The Hell Creek Rocks of the Dakotas, 

Montana, and Wyoming

Learn About:

The Ocean That Once Split North America 

in Two ∙ What Life Was Like When 

Dinosaurs Lived ∙ What a Dinosaur 

Excavation Is Really Like

Read About:

The Age of Dinosaurs ∙ 

Valuable Clues Hidden in the 

Rocks ∙ The Asteroid That 

Changed the World

But Wait! 

There’s More: 

Puzzles ∙ Coloring

Pages ∙ Activities

Forest Service

U.S. DEPARTMENT OF AGRICULTURE

FS-1224 | July 2023

 THE DINOSAURS LAST OF
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1

The Hell Creek Formation and 

the End of the Age of Dinosaurs

Imagine a peaceful low-lying plain near a vast inland 

ocean. A pair of duck-billed hadrosaurs are tending to 

their nests as a lazy river meanders by. The climate is 

warm and humid. The vegetation is tropical and lush. 

Suddenly and without warning the bright flash of an 

immense meteor streaks across the sky! 

The meteor is a giant asteroid that survives its fiery 

journey through the Earth’s atmosphere and slams 

into the Caribbean Sea. The massive impact creates 

a mushroom cloud of dust and debris that spreads 

through the atmosphere and covers the Earth. The dust 

cloud darkens the sky for years, blocking sunlight and 

creating winter-like conditions. Plants die from the 

cold and lack of sunlight. Herbivores that rely on plants 

for food begin to starve in great numbers, followed by 

carnivores that rely on eating herbivores. The food chain 

collapses, and 75 percent of life on Earth goes extinct. 

Some birds and small mammals are among the lucky 

survivors, but gone are the giant dinosaurs. These 

events changed the world forever, and the rocks of the 

Hell Creek Formation record this catastrophe.

As you read this activity book and work the puzzles, you 

will learn about the Hell Creek Formation, the incredible 

fossils in its rocks, and the story they tell about the end 

of the Mesozoic Era: The Age of Dinosaurs.
2

The Age of the Earth as the Distance

Around a Track

Think of the age of the Earth as the distance around 

a stadium track with the outside lane approximately 

450 meters long (1,476 feet). The age of the Earth is 4.5 

billion years. The start/finish line marks the beginning 

of the track as you run counterclockwise around the 

entire distance of the track, starting at the origin of the 

Earth 4.5 billion years ago and ending at the present 

day. Every marker ( ) represents 250 million years 

(250,000,000 years). Major events in the history of the 

Earth and the development of life are shown as you 

proceed around the track.

The ages of the geologic eras (Paleozoic, Mesozoic, and 

Cenozoic) are shown as shaded portions near the end of 

the race. The Hell Creek Formation contains remarkably 

diverse fossilized life forms at the close of the Mesozoic 

Era, the Age of Dinosaurs. The Mesozoic Era lasted for 

165 million years, from 230 million years ago to 65 

million years ago when the dinosaurs became extinct 

and the Cenozoic Era, the Age of Mammals, began. 

Humans appeared on this track just inches from the 

finish line.

FORMATION OF EARTH

4.5 BILLION YEARS

FIRST HUMANS

2.3 MILLION YEARS

FIRST MAMMALS

210 MILLION YEARS

FIRST DINOSAURS

230 MILLION YEARS

FIRST LAND PLANTS

450 MILLION YEARS

FIRST MARINE ANIMALS

WITH HARD BODY PARTS

575 MILLION YEARS

MULTI-CELLED ORGANISMS

1.1 BILLION YEARS

COMPLEX CELL STRUCTURE

2.1 BILLION YEARS

EARLIEST FORMS OF LIFE

BACTERIA, STROMATOLITES (DOME-LIKE 

LAYERED ALGAL STRUCTURES)

3.5 BILLION YEARS MESOZOIC ERA

LASTED FOR

165 MILLION YEARS

START / FINISH

4.25 BILLION 4.0 BILLION

2.0 BILLION

1.25 BILLION

1.5 BILLION

2.25 BILLION 1.75 BILLION 2.5 BILLION

1.0 BILLION

3.0 BILLION

750 MILLION

2.75 BILLION

500 MILLION

3.25 BILLION

4.5 BILLION / PRESENT

3.5 BILLION

3.75 BILLION

250

MILLION

DEPOSITION OF THE HELL CREEK

 FORMATION

CENOZOIC ERA

65 MILLION YEARS

TO THE PRESENT

PALEOZOIC ERA

LASTED FOR

370 MILLION YEARS

Number Crunch

If a 450-meter-long track represents the 4.5-billion-year-old Earth, 

how many years does 1 meter of the track represent? How many 

years is 1 centimeter? Or 1 millimeter? (Hint: There are 100 

centimeters in a meter and 1,000 millimeters in a meter.)
3

The Phanerozoic Eon:

Fossils Reveal the History of Life

The Phanerozoic Eon is represented by the last 60 

meters (200 feet) of the 450-meter (1,475-foot) track. 

Geologists divide it into three eras which are further 

divided into periods as shown in the table below. We 

know more about the Phanerozoic Eon than any other 

time in Earth’s history. Why? Because of fossils.

Life with hard parts—in the form of shells or outer 

skeletons—first appears at the beginning of the 

Phanerozoic. The hard parts of animals are most likely 

to be preserved as fossils in rock layers. Geologists study 

the rock layers and the fossils in them to learn about the 

history of the Earth. 

Geologists know that limestone is deposited on an 

ocean floor, sand collects on a beach or desert, coal is 

formed in swamps, and ash and lava are ejected from 

volcanoes. Younger sediments are deposited on top of 

older sediment layers and if they are undisturbed, they 

stay this way as they harden into rock layers. 

Geologists interpret the history of the Earth from this 

and other information. Let’s now go on an adventure 

to learn the geologic and fossil history of the areas that 

are present-day national forests and grasslands in North 

Dakota, South Dakota, Montana, and Wyoming.

HOLOCENE PRESENT TO 10,000

PLEISTOCENE 10,000 TO 2 M.Y.

PLIOCENE 2 M.Y. TO 6 M.Y.

MIOCENE 6 M.Y. TO 22.5 M.Y.

OLIGOCENE 22.5 M.Y. TO 36 M.Y.

EOCENE 36 M.Y. TO 58 M.Y.

PALEOCENE 58 M.Y. TO 65 M.Y.

CRETACEOUS K 65 M.Y. TO 141 M.Y.

JURASSIC J 141 M.Y. TO 195 M.Y.

TRIASSIC T 195 M.Y. TO 230 M.Y.

PERMIAN P 230 M.Y. TO 280 M.Y.

PENNSYLVANIAN 280 M.Y. TO 310 M.Y.

MISSISSIPPIAN M 310 M.Y. TO 345 M.Y.

DEVONIAN D 345 M.Y. TO 395 M.Y.

SILURIAN S 395 M.Y. TO 435 M.Y.

ORDOVICIAN O 435 M.Y. TO 500 M.Y.

CAMBRIAN C 500 M.Y. TO 600 M.Y.

CENOZOIC

(MEANS NEW LIFE)

MES

OZOIC

(MEANS MIDDLE LIFE)

PALE

OZOIC

(MEANS ANCIENT LIFE)

PRECAMBRIAN p C LIFE EXPLODES IN THE SEAS AGE OF MAMMALS AGE OF DINOSAURS TERRESTRIAL LIFE FISH

 ERA PERIOD EPOCH SYMBOL APPROXIMATE AGE GEOLOGIC EVENTS IN THE DAKOTA PRAIRIES

P

THE “RECENT;” HUMANS ARRIVE IN N. AMERICA

THE ICE AGE, GLACIERS CROSS NORTH 

AMERICA AND DEPOSIT SEDIMENTS 

ON THE PLAINS

VOLCANIC ERUPTIONS TO THE WEST

EXTENSIVE DEPOSITION ON DAKOTA PRAIRIES 

FROM VOLCANIC ACTIVITY AND EROSION OF 

MOUNTAINS TO THE WEST

REGIONAL UPLIFT, MOUNTAIN BUILDING TO THE 

WEST, DEPOSITION TO THE EAST

VOLCANIC ACTIVITY TO THE WEST, DEPOSITION 

OF ASH ACROSS THE PLAINS

LARAMIDE OROGENY: EPISODE OF MOUNTAIN 

BUILDING THAT CREATED THE PRESENT-DAY 

STRUCTURE OF THE ROCKY MOUNTAINS

COASTAL AND MARINE DEPOSITION FROM A 

SHALLOW INLAND SEA AND DEPOSITION OF THE 

HELL CREEK FORMATION

ERODED LAND SURFACE

MOUNTAIN BUILDING TO THE WEST, UPLIFTED

AND ERODED LAND SURFACE TO THE EAST

EROSION OF LAND SURFACE TO THE WEST WITH 

DRY ARID CONDITIONS IN THE DAKOTAS

SHALLOW SEA, SWAMPS, AND SHORELINE

OUACHITA OROGENY: EPISODE OF MOUNTAIN 

BUILDING TO THE SOUTH

LOW ELEVATION LAND THEN SHALLOW SEA 

LOW LAND SURFACE AND EROSION

MARINE DEPOSITION IN A SHALLOW SEA

ERODED LAND SURFACE THEN MARINE 

DEPOSITION

GRANITE “BASEMENT” ROCK

R

PALEOGENE NEOGENE

QUATERNARY

Bold words are defined in the glossary on page 15.

M.Y. = million years
4

The Cretaceous Period: 

When Dinosaurs Lived Next to an Inland Sea!

Welcome to the Cretaceous! The environments of Montana, North 

Dakota, South Dakota, and Wyoming were like present-day Florida 

during this time. The climate was humid and subtropical forests, 

swamps, and rivers covered the land. The landscape teemed with 

crocodiles, insects, fish, flowering plants, small mammals, and 

dinosaurs! Dinosaurs dominated land habitats at the end of the 

Cretaceous, like the large mammals of today.

An ocean known as the Western Interior Sea covered what is now 

the central United States. Throughout the Cretaceous Period the sea 

spread and shrank in cycles, depositing marine mud layers as the 

seas spread over areas that were formerly dry land. These marine mud 

layers buried and preserved the remains of marine animals as well as 

the remains of terrestrial plants and animals that lived near the coast 

when the sea rose and spread over the land. Rivers snaked across the 

landscape leading to the shore, building up thick deposits of terrestrial 

mud and sand over areas inland of the sea deposits.

Distant volcanoes would sometimes erupt and winds would blanket 

the land and the sea with volcanic ash. Animals that breathed or 

swallowed the volcanic ash floating in the air and water suffered 

damage to lungs and gills resulting in periodic deaths of entire animal communities. Plant and animal communities 

would recover following volcanic ash-falls, and this cycle repeated itself many times during the Cretaceous Period. 

The combination of river and floodplain sediments, nearshore sea sediments, and volcanic ashfalls are grouped 

together by geologists in a rock unit named the Hell Creek Formation in North and South Dakota, Wyoming, and 

Montana. Remains of plants and animals that were buried quickly by river or marine sediments became fossilized and 

left behind a record of past life in the Hell Creek Formation. Volcanic ash layers are important because radioactive 

minerals in the ash can be dated by lab tests, which allow scientists to determine the age of the rocks and fossils in the 

Hell Creek Formation.

Cretaceous Critter Puzzle 

Draw a line from the name to the drawing of the Cretaceous fossils

found in the Hell Creek Formation. Read page 5 for hints.

Ankylosaurus 

Edmontosaurus 

Pachycephalosaurus

Pteranodon 

Triceratops 

Tyrannosaurus

Answers on back page. 

Map Source:USGS public domain
5

Dinosaurs of the Hell Creek Formation

The Hell Creek Formation contains fossils of 

many of the most famous dinosaurs known. 

Duck-billed hadrosaurs such as Edmontosaurus

roamed in great herds. Hundreds of hadrosaur 

nest sites clustered together with abundant 

eggshell fragments tell us that hadrosaurs were 

social creatures that nested together like some 

modern birds. 

Many kinds of horned and frilled ceratopsian 

dinosaurs are also present, including the wellknown Triceratops. Horns served as highly 

effective defensive weapons against large 

predators. Horns and frills may also have played 

important roles in recognizing herd members as 

well as competition among rivals, like horns and 

antlers do in many living large mammals.

Pachycephalosaurs (meaning “thick-headed 

reptile”) are unique among dinosaurs because of 

their thickened, dome-like skulls that are often surrounded by large bumps or spikes. Pachycephalosaurs are relatively 

small (2–3 meters long) herbivorous or omnivorous dinosaurs with short, thick necks on thick bodies that walked on 

their hind legs. Did pachycephalosaurs use their thick skull roof domes in head-butting behavior like modern bighorn 

sheep? Many paleontologists think so, and studies of fossilized injuries to pachycephalosaur skull bones support 

this theory.

Armored dinosaurs like Ankylosaurus are among the most ominous looking creatures from the Hell Creek Formation. 

Ankylosaurus was a fearsome adversary for any would-be predator, with a head and torso armored with bony plates 

beneath the skin, massive spikes along its sides, and a large bony club on the end of its tail. The small, flat teeth of 

these dinosaurs indicate an herbivorous diet.

The Hell Creek Formation is also the burial place of the most 

famous dinosaur ever: Tyrannosaurus rex! Tyrannosaurus was 

the top carnivore on the Cretaceous landscape, and some 

scientists think females were larger than males like many of 

their living relatives, the birds. Big and mean, with the fiercest 

being the tyrant queens! Tyrannosaurus had a huge skull with 

massive 6-inch-long teeth and an immense bone-crunching 

bite force of 8,000–12,000 pounds per square inch! 

Remarkably small forelimbs, along with other curious 

features, wrapped into a 6-ton, 40-foot-long package raise 

many unanswered questions about Tyrannosaurus. Was T. 

rex the only large predator in the late Cretaceous? Were they 

social animals that hunted in packs? Were they hunters at all, 

or were they just scavengers? Could T. rex truly run, or was 

the animal so heavy that it was limited to fast walking? Much 

of the dinosaur research by paleontologists today tries to 

answer these and other questions about fossils buried in the 

Hell Creek Formation.

Did You Know?

Scientists sometimes use a special 

type of name for plants and animals, 

called a binomial. These names have 

two terms (notice the "bi" prefix) and are 

always written in italic (slanted) font. The 

first name is the genus and the second 

is the species, which are used to group 

organisms by their common traits. How 

many genera (plural of “genus”) are 

mentioned on this page?

Look up a binomial for a common 

plant or animal that you know!
6

Not Just Dinosaurs: Reptiles, Amphibians, 

Fish, and Insects Lived There, Too!

Some very remarkable animals lived alongside 

and beneath the feet of dinosaurs during the late 

Cretaceous, including the ancestors of living alligators. 

The Hell Creek Formation is famous for skeletons of 

large dinosaurs, but it is also rich with deposits that 

have tiny bones, teeth, and scales. These deposits are 

called vertebrate microsites, and are places where 

rivers concentrated very small fossils on sand bars. 

Microsites may have fossil bones of lizards, snakes, 

turtles, frogs, and salamanders, as well as early birds 

and small dinosaurs. Fish fossils are very common in 

microsites, especially the thick, shiny scales of gar fish. 

Bony plates from turtle shells are also common, and 

scientists have found small teeth of early mammals. 

Most mammals in the late Cretaceous were smaller than 

rabbits but were abundant in the Hell Creek landscape. 

Different species lived in many habitats and adapted 

to many different lifestyles, from ground-burrowing to 

tree-dwelling. 

Rare amber (fossilized tree sap) in the Hell Creek 

Formation may contain insects, including flies, 

mosquitos, dragonflies, and early bees and butterflies. 

Tree-living animals such as baby birds and small 

lizards have been discovered in amber deposits 

from other locations.
7

Plants of the Cretaceous

A rule that paleontologists use is that “the present is the key to the past.” 

Paleontologists use present-day examples of geological and biological processes 

to interpret the fossil evidence they study. Today, much of the world depends on 

flowering plants. So, the appearance of the first flowering plants (angiosperms) in 

the fossil record was a very important event in the Cretaceous Period. 

The repeated flooding and volcanic events preserved a detailed fossil record of plant 

fossils and their spread in the Hell Creek environment. Fruits, cones, pollen, amber, 

and seeds from plants are known from fossils in the Hell Creek Formation. Because 

plants are rooted and were fossilized where they grew, plant fossils are excellent 

indicators of local habitat and climate. Some fossil leaves are so well preserved that 

insect chew marks can be used to identify the ancient creatures that fed on them.

Fossil plants show that the climate from 100 to 65 million years ago in the late Cretaceous world was much warmer 

and wetter than it is today. Tropical plants and ferns are found in Cretaceous sediments all over the globe, even in 

places that today are much colder and frozen year-round (permafrost). 

Activity: Make your own fossil!

A fossil is evidence of past life preserved in sediments and rocks. You can 

make a fossil by taking some mud, rolling it flat, and laying a leaf on top. 

Carefully press the leaf into the soft mud. Allow the mud and leaf to dry 

(mud will dry within days, and then the leaf can be peeled away). If you 

leave the leaf on the dried mud, it will decay and disintegrate after a while, 

but the leaf impression will remain. You now have a fossil leaf impression! 

Try the same process in sand or sandy gravel. Can you make a leaf 

impression? It doesn’t work as well because the sediment grains in sand 

and gravel are too large for a leaf impression to be made. The very finegrained sediments of the Hell Creek Formation were perfect for preserving 

details of plant leaves and other fossils.

Sequoia Ficus Barberry Magnolia

Palm Ginkgo Tulip Tree Buckthorn

Plane Tree
8

The Western Interior Sea Swarmed With Life!

The Pierre Shale is a marine rock unit. It formed at the 

same time as the Hell Creek Formation, but the Pierre 

Shale was in a shallow sea. Pierre Shale fossils show that 

marine life during the Cretaceous compared to marine 

life today was as different as the terrestrial lifeforms of 

then and now. Some sharks, rays, and bony fish were 

like their living relatives, but the ocean also swarmed 

with ammonites, which were octopus-like animals living 

inside coiled shells. Giant sea turtles like the car-sized 

Archelon cruised the depths. 

Scientists have not yet discovered any dinosaurs that 

lived in oceans, so the dominant predators of the sea 

were not dinosaurs. The largest and fiercest predators 

in Cretaceous oceans were the mosasaurs. Mosasaurs 

were huge (up to 30 feet long) ocean-dwelling lizards. 

Mosasaurs developed long, powerful tails to propel 

them through the water, and their limbs had 

evolved into flippers to quickly change direction 

while swimming.

Other remarkable reptiles found unexpectedly in sea 

deposits are the giant-winged pterosaurs (meaning 

“winged reptile”) like Pteranodon. Some pterosaurs had 

wingspans up to 24 feet or more. They glided over the 

ocean waves feeding on schools of fish or other marine 

animals, like modern pelicans. They lived along the 

shorelines, and sometimes after death their bodies were 

swept out to sea and sank to the bottom where they 

were buried and preserved. 
9

The Big Extinction:

Ending the Cretaceous With a Bang! 

Iridium is a very hard, brittle, silver-white metallic 

element. It is one of the rarest elements in the Earth’s 

crust, but it is common in rocks from space (asteroids). 

It is usually found on the Earth in meteorites. In 1980, 

scientists discovered a rock layer rich in iridium that 

formed at the end of the Cretaceous. They suggested 

that the layer was created by a large asteroid crash on 

Earth (scientists call this crash an “impact”). 

Since that time, other geologists have found an iridiumrich layer separating Cretaceous rocks from Cenozoic 

layers at many locations around the world (see the table 

on page 3 for the order of geologic time). The layer also 

contains glassy particles and microscopic diamonds 

that form only under conditions of high temperature 

and great pressure; a large asteroid impact on Earth can 

create those conditions. 

There is a giant crater beneath the seafloor of the 

Caribbean Sea. Evidence shows that a large asteroid 

impacted near the present-day Yucatan Peninsula 

near Chicxulub, Mexico. The Chicxulub asteroid is 

estimated to have been 6 to 9 miles wide! The asteroid 

crash left a crater 93 miles wide, which makes it one of 

the largest known impact structures on Earth! 

The impact ejected pulverized rock particles and dust 

containing iridium into the atmosphere. The cloud of 

impact dust surrounded the Earth for several years, 

blocking sunlight and causing global temperatures 

to cool during what some geologists call a “long dark 

winter.” This caused nearly 75 percent of animal and 

plant species to go extinct at the end of the Cretaceous 

Period, including the dinosaurs—although their 

descendants survived as birds!

Because of the drastic changes in life on Earth from 

this event, this iridium layer is commonly referred to 

as the “K-Pg” boundary. The symbol “K” represents the 

Cretaceous Period and the symbol “Pg” represents the 

Paleogene Period (see the table on page 3). This is the 

boundary line between when dinosaurs stopped being 

the dominant life forms and when mammals started 

becoming the dominant life forms on Earth.

Hell Creek Formation Outcrop

National Forest/Grassland
10

Life Goes on After the Asteroid: 

The Cenozoic Era and the “Age of Mammals”

The asteroid impact at the end of the Cretaceous 

caused a global extinction that changed the world. The 

landscape started to recover as the atmosphere began 

to clear following the “long dark winter” and plants 

could grow again. Ferns gradually became dominant 

plants that flourished in the carbon dioxide-rich 

atmosphere, like growing in a greenhouse (geologists 

call warm climates caused by high carbon dioxide 

“greenhouse climates”). 

Many groups of sharks and other fish survived 

extinction, as did smaller cold-blooded animals such 

as turtles, snakes, lizards, and crocodilians. All warmblooded dinosaurs except birds went completely 

extinct during the “long winter,” as did many groups 

of large reptiles such as the marine mosasaurs and 

flying pterosaurs. Some scientists suggest that animals 

who ate seeds, like small mammals and birds, avoided 

extinction and survived. 

The surviving mammals and birds rapidly evolved and 

diversified to fill the vacant habitats left by the extinct 

dinosaurs and marine reptiles. Paleontologists call such 

bursts of rapid evolution an “adaptive radiation.” The 

early Cenozoic Era is referred to as the “Dawn of the Age 

of Mammals” because mammals started to become the 

dominant life on Earth during that time. Following the 

mass extinction caused by the asteroid impact at the 

end of the Cretaceous, mammals evolved from small 

rodent-like creatures into animals like elephants, cats, 

and rhinos on the land, and seals, dolphins, and whales 

in the oceans.
11

The Incredibly Fun Science of

Dinosaur Fossil Excavations!

An animal or plant begins to fossilize after it dies and 

is covered by sediment. Body parts like bones and 

teeth are porous (having many small openings) and 

act like sponges when mineral-rich water in the soil 

and sediment soaks through them and leaves minerals 

behind. Over time, the openings in the bone become 

filled with minerals that typically make the bone harder, 

heavier, and different colors.

The scientific gathering of information begins once 

someone finds a fossil. Paleontologists examine the 

ground for more fossil pieces, which they bag and label. 

These scientists spend a lot of time writing careful notes. 

Valuable information about fossil excavations comes 

from writing down observations and collecting samples 

of the rock layers that fossils are found in. Rock samples 

provide clues about the environment the bones were 

buried in. Some rocks can be dated if they contain 

radioactive minerals, telling scientists exactly how old 

the rocks and bones are. 

Paleontologists draw maps and sketches showing how 

bones in a quarry are arranged. Sometimes bones 

can be part of the skeleton from a single animal, and 

sometimes many skeletons of animals are buried 

together by large floods, sandstorms, and other natural 

catastrophes. Modern scientific equipment allows 

paleontologists to record information about fossils in 

new and more precise ways than in the past. 

Complete fossil skeletons are rare, and the reasons why 

are often provided in clues from the excavation site. 

Predators and scavengers eating prey and scattering 

bones prior to burial result in broken skeletons with 

missing parts and may leave behind bite marks on bone 

or broken teeth.

If the fossil is a large bone or complete skeleton, 

workers dig around the fossil using small hand tools 

and brushes, leaving the bone safely supported on top 

of a pedestal of rock. Wet tissue paper is put onto any 

exposed fossil bone and is pressed gently onto the bone 

surface with a paint brush. Scientists mix plaster until 

it is as thick as pancake batter, and strips of burlap (a 

very rough cloth) are soaked in the plaster. Next, they 

tightly wrap the plaster-soaked burlap strips around the 

fossil and its pedestal of supporting rock, creating what 

paleontologists call a “field jacket.” A field jacket is like a 

hard cast around a person’s broken bone, and like a cast 

it protects the fossil bone inside. 

Once the plaster is dry, workers carefully break the 

pedestal away from the rock beneath, and the field 

jacket is flipped over so the bottom can be covered in 

plaster-soaked burlap to complete the field jacket. The 

completed field jacket looks like a hard cocoon that 

protects the fossil inside when it is moved to a museum.
12

Tools of the Trade

You just learned about some of the tools that paleontologists use. Now it’s time to test your knowledge. In the space 

below each tool, take your best guess of what each tool is used for on the excavation site.

Plaster:

Brush:

Whisk broom:

Chisel:

Photo scale:

Dental pick:

Trowel:

Tissue paper:

Picks and shovels:

Plaster Brush Whisk Broom Chisel Photo scale

Tissue paper Picks and shovels

Trowel

Dental Pick
13

Is This Really a Fossil?

Fossils are any evidence of past life—both plants and 

animals—and are usually preserved in rock. Common 

fossils include leaf imprints and shells, such as clams, 

ammonites, and corals. All of these are the preserved 

hard parts of ancient organisms. However, there are also 

rocks that look like fossils but are not. 

Can you guess which of the following things are true 

fossils and which are just rocks? Circle either TRUE or 

FALSE for each picture. Check your answers on the back 

page and learn about unique fossil evidence of former 

life.

Gastrolith

TRUE | FALSE

Ripple marks

TRUE | FALSE

Shaped rocks

TRUE | FALSE

Dendrite

TRUE | FALSE

Coprolite

TRUE | FALSE

Tracks

TRUE | FALSE

Tube shapes in rock

TRUE | FALSE

Small shallow holes

TRUE | FALSE
14

The Secrets Are in the Rock Layers

In this booklet, you learned about sediment 

deposition, volcanic events, and the spread and 

retreat of shallow seas across the landscape. 

Paleontologists know these events happened 

because of the clues left behind in the geologic record. 

Sediments deposit in a predictable way, like building a 

cake: the first layer is on the bottom and is older than 

the layers above. Over time, younger top layers squeeze 

the sediments below, and the pressure slowly turns 

the sediments to rock. Geologists call this lithification. 

Geologists use the word stratification to describe 

stacked layers of rock, one upon the other. Each layer 

is a stratum. A graphic of multiple stratum is called a 

stratigraphic column, and it can be used to describe the 

geologic history of an area. 

This illustration shows a stratigraphic column of rock 

units from just below and above the iridium-rich layer 

at the end of the Cretaceous, including the Hell Creek 

Formation. Use the explanation below the figure, called 

the legend, to find out how each stratum was deposited. 

The answers come from studying the sediments that 

make up each stratum.

IRIDIUM LAYER

CRETACEOUS/PALEOGENE BOUNDARY

(K/Pg BOUNDARY)

END OF THE AGE

OF DINOSAURS

BEGINNING OF THE 

AGE OF MAMMALS

FORT UNION

FORMATION

NIOBRARA

FORMATION

CARLILE

SHALE

PIERRE

SHALE

HELL CREEK

FORMATION

FOX HILLS

SANDSTONE

LEGEND

COAL (SWAMPS)

LIMESTONE (MARINE)

SHALE (MARINE)

MUDSTONE (MARSHES)

SILTSTONE (SLOW STREAMS)

SANDSTONE (STREAMS AND BEACHES)

C R E T A C E O U S

WASATCH

FORMATION

CENOZOIC

P A L E O G E N E

MESOZOIC
15

Glossary

deposition (de-pə-zi-shən): an act or process of laying something down

element (e-lə-mənt): any of the fundamental substances that consist of atoms of only one kind and that singly or in 

combination constitute all matter

geologist (jē-ä-lə-jist): a scientist who studies geology, the science that deals with the history of the Earth and its life 

especially as recorded in rocks

sediment (se-də-mənt): the matter that settles to the bottom of a liquid; material deposited by water, wind, or glaciers

marine (mə-rēn): of or relating to the sea

meteorite (mē-tē-ə-rīt): a meteor that reaches the surface of the Earth without being completely vaporized

paleontologist (pā-lē-än-tä-lə-jist): a scientist who studies paleontology, the science dealing with the life of past 

geologic periods as known from fossil remains

porous (pȯr-əs): possessing or full of pores; capable of absorbing liquids

terrestrial (tə-re-st(r)ē-əl): of or relating to land as distinct from air or water

vacant (vā-kənt): being without content or occupant; not lived in

vertebrate (vər-tə-brət): of or relating to animals with a spinal column

Accented syllables are in bold. Marks and definitions are from the Merriam-Webster dictionary and are limited to the 

word’s meaning in the booklet.

What Is the Forest Service?

The Forest Service is part of the U.S. Department of Agriculture (USDA). The 

Forest Service manages 154 national forests and 20 national grasslands. These 

are large areas of land with trees, streams, and prairies. National forests are 

similar in some ways to national parks. Both are public lands, meaning they 

are owned by the public and managed for the public’s use and benefit. Both 

national forests and national parks provide clean water, homes for the animals 

that live in the wild, and places for people to do fun things in the outdoors. 

National forests also provide resources for people to use, such as trees for 

lumber, minerals for batteries and phones, and plants used for medicines. 

These public lands also protect the geology and fossils that you learned about 

in this booklet. Some people in the Forest Service are scientists who look for 

and help us learn about fossils. Forest Service scientists work to protect and 

preserve these amazing resources!
16

Activity: Race you to the bottom!

Who wins the particle size race? The Hell Creek Formation is made up of sediments with very tiny grains (much 

smaller than sand grains on a beach) that were deposited by very slow-moving streams in the environment. Faster 

flowing waters deposit larger sediment grains like sand, gravel, and pebbles. What does this mean? Here is an activity 

to learn! 

Take a large jar with a tight-fitting lid (a quart-size canning jar works well, but any large clear jar will work). Go outside 

and dig up some sediments. Try to get different sizes including some small pebbles, some sand, and some clayey goo. 

Fill your jar about half full of this mixture and add water, almost to the top. Stir to mix all the sediments together. 

Screw the top on tight and gently shake your sediment mixture, being careful not to drop the jar. Set the jar on a flat 

surface and watch as the sediments settle out. Which size falls to the bottom first? Leave the jar alone for several days 

until all the sediments have sunk through the water to the bottom of the jar.

What do you see and what does this mean? When you shake the jar, you create a “high energy” environment 

in which the small pebbles are mixed in with the sand and smaller-grained material. Setting the jar down creates 

a “low energy” environment. The pebbles and sand-size particles soon settle at the bottom. The smallest particles 

like the clay take a very long time to settle after the jar is left undisturbed. Think about lazy streams, marshes, or 

swampy areas where water flows very slowly, and you have the low energy environments where very fine particles are 

deposited. These are the types of environments where the sediments of the Hell Creek Formation were deposited!
17

Word Search

Try to find the following words in the grid below. Answers are in the back of the book.

FORMATION 

CRETACEOUS 

SWAMP 

DINOSAUR 

MAMMAL 

CROCODILE 

CLIMATE 

FOSSIL 

IRIDIUM 

EXTINCTION 

ASTEROID 

FERNS 

DEPOSITION 

VOLCANIC 

SEDIMENTS 

BONE 

TEETH 

EXCAVATE 

TRICERATOPS 

PEDESTAL 

TYRANNOSAURUS 

MAGNOLIA 

PLASTER 

TURTLE 

ASH 

COPROLITE 

RAY 

PLANTS 

PALM 

GINKGO 

SEAWAY 

AMPHIBIAN 

REPTILE

N J D R E E A W P T Y R A N N O S A U R U S

C M K U S L R U X R R S Y D B G Y V X B C U

N C K A V I C B W P L K F I E K E H C E Z O

L A T S E D E P O X L P G X J N S V L I K E

E Z Y O T O E E M L A P X Q O I D K I P A C

L M H N U C B P A R A Y R B I G M E M A R A

I A E I R O N U M M U I D I R I A I A E Z T

T G X D T R W O L A X V K K P W M A T Z M E

P N C Q L C A S I F W V O T X J M S E C T R

E O A V E S L B V T N S W L F F A T S F V C

R L V G R D Z B I C C B I O C L L E Y O S K

S I A A M P H I B I A N R P P A A R A S Q C

T A T B U X L X X X M M I F L W N O C S X V

N W E P M R Y D C J A C V T A A Y I B I D G

E I M F R T O W U T Z K J Y X K N D C L X F

M L Y G O E Y T I L Q Y Z Q I E Z T T Y Q E

I H U I D E P O S I T I O N I A J K S Y P R

D Y O W E T N R C O P R O L I T E K X Q P N

E X A S H H P O W L I Q W O R W W I G A Y S

S I F D Q D Y S P O T A R E C I R T O J R J
18

Scrambled Words

The creatures shown below are all fossils in the Hell Creek Formation, but their names are scrambled. Try to 

unscramble each name and then connect the name to the drawing. As a hint, there is a drawing of each creature. 

Answers are in the back of the book.

1. GROF 

2. AAEANLMRSD 

3. SLSAARNOKUYU 

4. LOOCCDRIE 

5. RSTUUONSRYAAN 

6. KHRAS 

7. RTRTEAOCSIP 

8. AOARRHSUD 

9. EUTTLR 

10. ORPDOTENAN 
19

Coded Messages

Try to decipher the secret messages below. Each letter in the alphabet represents another letter. Write your answer 

below the code. A clue for each coded message is given at the bottom of the page. 

Answers are in the back of the book.

Message 1:

L T M T M N N O I M M W Q R I G S L U R E U A E S G M J Q R I U L A 

RCLOIRB SNREV TMNN OIMMW UE GRELSES

___ ____ _____ 

_________ __ _____ 

___ ___ _______ _____ 

____ _____ __ ________

Message 2:

K Q O C M Z Z T X Z T B K Q O Q O X X N F O O D C M F R E K T M B 

J F M P T L O T B Z T S Q K T B K M X T C O T B K Q O X E K O 

NFOKENOMAZ

___ _______ __ ___ 

____ _____ _________ 

_______ _______ 

____ ____ __ ___ ____ 

__________ CLUES - Message #1: N represents the letter L; Message #2: T represents the letter I
20

Answers

Creteceous Critter Puzzle 

Ankylosaurus 

Edmontosaurus 

Pachycephalosaurus

Pteranodon

Triceratops 

Tyrannosaurus

Is this really a fossil?

(A) Gastrolith: TRUE. A gastrolith is a gizzard stone, like gizzard stones found in modern chickens and turkeys. 

Some birds, reptiles, and dinosaurs swallowed rocks to help grind food. A gastrolith is a rock that was deliberately 

swallowed by an animal which became very smooth and polished from rubbing against other rocks in the creature’s 

gut. Not all polished rocks are gastroliths. Only polished rocks that are found very close to or inside fossil skeletons 

might be gastroliths.

(B) Dendrite: FALSE. Although it looks like a plant, a dendrite is a mineral deposit on stone.

(C) Tube shapes: TRUE. Prehistoric worms burrowed through soft sediments just like modern worms do. They left 

traces of their passing as rounded tubes crisscrossing through rock. The soft body of the worm is not preserved, only 

the traces of its tunnels. This is an example of a trace fossil, and trace fossils are a record of animal behavior.

(D) Ripple marks: FALSE. Ripple marks are made by running water or wind. You can see these today along streams, 

lakes, the seashore, and in sand dunes. Ripple marks are not fossils because they are not evidence of former life. 

(E) Coprolite: TRUE. A coprolite is fossilized feces (poop!). Scientists find valuable information about the diet of 

prehistoric animals by studying coprolite contents. Sometimes fossilized bits of plants or animal bones are found in 

coprolites.

(F) Holes in a rock: FALSE. Rocks with many holes may be preserved rain drop prints or caused by natural weathering 

of the rock. Although interesting, many rocks with holes may not be fossils.

(G) Shaped rock: TRUE. Small rocks with peculiar shapes, especially when found in groups, could be fossils. These are 

prehistoric fish scales.

(H) Tracks: TRUE. Tracks left by fossil animals and preserved in stone are fossils and tell us about the behavior of the 

creatures that left them.
21

Word Search

N J D R E E A W P T Y R A N N O S A U R U S

C M K U S L R U X R R S Y D B G Y V X B C U

N C K A V I C B W P L K F I E K E H C E Z O

L A T S E D E P O X L P G X J N S V L I K E

E Z Y O T O E E M L A P X Q O I D K I P A C

L M H N U C B P A R A Y R B I G M E M A R A

I A E I R O N U M M U I D I R I A I A E Z T

T G X D T R W O L A X V K K P W M A T Z M E

P N C Q L C A S I F W V O T X J M S E C T R

E O A V E S L B V T N S W L F F A T S F V C

R L V G R D Z B I C C B I O C L L E Y O S K

S I A A M P H I B I A N R P P A A R A S Q C

T A T B U X L X X X M M I F L W N O C S X V

N W E P M R Y D C J A C V T A A Y I B I D G

E I M F R T O W U T Z K J Y X K N D C L X F

M L Y G O E Y T I L Q Y Z Q I E Z T T Y Q E

I H U I D E P O S I T I O N I A J K S Y P R

D Y O W E T N R C O P R O L I T E K X Q P N

E X A S H H P O W L I Q W O R W W I G A Y S

S I F D Q D Y S P O T A R E C I R T O J R J

Coded Messages

Message 1: THE HELL CREEK FORMATION IS NAMED FOR HELL CREEK IN MONTANA

Message 2: THE FOSSILS IN THE HELL CREEK FORMATION TELL US ABOUT LIFE IN THE LATE CRETACEOUS

Scrambled Words:

1. FROG

2. SALAMANDER

3. ANKYLOSAURUS

4. CROCODILE

5. TYRANNOSAURUS

6. SHARK

7. TRICERATOPS

8. HADROSAUR

9. TURTLE

10. PTERANODON