Worksheet

Footprints in the Dry Riverbed: Placing Layers in Deep Time

Grade 8 geologic time practice: use a riverbank's layers, dinosaur tracks and fossil age ranges to date rocks, find missing time and explain era boundaries.

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1 NGSS standard
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11 questions
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Format: Interactive · Printable
Grade: Grade 8
Subject: Science
Question types: Fill in the blank, Multiple choice, Circle the answer, Sorting, Short answer

About this worksheet

What students practice

Students use layer order and fossil age ranges to place rock layers in geologic eras.
Students find a gap in the rock record and explain how era boundaries are set by changes in fossils.
Students compute how long parts of Earth's 4.6-billion-year history lasted.

How to use

Read the passage together and model how to use one row of Table 2 before students start. Part 2 works well as a partner activity; finish with the open response.

Curriculum

NGSS standard covered

MS-ESS1-4

Explain how rock strata and fossils are used to organize Earth's 4.6-billion-year history.

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Footprints in the Dry Riverbed: Placing Layers in Deep Time

Use rock layers, fossil ranges and the geologic time scale to date a riverbank.

Part 1: The riverbank layers

Read the passage, Table 1 and Table 2, then answer questions 1–6.

The Summer the River Dropped

1Last summer was the driest that the wheat-growing town of Corliss had seen in decades. Cattle ponds shrank to mud, and the river that runs past the town dropped so low that a long stretch of its bed dried out. One August evening, eighth grader Beatriz Halvorsen walked the dry riverbed with her uncle, who farms wheat and raises cattle on the land above it. On a flat shelf of gray limestone, she found a line of large three-toed footprints, each longer than her forearm.

2A geologist from the state geological survey, Dr. Winifred Osei-Brandt, came out the next week to measure and photograph the tracks before the river rose again. She explained that footprints are trace fossils. They do not preserve the animal's body, but they record something it did. The tracks show that a large three-toed dinosaur once walked across soft, limy mud here. Oyster shells in the same limestone suggest that the mud lay near the edge of shallow, salty water. Later the mud hardened and was buried under more layers.

3Above the riverbed rises a bluff about 16 m high. Dr. Osei-Brandt described its layers from the top down (Table 1). The layers lie flat, one on top of another, and are not folded, tilted or broken. In a quarry 5 km away, the same track limestone sits on top of a darker limestone that does not crop out at the river, so she added it as the last row of the table. She split open loose chunks of each layer with a rock hammer, wearing safety glasses, and searched them for fossils. In layer C she found ripple marks but no fossils.

4Dr. Osei-Brandt also brought a chart of age ranges for some fossils (Table 2) and the main divisions of the geologic time scale (Table 3). Scientists divide Earth's 4.6-billion-year history into long spans of time. She told Beatriz to look closely at where the eras begin and end, and at which fossils first appear or last appear near those times. In October, heavy rains finally came, and the river covered the tracks again, but the survey's photographs keep a record of them.

Table 1: Layers at the River, Top to Bottom
Layer	Rock	Thickness	Fossils found
A (top)	Sand and gravel	2 m	Mammoth tooth
B	Gray shale	6 m	Ammonite shells
C	Sandstone	8 m	None found
D (riverbed)	Gray limestone	3 m	Dinosaur tracks, oysters
E (quarry only)	Dark limestone	10 m	Trilobites

Table 2: Fossil Age Ranges (Ma = millions of years ago)
Fossil group	First appears	Last appears
Trilobites	521 Ma	252 Ma
Ammonites	409 Ma	66 Ma
Dinosaurs (not birds)	233 Ma	66 Ma
Mammoths	5 Ma	about 0.004 Ma

Use Tables 1 and 2.

Based only on its dinosaur tracks, layer D formed no more than million years ago and no less than million years ago.

Layer C has no fossils. Use Tables 1 and 2. What is the best way to tell when layer C formed?

A. It formed at the same time as layer E, since both lack ammonite shells.
B. It lies on top of layer D and under layer B, so it formed after D and before B.
C. It formed after layer A, because the river cut through it most recently.
D. It formed before layer D, because sandstone takes longer to form than limestone.

Use Tables 1 and 2. Layer B is at least as old as the youngest ammonites, and layer A is no older than the oldest mammoths.

Layer A was laid down at least million years after layer B.

Layer A sits right on top of layer B, yet millions of years separate them. Which explanation fits the bluff best?

A. Layer A sank down through the shale until it rested on top of layer B.
B. The mammoth tooth was carried down from a younger layer that once sat above A.
C. For a long time, layers were worn away here or were not laid down at all.
D. Shale forms so slowly that layer B took about 61 million years to build up.

Use paragraph 2 and Table 1. Which conclusion about this place is best supported by layer D?

A. The tracks were made last summer, when the dry riverbed was still soft mud.
B. The dinosaur that made the tracks lived at the same time as the trilobites in layer E.
C. When layer D formed, a large dinosaur walked on soft mud near shallow, salty water here.
D. When layer D formed, this place was dry wheat land much like it is today.

Part 2: Fitting the layers into the time scale

Use Table 3 together with Tables 1 and 2 to answer questions 6–11.

Table 3: Main Divisions of Earth's History
Division	Began	Ended
Cenozoic Era	66 Ma	Today
Mesozoic Era	252 Ma	66 Ma
Paleozoic Era	539 Ma	252 Ma
Precambrian time	4,600 Ma	539 Ma

Sort each layer into the era in which it formed.

Layer ALayer BLayer CLayer DLayer E

Cenozoic	Mesozoic	Paleozoic

Use Table 3. Round to the nearest whole percent.

Precambrian time covers about % of Earth's 4,600-million-year history.

Use Table 3. Round the last answer to the nearest tenth.

The Mesozoic Era lasted million years. That is about times as long as the Cenozoic Era so far.

Compare Tables 2 and 3. Why does the time scale most likely put the end of the Mesozoic Era at 66 Ma?

A. That is when several groups, like ammonites and dinosaurs, vanish from the rocks.
B. That is when the oldest rocks on Earth formed, so the counting starts there.
C. That is about halfway through Earth's history, so it splits time into two equal parts.
D. That is when mammoths first appear, so a new era begins with them.

10.

Dr. Osei-Brandt wants a fossil that, if found in a layer, would show the layer is Paleozoic. Use Tables 2 and 3. Circle each fossil group whose whole age range lies inside the Paleozoic Era.

TrilobitesAmmonitesDinosaurs (not birds)Mammoths

11.

Beatriz's uncle says, "The mammoth and the dinosaur that made these tracks must have lived at about the same time. Their fossils are in the same bluff, only about 15 m apart." Make a claim about this idea. Use the order of the layers and at least two numbers from Tables 2 and 3 as evidence, and explain your reasoning.