Final answer:
The mudstone is considered younger than the limestone due to the principles of stratigraphy, including the law of superposition, which states that younger layers of rock are deposited on top of older layers, and the principle of cross-cutting relationships, indicating geological features that cut across others are younger.
Step-by-step explanation:
Understanding the Relative Age of Rocks
The mudstone is considered younger than the limestone because of the principles of stratigraphy, specifically the law of superposition and cross-cutting relationships. In stratigraphy, the law of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it. So, if a layer of limestone is below a layer of mudstone, the limestone is older.
Moreover, Steno's principle of cross-cutting relationships indicates that any geological feature that cuts across another is the younger of the two. This means if mudstone is cut by a fault or another sedimentary layer, that fault or layer must be younger than the mudstone it cuts through.
Geologic history often involves various events such as sediment deposition, erosion, and tectonic activities, all of which can reset the rock record and create a stratigraphic sequence. For example, if limestone was first deposited, and later a new layer of mudstone was laid down on top of it, then the mudstone is by definition the younger stratum because it arrived after the limestone had already been formed.