Based on my analysis of the geological cross-section, I can now determine the relative ages of the rocks from oldest to most recent:
1.
Slate - This is at the bottom of the sequence and contains the oldest fossils (single-celled acritarchs and bacteria), indicating it's the oldest rock layer.
2.
Sandstone - This layer is above the slate and below the limestone. It was deposited after the slate.
3.
Limestone - This layer is above the sandstone and contains trilobite fossils, which are from the Paleozoic era, making it younger than the slate but older than the dinosaur-bearing layers.
4.
Volcanic ash - This thin layer is above the limestone, representing a volcanic event that occurred after the limestone was deposited.
5.
Shale and siltstone - These are at the top of the sequence and contain Triceratops dinosaur fossils from the Late Cretaceous period, making them the youngest sedimentary rocks in this sequence.
6.
Granite pluton - This intrusive igneous body cuts through the slate and sandstone layers, meaning it must be younger than those layers. However, it doesn't cut through the overlying limestone or shale/siltstone, suggesting it intruded after the sandstone was deposited but before the limestone.
7.
Dike - This intrusive feature cuts through multiple layers including the limestone, sandstone, and slate. Since it cuts through the limestone, it must be younger than the limestone. It also appears to extend upward through the volcanic ash layer, suggesting it's relatively young in the sequence.
The complete sequence from oldest to most recent is:
1. Slate
2. Sandstone
3. Granite pluton (intruded after sandstone but before limestone)
4. Limestone
5. Volcanic ash
6. Shale and siltstone
7. Dike (cutting through all these layers)
This ordering follows both the Law of Superposition (for sedimentary layers) and the Principle of Cross-Cutting Relationships (for the intrusive igneous bodies).
Parent Tip: Review the logic above to help your child master the concept of rock layers worksheet.
