Learning target: I can use evidence from a rock-layer record to argue where the boundaries of a time scale belong and explain why scientists organize Earth's history by events rather than by thickness or by convenient dates.
Read the scenario and the three proposals. Study the layer table; it is a teaching dataset, invented to be realistic. Answer the three warm-up questions first; they organize your evidence. Then write your Claim, Evidence and Reasoning in the boxes. A strong response names layers by number, uses the fossil counts and the dates, and explains the rule that scientists use when they place boundaries on the geologic time scale.
You will need: This worksheet, Pencil, Calculator, Scrap paper for the evidence grid
A county museum is building a walk-along exhibit beside a 124-meter cliff of layered rock. Visitors will climb a stairway from the oldest layer at the bottom to the youngest at the top, and the museum wants to paint the cliff's history as chapters, with a sign at each chapter boundary. Three staff members have proposed different ways to divide the cliff. The director has asked a geology student, Priya, to decide which plan matches the way scientists divide Earth's history on the geologic time scale, and to explain why with evidence from the cliff itself.
| Layer (1 = bottom) | Rock | Thickness (m) | Kinds of fossils | Date in years, if any |
|---|---|---|---|---|
| 1 | lava flow (volcanic rock) | 3.0 | 0 | 480 million years |
| 2 | limestone, sea shells | 15.0 | 24 | none |
| 3 | shale, sea shells | 12.0 | 25 | none |
| 4 | thin dark line | 0.1 | 25 below, 9 above | none |
| 5 | sandstone, sea shells | 30.0 | 9 | none |
| 6 | white ash band | 0.2 | 9 | 470 million years |
| 7 | limestone, sea shells | 20.0 | 10 | none |
| 8 | mudstone, sea shells | 8.0 | 12 | none |
| 9 | thin dark line | 0.1 | 12 below, 5 above | none |
| 10 | sandstone, sea shells | 25.0 | 5 | none |
| 11 | gray ash band | 0.3 | 5 | 380 million years |
| 12 | sandstone, fern leaves and footprints | 10.3 | 6 | none |
On scrap paper, make a grid with Plans A, B and C across the top and three rows: 'fossil changes', 'thickness versus time', and 'dated layers'. In each box write one fact from the table that supports or hurts that plan (for example, under Plan A and 'thickness versus time': '57 m took 10 million years but 53 m took 90 million years'). The plan with support in every row is your claim; the facts in the other columns are your evidence against the rejected plans.
The museum's second exhibit is not about chapters but about when things happened: a sign every 10 million years. Explain why the dated layers would now be essential, how you would estimate where the 400-million-year sign goes using the two ash bands (layers 6 and 11) and the thicknesses between them, and why that estimate could be badly wrong. Then write two sentences on how the real geologic time scale uses both kinds of information: events for boundaries, dated layers for numbers.