principle of horizontality

geologic history

Youre standing in the Grand Canyon and you see rocks like those in the Figure 1.1. Using the rules listed above, try to figure out the geologic history of the geologic column. The Grand Canyon is full mostly of sedimentary rocks, which are important for deciphering the geologic history of a region. In the Grand Canyon, the rock layers are exposed like a layer cake. Each layer is made of sediments that were deposited in a particular environment - perhaps a lake bed, shallow offshore region, or a sand dune. (a) The rocks of the Grand Canyon are like a layer cake. (b) A geologic column showing the rocks of the Grand Canyon. In this geologic column of the Grand Canyon, the sedimentary rocks of groups 3 through 6 are still horizontal. Group 2 rocks have been tilted. Group 1 rocks are not sedimentary. The oldest layers are on the bottom and youngest are on the top. The ways geologists figure out the geological history of an area will be explored more in the chapter Earth History. Click image to the left or use the URL below. URL:

textbook_image

sedimentary rock rules

Sedimentary rocks follow certain rules. 1. Sedimentary rocks are formed with the oldest layers on the bottom and the youngest on top. 2. Sediments are deposited horizontally, so sedimentary rock layers are originally horizontal, as are some vol- canic rocks, such as ash falls. 3. Sedimentary rock layers that are not horizontal are deformed. Since sedimentary rocks follow these rules, they are useful for seeing the effects of stress on rocks. Sedimentary rocks that are not horizontal must have been deformed. You can trace the deformation a rock has experienced by seeing how it differs from its original horizontal, oldest- on-bottom position. This deformation produces geologic structures such as folds, joints, and faults that are caused by stresses.

instructional diagrams

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questions

sedimentary rocks are laid down

a) vertically

b) diagonally

-->  c) horizontally

d) none of the above

the classic location to see layer cake geology is

-->  a) the grand canyon

b) the himalayas

c) hawaii

d) the central valley

a lake fills with sediment and then is buried and the sediment is lithified into sedimentary rock. the first sediment to fall to the lakebed will be

a) in the rocks that are on the top.

b) swept away by bottom currents.

c) in rocks spread throughout the layers.

-->  d) in the rocks that are on the bottom.

some volcanic rock types are laid down horizontally.

-->  a) true

b) false

if a group of sedimentary rocks is lying diagonally, they

a) were laid down diagonally.

b) were laid down vertically, but were tilted.

-->  c) were laid down horizontally, but were tilted.

d) were laid down randomly, but were tilted.

sedimentary rocks are useful for deciphering the geological history of an area.

-->  a) true

b) false

the grand canyon is a favorite location for geologists because

a) it has a tremendous amount of mineral resources.

-->  b) it exposes the geologic history of the region like a storybook.

c) it is a fabulous site for fossils.

d) all of the above

deformation can

a) tilt a group of horizontal sedimentary layers to another position.

b) produce geologic structures such as folds.

c) cause earthquakes.

-->  d) all of the above

the rocks at the bottom of the grand canyon do not have layers. they are sedimentary rocks that were laid down vertically.

a) true

-->  b) false

fossils found in the top layer of sedimentary rocks are the oldest.

a) true

-->  b) false

diagram questions

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