Chapter 17 Unraveling Geologic History

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386 CHAPTER 17: UNRAVELING GEOLOGIC HISTORY Niagara River Lockport dolostone Rochester shale Irondequoit limestone Reynales limestone Thorold sandstone Grimsby sandstone Power Glen shale Whirlpool sandstone Queenston shale Figure 17-1 The profile of Niagara Falls shows the oldest rocks are at the bottom and the layers become younger toward the top. This illustrates the law of superposition. younger sediments can be deposited on top of them. Superposition is the concept that unless rock layers have been disturbed, each layer is older than the layer above it and younger than the one below it. Therefore, when geologists assign relative ages to rocks, the oldest rocks are on the bottom and the layers become progressively younger toward the top. Figure 17-1 is a profile of the rock layers at Niagara Falls. Each rock layer is named for the location where it can be observed and studied. For example, the hard cap rock at the top that is responsible for the falls is the Lockport Dolostone. If you want to observe this layer in the field, there is a bedrock exposure near Lockport, New York. The first layer deposited at Niagara Falls is the layer on the bottom, the Queenston Shale. After this layer was deposited, sand was deposited that became the Whirlpool Sandstone. One by one, each layer was deposited before the layer above it. Superposition does not always apply. In some places, folding has overturned the rocks or faulting has pushed older layers on top of younger layers. These exceptions to the law of superposition will be explained in the next section. However, if you are looking at rock layers or a diagram of rock layers, unless you see evidence for overturning or faulting you should assume that the law of superposition can be applied.The older layers are those on the bottom. Exceptions are unusual.
Original Horizontality HOW CAN WE DETERMINE THE SEQUENCE OF EVENTS? 387 Sedimentary rock is usually the result of deposition of sediments in layers. In some places, the layers are tilted rather than horizontal. The principle of original horizontality states that no matter the present angle or orientation of sedimentary layers, it is almost certain that the layers were originally horizontal and were tilted after deposition. The cause of the tilting could be folding or an uneven regional uplift. Figure 17-2 shows a sequence of events in the creation of a bedrock outcrop. An outcrop is a place where bedrock is exposed at the surface. In diagram 1, sandy sediments are washed into a curved basin. Most sedimentary rocks are formed from deposits in water. The low spots in the basin are the first to fill with sediment. Therefore, no matter what the shape of the basin, the layers of sediment are flat and hori- Figure 17-2 When sediments are deposited in a basin, they accumulate as flat, horizontal layers as shown in diagrams 1 and 2. This occurs no matter what the original shape of the basin. Diagrams 3 and 4 represent a later time and a bedrock outcrop that is from the center of diagram 2. When geologists see tilted sedimentary layers as in diagram 4, they usually infer that the layers were deposited flat and horizontal.
Page 1 and 2: Chapter 17 UNRAVELING MYSTERIES Unr
Page 3: have been able to piece together a
Page 7 and 8: Figure 17-3 It is clear that this f
Page 9 and 10: Contact Metamorphism Metamorphism i
Page 11 and 12: Figure 17-6 Formation of an unconfo
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Page 15 and 16: Intrusion and extrusion at the surf
Page 17 and 18: Figure 17-9 Carbon atoms always con
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Page 23 and 24: Base your answers to questions 2-4
Page 25 and 26: Which sequence of events below is b
Page 27 and 28: 12. The graph below represents data
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Chapter 17 Unraveling Geologic History

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