The geologic principle that explains why rock layers typically form parallel to each other is the principle of original horizontality.

Here's why:

* Sediment Deposition: Sediments, like sand, silt, and clay, are deposited in horizontal layers by water, wind, or ice. Gravity ensures that these layers settle out in a relatively flat manner.

* Compaction and Cementation: Over time, the weight of overlying sediment compacts the lower layers, squeezing out water and air. Minerals dissolved in the water then precipitate out, cementing the sediment grains together, forming solid rock.

Therefore, since sediments are originally deposited horizontally, the resulting rock layers also tend to be horizontal.

It's important to note that these layers can be tilted, folded, or broken later due to tectonic activity, but the original horizontal arrangement serves as a starting point for understanding the history of rock formations.