1. Relative Dating

2. History of Geology
In 1795 James Hutton (Figure 1) published a book that helped explain some of the leading questions many scientists had at the time.
Like many wealthy scientists from that time period, Hutton was interested in everything, from medicine to engineering and designing new canals. However, what he was most interested in was geology.
Much of science at this time was focused on biology, chemistry, and physics. However, many people were interested in Earth’s geologic history. One of the fundamental questions people had was trying to explain how fossilized seashells turned up in the most unexpected places, like on the top of Mount Everest.
Geologists who thought they knew the answer to this riddle fell into two opposing camps. The first group thought that the Earth’s landscape was basically unchanging and that changes in sea levels explained how fossilized marine organisms ended up on top of mountains.
However, the other group of scientists didn’t accept this hypothesis. They correctly questioned where all the water went during times between global flooding events. They also pointed out that there didn’t seem to be any evidence of global flooding. Instead they hypothesized that the Earth is always changing. Hutton made simple observations on his farm in Scotland, and concluded that the soil in his farm was the result of the weathering rock below it. He could also see in a single lifetime how the hills around his farm had slightly changed. Therefore he suggested that mountains grow because the heat inside the Earth pushes rock upwards, bringing the seafloor with the clamshells up with it.
To accept this line of thought you also needed to except something else that very few people understood at the time. The Earth is very old. Hutton correctly pointed out that large canyons made of thick layers of sedimentary rocks, like the Grand Canyon (Figure 2), would take 10’s to 100’s of millions of years. Few people, if any, had suggested that the Earth existed for this long.
James Hutton planted the seed of geology. I’m sure this next statement is very hard to believe, however for the next century geology excited the scientific community in a way that perhaps no other science every has or will.
Figure 1. James Hutton considered the “father” of geology.
Figure 2. The Grand Canyon showing thick layers of sedimentary rocks.

3. Relative Dating
There are two different ways in which something can be dated. These different methods are referred to as relative dating and absolute dating.
Absolute dating provides a numerical value, such as the Earth is 4.54 billion years old. However, relative dating, as the term implies, refers to the age of a rock in relation to other rocks. Therefore when you date rocks using relative dating techniques you are trying to put rocks and other geologic events into chronological order (i.e. what happened first, second, third, etc.) .
Because it can be very difficult and many times impossible to date the exact age of a rock or geologic event, relative dating is a very important tool used to learn about the geologic history of the Earth.
To use this method of dating, rocks are one of the most important tools. The exposed pieces of bedrock used to help better understand the geologic history of Earth are called outcrops (Figure 3.) Anytime you have looked out the car window and saw exposed bedrock along the side of the road you were looking at an outcrop.
Figure 3. Exposed piece of bedrock called an outcrop.

4. Relative Dating – Guidelines
Superposition
Because sedimentary rocks form from the deposition of sediment settling either on the surface of the earth or more commonly in the water, the rocks on the bottom are typically older than the rocks on the top (Figure 4). This is called the Law of Superposition.
Like most rules, there are exceptions. This only applies to an undisturbed sequence of rocks. If the rock strata has experienced some sort of tectonic activity resulting in tilting, faulting, or folding this principle may not apply (Figure 5). Furthermore, because igneous rocks form from magma which comes from deep inside the earth, younger igneous intrusions can found below older rock.
Figure 4. Using the Law of Superposition, you can assume that younger rocks sit on top of older rocks.
Figure 5. As you can see in the outcrop above, older rocks can be found on top of younger rocks when extensive folding occurs.

5. Relative Dating – Guidelines
Cross-Cutting Relationship
The cross-cutting relationship says that things that cut across rocks, like faults and igneous intrusions (Figure 6), must be younger than the rocks that they cut across. B
Figure 6. The fault in A and the dike (igneous intrusion) in B are younger and therefore occurred more recently than the rocks that they cut across.

6. Relative Dating – Guidelines
Law of Original Horizontality
Because sedimentary rocks form from the deposition of sediments, they form in horizontal layers (Figure 8).Therefore, if you see tilted or folded layers you can infer that the tectonic activity that folded the rocks must have occurred after the rocks formed.
Figure 8. Shale outcrops from Treman State Park in Ithaca, NY.

7. These red sandstones where over 5,000 feet from the seafloor as a result of the uplift creating the Rocky Mountains.

8. Relative Dating – Guidelines
Contact Metamorphism
It can be difficult to determine the relative age of an igneous intrusion, because sometimes it isn’t obvious if magma intruded into surrounding rock, like Figure 9 and 10 are showing, or if a layer of rock formed from a surface flow during a volcanic eruption.
Looking for contact metamorphism can help determine the relative age of a rock layer or geologic event.
If a layer of rock has been metamorhpisized it is older than the igneous intrusion that is suppling the heat for contact metamorphism to occur.
Figure 9. Heat from the magma chamber has metamorphisized some of the older bedrock.
Figure 10. In this outcrop, layer D is older than layer C. The fact that the bottom layer of D has been metamorphisized, indicates it was there before the igneous intrusion that created layer C.

9. Missing Rock Layers
Because rocks are constantly being eroded, it is common to find large gaps in the rock record. For example, due to the tremendous amount of erosion that has occurred as the result of the advancing and retreating glaciers from the ice age, massive amounts of rock have been removed from most of New York State.
These gaps in the rock record are known as unconformities.
Unconformities represent buried erosional surfaces, and because they are usually the result of periods of uplift, followed by erosion, and deposition of new rock, unconformities represent large passages of time (Figure 11.)
Figure 11. The tilted sedimentary rock layers suggest that the unconformity shown in the outcrop above was created as the result of some sort of tectonic activity that pushed these rock layers up, exposing them to weathering and erosion. This erosional surface then subsided, probably underwater, were more sediment buried it.