1. Contact Relationship:
* Sharp Contact: If the sandstone and basalt have a sharp, distinct boundary, it suggests that they were deposited separately. This is because a sharp contact usually indicates an erosional surface, meaning the basalt was eroded before the sandstone was deposited.
* Gradational Contact: If the contact between the two rocks is gradational, meaning there's a gradual transition from basalt to sandstone, it suggests that the sandstone was deposited *directly* on top of the basalt. However, this doesn't necessarily mean the sandstone was directly deposited on top of the basalt. The basalt could have been weathered, eroded, and then the sandstone deposited on top of the eroded basalt.
2. Angular Unconformity:
* Tilted Basalt: If the basalt is tilted or folded, and the sandstone sits horizontally on top, this indicates an angular unconformity. This means the basalt was deposited, tilted, eroded, and then the sandstone was deposited on top. This is a clear sign that the sandstone was not deposited directly on top of the basalt.
3. Fossil Evidence:
* Fossil Discontinuity: The types of fossils found in the sandstone and basalt layers can help determine their relative ages. If the fossils in the sandstone are significantly younger than those in the basalt, it indicates a time gap between their formation, suggesting the sandstone was not deposited directly on top of the basalt.
4. Weathering and Erosion:
* Basalt Weathering: Basalt is typically more resistant to weathering than sandstone. If the basalt shows signs of significant weathering (like rounded edges, cracking, or soil formation), it suggests that it was exposed to the elements for a considerable period before the sandstone was deposited.
5. Mineralogical Differences:
* Chemical Composition: The chemical composition of the basalt and sandstone can provide clues. If the sandstone contains minerals that are not found in the basalt, it suggests that they formed in different environments and were deposited separately.
Important Note: It's crucial to consider the geological context. The best way to determine if the sandstone was deposited on top of the basalt is to conduct a thorough geological investigation.
