Rock layers follow a consistent, readable order across regions

Smith's foundational insight, developed while surveying canal routes and coal mines, was that sedimentary rock does not appear in random configurations from place to place. Instead, distinct layers, or strata, stack in the same relative sequence wherever they are found, even when separated by great distances or interrupted by folding and faulting. This meant that a layer exposed in one county could be matched with confidence to the same layer exposed somewhere else, simply by its position relative to other layers. Before this, geology lacked a systematic way to correlate rock formations across a landscape; each outcrop was treated as its own isolated puzzle. Smith's contribution was recognizing that the puzzle pieces belonged to one large, orderly picture. Takeaway: what looks like scattered geological chaos often follows a hidden, learnable sequence.