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fossils and geological time

^ Before evolution was understood, Pioneer geologists realized that fossils of animals such as shellfish were clues to the age of rocks.

They guessed that all shellfish of one type lived at the same time. So all rocks containing them must be the same age.

^ By gathering fossils from rock layers at one site, they identified fossil groups that were typical of each layer.

If they found the same grouping in another rock outcrop far away, this showed that the rock was the same age and even part of the same layer.

This method was used to produce the first accurate geological map, published by English geologist William Smith in 1815.

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Hot plates

•Earth’s crust is divided into huge plates that float on the hot rock below.

•When these plates

push together, one slides beneath the other in a series of jerks that cause earthąuakes and erupting volcanoes.

•Where plates are moving apart, continents split to create new oceans. Molten rock erupting from the rifts forms new ocean floors and ridges.

Sedimentary rocks

Formed from beds of soft sediments such as sand and mud, these rocks harden into fiat, solid layers called strata.

Old at heart

Different types of sediment form strata of various colors, thicknesses, and hardness. They lie in horizontal beds, with the youngest on top.

• Over millions of years— and at a very slow ratę— this has created and destroyed oceans and moved the continents into many different arrangements.

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Buckie up

If strata are squeezed by Earth movements, they buckie and fold. Often, only one side of the fold is seen as sloping or even vertical strata.

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Fault linę

Extreme pressure or tension can snap the strata, so that they slip out of alignment. This is called a fault.

Blasts from the past

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Geology 40 41