Final answer:
The conclusion is that the siltstone and limestone layers were originally deposited horizontally, and a geological event caused the folding after their formation. The folding indicates a syncline, which suggests a geological event affecting a continuous sequence of sediment.
Step-by-step explanation:
Based on the geologist's observation that the layer of siltstone is younger than the layer of limestone above it in folded rock layers, we can conclude that a geological event occurred after the layers were formed. This is in line with the principle of original horizontality, which states that sedimentary rocks are originally deposited in a horizontal manner, and any folding or tilting occurs after the deposition.
Furthermore, if in a stratigraphic sequence, the rocks get younger towards the inside of a fold, it indicates the fold is a syncline. This can also be an example of the principle of lateral continuity, implying that the folding event occurred post the formation of the layers and affected a continuous and previously undisturbed sedimentary sequence.