Based on my analysis of the geological cross-section, I can now determine the relative ages of the rock units from oldest to most recently formed by applying the principles of stratigraphy:
1.
Shale (item 8) - This is the bottommost layer in the sequence, so according to the principle of superposition, it's the oldest.
2.
Sandstone (item 6) - This layer is deposited directly above the shale, making it younger than the shale but older than the layers above it.
3.
Limestone (item 5) - This layer is deposited above the sandstone, making it younger than both the shale and sandstone.
4.
Granite (item 10) - The granite intrusion cuts through the shale, sandstone, and limestone layers. According to the principle of cross-cutting relationships, the granite must be younger than all these layers it intrudes.
5.
Pegmatite (item 7) - The pegmatite vein appears to cut through the shale and sandstone layers but is itself cut by the basalt dike. It's also below the erosion surface, so it's older than the erosion surface but younger than the shale and sandstone it cuts through.
6.
Basalt dike (item 4) - This dike cuts through multiple layers including shale, sandstone, limestone, and pegmatite. It must be younger than all these layers it cuts through.
7.
Erosion surface (item 9) - This erosion surface cuts across the granite, basalt dike, and other layers, indicating it formed after these features were in place.
8.
Erosion surface (item 3) - This is another erosion surface located higher in the sequence, above the limestone layer and below the shale and siltstone.
9.
Shale and siltstone (item 1) - These layers are deposited above the lower erosion surface, making them younger than that erosion surface and all the layers beneath it.
10.
Volcanic ash (item 2) - This is the topmost layer in the sequence, deposited last, making it the most recently formed.
Therefore, listing the rocks from oldest to most recently formed:
8. Shale
6. Sandstone
5. Limestone
10. Granite
7. Pegmatite
4. Basalt dike
9. Erosion surface
3. Erosion surface
1. Shale and siltstone
2. Volcanic ash
Parent Tip: Review the logic above to help your child master the concept of relative ages of rocks worksheet.
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