INTRODUCTION

You and your fellow classmates are geologists living in the late 18 th and early 19 th century. Scientists are using direct evidence from observations of the rock layers or strata to help determine the relative age of rock layers. Specific rock formations are indicative of a particular type of environment existing when the rock was being formed. For example, most limestones represent marine environments, whereas, sandstones with ripple marks might indicate a shoreline habitat or a riverbed.

The Law of Superposition , which states that in an undisturbed horizontal sequence of rocks, the oldest rock layers will be on the bottom, with successively younger rocks on top of these, helps geologists correlate rock layers around the world. This also means that fossils found in the lowest levels in a sequence of layered rocks represent the oldest record of life there. By matching partial sequences, the truly oldest layers with fossils can be worked out. William Smith, an important fellow scientist, recently presented his paper about the succession of different fossils based on their distribution in a sequence of sedimentary rocks in England .

By correlating fossils from various parts of the world, scientists are able to give relative ages to particular strata. This is called relative dating . Relative dating tells scientists if a rock layer is "older" or "younger" than another. In reading earth history, these layers would be "read" from bottom to top or oldest to most recent. If certain fossils are typically found only in a particular rock unit and are found in many places worldwide, they may be useful as index fossils in determining the age of undated strata.

By using this information from rock formations in various parts of the world and correlating the studies, scientists have been able to establish the geologic time scale . This relative time scale divides the vast amount of earth history into various sections based on geological events (sea encroachments, mountain-building, and depositional events), and notable biological events (appearance, relative abundance, or extinction of certain life forms).

PART A: Practice!!!!!

1. Cut out the set of 8 card “A's” located on the card sheet . Each card will represent a different rock layer and the letters represent fossils found within the layers.
2. The first card in the sequence has contains the letters “TC” and represents the bottom of the sequence. If the letters "T" and "C" represent fossils in the oldest rock layer, they are the oldest fossils, or the first fossils formed in the past for this sequence of rock layers.
3. Now, look for another card that has either a "T" or "C" written on it. Since this card has a common letter with the first card, it must go on top of the "TC" card. The fossils represented by the letters on this card are "younger" than the "T" or "C" fossils on the "TC" card which represents fossils in the oldest rock layer. Sequence the remaining cards by using the same process. When you finish, you should have a vertical stack of cards with the top card representing the youngest fossils of this rock sequence and the "TC" card at the bottom of the stack representing the oldest fossils.
4. Tape the cards vertically on a sheet of paper. Be sure that the oldest rock layer is on the bottom and the youngest is on the top. 

Answer the following questions about Part A on the paper with the cards .

1. How do you know that fossil “X” is olders than fossil “M”?

2. Explain why fossil “D” in the rock layer represented by DM is the same age as fossil “M”.

3. Explain why fossil “D” in the rock layer represented by OXD is older than fossil “D” in the rock layer represented by DM.

PART B:

1) Cut out the following 8 cards. Each card represents a particular rock layer with a collection of fossils that are found in that particular rock stratum. All of the fossils represented would be found in sedimentary rocks of marine origin. The Table of Marine Organisms gives some background information on the individual fossils.

2) The oldest rock layer is marked with the letter "M". The letters on the other cards have no significance to the sequencing procedure and should be ignored at this time. Find a rock layer that has at least one of the fossils you found in the oldest rock layer. This rock layer would be younger as indicated by the appearance of new fossils in the rock stratum.

3) Keep in mind that extinction is forever. Once an organism disappears from the sequence it cannot reappear later. Use this information to sequence the cards in a vertical stack of fossils in rock strata. Arrange them from oldest to youngest with the oldest layer on the bottom and the youngest on top.

4) Tape the cards vertically on a sheet of paper. Be sure that the oldest rock layer is on the bottom and the youngest is on the top

Answer the following Questions based on part B:

1. Which fossil organisms could possibly be used as index fossils ? Use the Marine Fossil key on the next page to help you.

2. Name three organisms represented that probably could not be used as index fossils and explain why.

3. State the Law of Superposition and explain how this activity illustrates this law.

4. How can the sequence of rock layers & fossils be used to determine the relative age of rock layers?

5. Why might the relative age of fossils found at a dig site be miss leading?

NAME: Trilobite
PHYLUM: Arthropoda
DESCRIPTION: Three-lobed body; burrowing, crawling, and swimming forms; extinct

NAME: Eurypterid
PHYLUM: Arthropoda
DESCRIPTION: Many were large (a few rare species were 5 feet in length); crawling and swimming forms; extinct

NAME: Graptolite
PHYLUM: Chordata
DESCRIPTION: Primitive form of chordate; floating form with branched stalks; extinct

NAME: Horn coral
PHYLUM: Coelenterata ( Cnidaria )
DESCRIPTION: Jellyfish relative with stony ( Cnidaria )(calcareous) exoskeleton found in reef environments; extinct

NAME: Placoderm
PHYLUM: Vertebrata
DESCRIPTION: Primitive armored fish; extinct

NAME: Foraminifera (microscopic type)
PHYLUM: Protozoa ( Sarcodina )
DESCRIPTION: Shelled, amoeba-like organism

NAME: Pelecypod
PHYLUM: Mollusca
DESCRIPTION: Clams and oysters; many living species

NAME: Ammonite
PHYLUM: Mollusca
DESCRIPTION: Squid-like animal with coiled, chambered shell; related to modern-day Nautilus

NAME: Icthyosaur
PHYLUM: Vertebrata
DESCRIPTION: Carnivore; air-breathing aquatic animal; extinct