John Michoian 5/7/15

Relative dating involves placing events in their proper chronological sequence, that is,

in the order of their occurrence. This type of dating tells us which geologic event happened

first, but does not give an exact date to which something happened. There are several

different methods that are used in relative dating. There are fundamental methods that are

used by geologists and scientists to gather information about the relative age of rock. These

principles are the principle of superposition, the principle of original horizontality, the

principle of cross-cutting relations, and uniformitarianism. The principle of superposition is

defined as: In the environment of an undisturbed layer of sedimentary rocks; the layers on

the bottom are older than the layers towards the top. The Principle of Original

Horizontality states that layers of sediment are originally deposited horizontally under the action

of gravity. The principle of cross-cutting relationships states that an igneous intrusion is always

younger than the rock it cuts across. The law of uniformitarianism basically states that the

present is the key to the past. With laws like these it is easier and more effective for geologist

to interpret, analyze, and a geological sequence.

In sequence A, siltstone, sandstone, and fossiliferous limestone were horizontally

deposited. Second, uplift occurs due to faulting. Next in the sequence is weathering and

erosion. In a geological sequence, weathering and erosion always follow after uplift. There

is a disconformity which is an unconformable surface that separates parallel strata. There is

a stream which basically explains the cause of the disconformity. Erosion of sediments by

the stream is the explanation of why there is a piece of the limestone is missing. Next,

subsidence needs to happen. This is because the other layers have to go back down to be

able to be deposited. The next step is horizontal deposition of shale and sandstone. Uplift

occurs after that, and finally, weathering and erosion occur and the sequence is above sea

level.

Sequence B begins with the extrusion of felsic rock. Then follows the deposition of

limestone and the siltstone layers. After that is uplift from the intrusion. Next, there is

subsidence and after that is the horizontal deposition of the shale and sandstone layers.

There is more uplift after the two layers were deposited. Last, there is more weathering and

erosion. There is a U-shaped valley in this sequence which shows possibility that a glacier

may have come through.

In sequence C, bituminous coal is formed in a swamp or marshland from compacted

plants. Later came the deposition of conglomerate, limestone and clay are which was

lithified to shale. Sequence C is made of metamorphic rocks, therefore, the sediments have

to be deposited first in order to be metamorphosed. Following that is uplift - regional

metamorphism. Following is folding which causes a syncline. Next, the metamorphosing

takes place. Bituminous coal converts to anthracite coal, conglomerate turns to

metaconglomerate, Limestone metamorphoses to marble, then finally shale goes to slate

which turns to schist. Afterwards, the igneous intrusion is coming up to the surface and is

exposed; making it an extrusion. Then, there is a fault on the left. Eventually, the xenolith

comes up. Lastly, in the final last steps of sequence C, there is weathering and erosion.

Then, it is currently above sea level.