principles of relative dating

the grand canyon

The Grand Canyon provides an excellent illustration of the principles above. The many horizontal layers of sedi- mentary rock illustrate the principle of original horizontality (Figure 1.3). The youngest rock layers are at the top and the oldest are at the bottom, which is described by the law of superposition. Distinctive rock layers, such as the Kaibab Limestone, are matched across the broad expanse of the canyon. These rock layers were once connected, as stated by the rule of lateral continuity. The Colorado River cuts through all the layers of rock to form the canyon. Based on the principle of cross- cutting relationships, the river must be younger than all of the rock layers that it cuts through.

more principles of relative dating

Other scientists observed rock layers and formulated other principles. Geologist William Smith (1769-1839) identified the principle of faunal succession, which recognizes that: Some fossil types are never found with certain other fossil types (e.g. human ancestors are never found with dinosaurs) meaning that fossils in a rock layer represent what lived during the period the rock was deposited. Older features are replaced by more modern features in fossil organisms as species change through time; e.g. feathered dinosaurs precede birds in the fossil record. Fossil species with features that change distinctly and quickly can be used to determine the age of rock layers quite precisely. Scottish geologist, James Hutton (1726-1797) recognized the principle of cross-cutting relationships. This helps geologists to determine the older and younger of two rock units (Figure 1.2). If an igneous dike (B) cuts a series of metamorphic rocks (A), which is older and which is younger? In this image, A must have existed first for B to cut across it.


stenos laws

Remember Nicholas Steno, who determined that fossils represented parts of once-living organisms? Steno also noticed that fossil seashells could be found in rocks and mountains far from any ocean. He wanted to explain how that could occur. Steno proposed that if a rock contained the fossils of marine animals, the rock formed from sediments that were deposited on the seafloor. These rocks were then uplifted to become mountains. This scenario led him to develop the principles that are discussed below. They are known as Stenos laws. Stenos laws are illustrated in Figure 1.1. Original horizontality: Sediments are deposited in fairly flat, horizontal layers. If a sedimentary rock is found tilted, the layer was tilted after it was formed. Lateral continuity: Sediments are deposited in continuous sheets that span the body of water that they are deposited in. When a valley cuts through sedimentary layers, it is assumed that the rocks on either side of the valley were originally continuous. Superposition: Sedimentary rocks are deposited one on top of another. The youngest layers are found at the top of the sequence, and the oldest layers are found at the bottom. (a) Original horizontality. (b) Lateral continuity. (c) Superposition.


relative age dating

Early geologists had no way to determine the absolute age of a geological material. If they didnt see it form, they couldnt know if a rock was one hundred years or 100 million years old. What they could do was determine the ages of materials relative to each other. Using sensible principles they could say whether one rock was older than another and when a process occurred relative to those rocks.

instructional diagrams

No diagram descriptions associated with this lesson


original horizontality is when true

-->  a) sediments are deposited fairly flat, horizontal layers.

b) sediments are deposited in continuous sheets that span across a body of water.

c) sedimentary rocks are layered one on top of another by age.

d) all of the above

according to nicholas steno, if a rock at the top of a mountain contains fossils of marine animals, that rock formed in the sea and was uplifted into a mountain.

-->  a) true

b) false

superpostion is when _____.

a) sediments are deposited fairly flat, horizontal layers.

b) sediments are deposited in continuous sheets that span across a body of water.

-->  c) sedimentary rocks are layered one on top of another by age.

d) all of the above

the oldest rocks are found ____ of a sequence.

a) at the top

b) in the middle

-->  c) at the bottom

d) none of the above

if the same sedimentary rock layer is found on either side of a valley

a) the rocks on one side were moved across by earthquake action.

-->  b) the rock formed in the same environment and the valley was cut into the rock later.

c) the rocks look the same, but they are two separate rocks formed in two separate ways.

d) there is something wrong, because this could never happen in nature.

which of the following is true about fossils?

a) some fossils types are never found together.

b) younger fossils display more modern features than older fossils.

c) fossil species with features that change distinctly and rapidly can be used to determine ages precisely.

-->  d) all of these.

fossils of human ancestors have been found with dinosaur fossils.

a) true

-->  b) false

if an igneous dike cuts across metamorphic rock layers,

-->  a) we know that the metamorphic rocks are older.

b) we know that the igneous rock dike is older.

c) we cannot know which formed first.

d) we know that they both formed at the same time.

in the grand canyon,

a) the horizontal rock layers are sedimentary rocks that were deposited horizontally.

b) the rock layers are found separated by valleys so they are laterally continuous.

c) the colorado river is younger than the rocks it cuts across.

-->  d) all of the above are true

feathered dinosaurs preceded birds in the fossil record.

a) true

-->  b) false

diagram questions

No diagram questions associated with this lesson