Learning target: I can use fossils and an ash layer as evidence to claim which layers in two distant road cuts formed at about the same time, and explain my reasoning with the law of fossil succession and the idea of an index fossil.
Read the scenario and study the two layer logs and the fossil guide. Answer the warm-up questions first; they will organize your evidence. Then write your Claim, Evidence and Reasoning in the boxes. A strong response names specific layers by their labels, says which fossils and which rule connect them, and explains why rocks that look different can still be the same age. Finish with the follow-up question.
You will need: This worksheet, Pencil, Scrap paper for lining up the logs
A county is planning a bike path beside a highway. Two road cuts along the route, the North Cut and the South Cut, are three kilometers apart. The contractor, Mr. Alvarez, looked at both and said, 'These rocks are completely different colors and types. They have nothing to do with each other, so we do not need the geologist to look at both.' The county geologist, Dr. Okafor, disagrees. She thinks some layers in the two cuts formed at about the same time, and that knowing which ones would help predict what the path's builders will dig into between the cuts. Her field logs and fossil guide are below. (The places and fossils are invented for this worksheet.)
| North Cut | Rock and thickness | Fossils | South Cut | Rock and thickness | Fossils |
|---|---|---|---|---|---|
| N1 | Yellow sandstone, 2 m | none | S1 | Pebbly conglomerate, 1 m | none |
| N2 | Gray limestone, 3 m | Fossil K | S2 | Red sandstone, 2 m | Fossil R |
| N3 | Brown mudstone, 1 m | Fossils R and K | S3 | White ash, 0.3 m | none |
| N4 | White ash, 0.2 m | none | S4 | Gray limestone, 2 m | Fossils K and M |
| N5 | Black shale, 2 m or more | Fossil M | S5 | Black shale, 1.5 m or more | Fossil M |
| Fossil | What it was | How long the species existed (million years) | Where it has been found |
|---|---|---|---|
| K | A ribbed clam | 150 | Five continents |
| R | A tightly coiled sea shell | 2 | Four continents |
| M | A small sea snail | 6 | Only in this region |
On scrap paper, write the North Cut layers in a column and the South Cut layers in a column beside it. Draw a line between any two layers that share a fossil, and a dashed line between the two ash layers. Then ask of each line: how long did that fossil species live? A short-lived species makes a tight connection; a long-lived species makes a loose one. The tightest connections are your strongest evidence.
A new cut is opened ten kilometers away. From top to bottom it shows: gray limestone with fossil K; white ash; brown mudstone with fossils R and K; black shale with no fossils. Decide whether the ash in this cut is the same ash as N4 and S3, and explain your answer using fossil succession. Then explain what it would mean if the lab found that this ash has a different chemical fingerprint from the other two.