are typically 1:1,000,000 scale or larger.

Geologic maps can be located using the National Geologic Map Database:

FGDC Digital Cartographic Standard for Geologic Map Symbolization

Symbols Used On Geological Maps


Geologic maps and geology field trip destinations in northern San Diego County region:

Oceanside 1:100,000 scale map and 1:24,000 scale maps (coastal northern San Diego County)
San Diego 1:100,000 scale map and 1:24,000 scale maps (coastal southern San Diego County)
(Maps modified from state and federal geologic maps) - very useful resources for studying regional geology in San Diego County!

Selected field trip destinations in San Diego County — Places to see regional geology.
Answer to the Cross Section Exercise in Figure 3-39.

The order of formation (from oldest to youngest) is: P - E - F - D - I - L - O - M - J - H - A - K- G - N - B - C

Note that it is sometime easiest to start at the top and work backward on such diagram before you recreate the "geologic history" of an cross section. Here is what happened here (starting from oldest to youngest).

Sedimentary layers were deposited (following the laws of superposition and original horizontality).
1. Pshaly sandstone was deposited as a horizontal layer.
2. E mudstone was deposited as a horizontal layer.
3. F layers of sandstone were deposited as horizontal layers.
4. D more mudstone and shaly sandstone layers were deposited as horizontal layers.
5. I represents a fault that formed during a period of mountain building where the older layers P, E, F, and D were folded, faulted and intruded by igneous rock.
----- The law of cross-cutting relationships depicts that the fault [ I ] is younger than sedimentary layers P ,E, F, and D.
6. L represents an unconformity, mountain building had ended, the landscape had eroded flat over a long period of time.
7. O sandstone was deposited as a horizontal layer on top of the ancient eroded landscape surface.
8. M shale was deposited as a horizontal layer on top of the sandstone [M].
9. J layers of limestone were deposited as horizontal layers on top of the shale [J].
10. H a fault formed, cutting through all the previously formed rock layers - as defined by the the law of cross-cutting relationships.
11. Athe letter A represents an unconformity - layers of limestone on the right side are thinner, suggesting there was either erosion and/or non-deposition.
12. K a layer of sandstone was deposited above the unconformable surface.
13. G a layer of shale was deposited on top of the sandstone.
14. N an igneous intrusion cut through all previously formed rock units, possibly forming a volcano.
15. B after the igneous intrusion {N] formed, erosion stripped away any evidence of a possible volcano, and a layer of sandstone [B] was deposited.
16. C another igneous intrusion formed, resulting in eruptions that formed the volcano still present on the land's surface.

Other things to consider on this cross section exercise.
Note the the igneous rocks with X markings on the right side of the diagram are truncated by the unconformity [L] - the same as the fault [ I ]. Which one is older? It is impossible to tell they are "relatively" the same age. In addition, all the boundaries between the rock formations could be unconformities, but their significance in age differences would have to be determined by some other means.

Unconformity [L] clearly demonstrates that it is an unconformity represents a long period of time passage. In some places it is an angular unconformity; over the intrusion [ with the X markings] it is a nonconformity.

The unconformity marked as letter A is somewhat problematic. It truncates the fault (letter H). The problem is that it would take further examination to determine whether or not fault H was forming at the same time that limestone J was being deposited!

The boundary between sedimentary layers G and B is also an unconformity that represents a significant passage of time at least long enough for a volcano to form and then be stripped away by erosion.

Chapter 3 - Quiz questions