DETERMINING AGE OF ROCKS AND FOSSILS - Free Printable

To solve this problem, we need to analyze the block diagram and use principles of relative dating (like superposition, original horizontality, cross-cutting relationships) along with fossil evidence and radiometric age data to determine the order of rock formation from oldest to most recent.

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Step 1: Understand the Rock Layers and Features

From the block diagram:

- The rocks are layered vertically:
- Shale and siltstone (top layer)
- Volcanic ash (thin layer below shale)
- Limestone
- Sandstone
- Slate (at the bottom)
- There are intrusions:
- Granite (a plutonic igneous rock) intrudes into the sandstone and slate.
- Dyke (a vertical intrusion) cuts through the granite and other layers.
- Erosion surfaces indicate periods where erosion occurred before deposition of overlying layers.
- Fossils:
- Coccolithophores (single-celled organisms like *coccolithophorids*) in slate.
- Trilobites in limestone.
- Tyrannosaurus rex fossils in shale and siltstone.
- Radiometric age: This is given as a clue for absolute dating.

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Step 2: Use Relative Dating Principles

#### Principle of Superposition:
In undisturbed sequences, the oldest rocks are at the bottom, and the youngest are on top.

So, slate → sandstone → limestone → volcanic ash → shale and siltstone would be the sequence if no disturbances occurred.

But there are intrusions and faults, so we must consider cross-cutting relationships.

#### Cross-Cutting Relationships:
- Granite intrudes into sandstone and slate, so it is younger than those rocks.
- The dyke cuts through the granite, so the dyke is younger than the granite.
- Erosion surfaces indicate that some layers were eroded before new ones were deposited.

Also, volcanic ash is a layer between limestone and shale/siltstone. It is likely a deposit from a volcanic eruption, so it is younger than the limestone but older than the shale/siltstone.

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Step 3: Analyze Fossils

Fossils help us determine relative age based on index fossils and evolutionary timelines:

- Coccolithophores (microscopic plankton): Found in slate. These existed in the Paleozoic Era, possibly earlier.
- Trilobites: Found in limestone. Trilobites lived in the Cambrian to Permian periods (~540–250 million years ago).
- Tyrannosaurus rex: Found in shale and siltstone. Dinosaurs lived in the Mesozoic Era, specifically the Cretaceous Period (~145–66 million years ago).

So:
- Slate (with coccolithophores) → very old, possibly Precambrian to Paleozoic
- Limestone (with trilobites) → Paleozoic
- Shale and siltstone (with T. rex) → Cretaceous (Late Mesozoic)

This suggests the shale and siltstone are the most recent.

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Step 4: Determine the Sequence

Now, let’s list all rock units and their relative ages:

1. Slate – oldest sedimentary layer; contains early microfossils.
2. Sandstone – above slate, so younger.
3. Limestone – above sandstone, contains trilobites → Paleozoic.
4. Granite – intrusive into sandstone and slate → younger than both.
5. Volcanic ash – layer above limestone → younger than limestone.
6. Shale and siltstone – topmost layer, contains dinosaur fossils → Cretaceous → youngest.
7. Dyke – cuts through granite → youngest of all.

Wait — the dyke is a vertical intrusion that cuts through the granite and older rocks, so it must be after the granite formed.

So the dyke is the most recent feature.

But the question asks: List the rocks from oldest to most recently formed.

Let’s clarify what "rocks" means here. We have:

- Slate
- Sandstone
- Limestone
- Granite
- Volcanic ash
- Shale and siltstone
- Dyke (igneous intrusion)

So the full list of rock units:

1. Slate – oldest sedimentary layer.
2. Sandstone – above slate.
3. Limestone – above sandstone.
4. Granite – intruded into sandstone and slate → after them.
5. Volcanic ash – deposited after limestone.
6. Shale and siltstone – deposited after volcanic ash.
7. Dyke – cuts through granite → youngest.

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Final Answer:

List the rocks from oldest to most recently formed:

1. Slate
2. Sandstone
3. Limestone
4. Granite
5. Volcanic ash
6. Shale and siltstone
7. Dyke

> Note: The dyke is the most recent rock formed because it cuts across the granite and other older rocks.

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Additional Notes:

- Fossils confirm the relative age:
- Coccolithophores → ancient (possibly Precambrian to Paleozoic)
- Trilobites → Paleozoic
- T. rex → Late Cretaceous → youngest fossil group
- Radiometric age could give absolute dates, but since not provided, we rely on relative dating.

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✔ Answer:

Oldest rock: Slate
Next: Sandstone
Next: Limestone
Next: Granite
Next: Volcanic ash
Next: Shale and siltstone
Most recent rock formed: Dyke

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Fill in the blanks:

```
Most recent rock formed: Dyke
...
...
...
...
...
Oldest rock: Slate
```