Final answer:
The most distinctive visible feature in a thick pile of sedimentary rocks is the stratification or layering, including observable beds and lamination. Cross-bedding and fossil imprints may also be visible, indicating the past sedimentary environment.
Step-by-step explanation:
The most distinctive VISIBLE feature observed in a thick pile of sedimentary rocks is typically the stratification, which refers to the layering noticed in these types of rocks. In particular, when sedimentary rocks have easily observable bedding, this marks a distinct characteristic. Beds are layers of sediment thicker than 1 cm, while thinner layers are known as laminations, often composed of fine-grained silt and clay.
Complex sedimentary structures like cross-bedding, ripple marks, and fossil imprints can also be indicative of the sedimentary environment, but may not always be visible in hand specimens and are more apparent in outcrops. Dark-colored, fine-grained sedimentary rocks, such as shales, might reveal a record of the past environment thanks to the presence of fossilized leaves and branches, even though no other structures are observed.
Lastly, the orientation of sedimentary layers can provide way-up indicators, which are features that clarify whether the rock is in the position it was originally deposited. This aspect can be especially important on a geological timescale where rock layers may have shifted or been disrupted.