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Chapter 10 Geologic Time

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Historical notes


Landscape developed by catastrophes

James Ussher, mid - 1600’s, concluded Earth was only a few thousand years old


Modern geology


Fundamental principle of geology

"The present is the key to the past."

James Hutton

Theory of the Earth

Late 1700’s

Sir Charles Lyell

Advanced modern geology

Principles of Geology



Relative dating

Placing rocks and events in sequence

Principles and rules of

Law of superposition

Principle of oringial horizontality

Principle of cross-cutting relationships




Law of superposition

Oldest rocks are on the bottom


Principle of original horizontality

Sediment is deposited horizontally



One rock contained with in another - rock containing the inclusions is younger



An unconformity is a break in the rock record


Types of unconformities

Angular unconformity - tilted rocks are overlain by fly-lying rocks

disconformity - strata on either are parallel


metamorphic or igneous rocks below

younger sedimentary rocks above


Correlation of rock layers

Matching rocks of similar age in different regions

Often relies upon fossils



Remains or traces of prehistoric life


Types of fossils

Petrified - cavities and pores are filled with precipitated mineral matter

Formed by replacement - cell material is removed and replaced with mineral matter

Mold - shell or other structure is buried and then dissolved by underground water

Cast - hollow space of a mold is filled with mineral matter


More types of fossils

Carbonization - organic matter becomes a thin residue of carbon

Impression - replica of the fossils’ surface preserved in fine-grained sediment

Preservation in amber - hardened resin of ancient trees surrounds an organism

Indirect evidence includes



Coprolites - fossils dung and stomach contents

Gastroliths - stomach stones used to grind food by some extinct reptiles


Conditions favoring preservation

Rapid burial

Possession of hard parts


Fossils and correlation

Principle of fossil succession

Fossils succeed one another in a definite and determinable order

Proposed by William Smith - late 1700’s and early 1800’s

Index fossils

Widespread geographically

Existed for a short range of geologic time