Fossils and Reading Graphic Logs

Reading the record in the rocks

Sedimentary rocks do more than pile up: they trap the remains of living things and record the world they formed in. Reading that record is one of the great skills of geology. This module works through it: - Fossils: what they are and how they form.\n- Reading environments: what different fossils tell you about the past.\n- Fossil groups: recognising the main kinds and index fossils.\n- Graphic logs: summarising a whole rock sequence as a column. The sedimentary rocks themselves, and the full dating of rocks, are covered in other modules. Here the focus is fossils and graphic logs.

Words for fossils and logs

Five terms you need before you use them. Learn what each one means.

Match each fossil clue to what it suggests

A body fossil against a trace fossil

Not every fossil is a piece of the creature. Some record what a creature did rather than what it was, and telling the two apart is a real skill.

What do reef corals tell you?

A rock contains fossil reef corals. What does this suggest about the environment when the rock formed?

• A warm, clear, shallow sea
• A cold, deep ocean floor
• A dry desert on land
• A fast mountain river

Tap the two trace fossils

Tap the TWO fossils that are trace fossils rather than body fossils.

• A fossilised burrow
• A fossilised footprint
• A fossil ammonite shell
• A fossil leaf

How a fossil forms

Most living things rot away and leave nothing behind. Becoming a fossil takes a lucky chain of events. Usually only the hard parts, such as shells and bones, survive, because soft flesh decays fast. Rapid burial in sediment protects the remains from scavengers and decay. Over long ages, minerals slowly fill or replace the buried material, turning it to stone. Sometimes no part of the creature is kept at all, yet its activity is: a burrow, a footprint or a feeding mark left in soft sediment can harden and survive as a record of a living thing at work.

Pick the true facts about fossils

Select the TWO statements that are true.

• Index fossils help match rocks of the same age in different places
• Trace fossils record the activity of an organism
• Only the soft body parts are usually preserved
• A graphic log records the price of a rock

Order how a graphic log is built

Put the steps of constructing a graphic log in a sensible order.

• Measure the thickness of each bed in the sequence
• Note the grain size of each bed
• Record any sedimentary structures and fossils
• Draw the beds as a column to scale
• Read the column to work out how the environment changed

Reading a cliff, bed by bed

Follow how a geologist read a coastal cliff from bottom to top to work out how the sea there changed. The lowest beds: coarse grains and rounded shells showed a shallow sea with moving water near the shore. Rising through the cliff: the grains grew finer and the remains changed to delicate forms, pointing to deeper, calmer water further from land. A telltale species: one widespread but short-lived creature appeared in a single thin bed, letting the geologist match this cliff to rocks of the same age far away. The story in a column: by drawing each bed to scale as a narrow column, with its grain size and remains beside it, the whole history could be read at a glance. The rock itself had recorded a rising, deepening sea, written from the bottom up.

Complete the fossil facts

The preserved remains or marks of a once-living organism is a _____. A fossil of activity such as a burrow is a _____ fossil. A widespread, short-lived fossil used to correlate rocks is an _____ fossil. A column summarising a rock sequence by bed thickness and grain size is a graphic _____.

Work out the deposition rate

A graphic log shows three beds with thicknesses of 12, 8 and 10 metres. The whole sequence was laid down over 5 million years. First add the bed thicknesses, then divide by the time to find the average deposition rate in metres per million years.

Build a fossils point

Choose the words that complete this statement about fossils.

Interpret the rock

Read each situation and choose the best interpretation, then think about why.

• A rock holds fossils of reef corals. What environment does this suggest?
• You find the same short-lived fossil species in two rock layers far apart. What can you conclude?
• You want to summarise a thick rock sequence clearly for a report. What do you draw?

Explain how fossils and graphic logs are used

A classmate missed these lessons. Explain how fossils and graphic logs are used using what this module has covered.

• Explain what a fossil is and how one usually forms
• Explain the difference between a body fossil and a trace fossil
• Explain how fossils can indicate the past environment
• Explain what an index fossil is used for
• Finish by explaining how a graphic log summarises a rock sequence