Final answer:
The law of original horizontality states that sedimentary rocks are originally deposited in flat, horizontal layers. It is one of the principles geologists use for relative dating.
Step-by-step explanation:
The law of original horizontality states that layers of sedimentary rocks are originally deposited horizontally. This means that when sedimentary rocks form, they are usually found in flat, horizontal layers. Any tilting or folding of the rock layers occurs after they have been deposited. The law of original horizontality is one of the principles geologists use for relative dating, along with the law of superposition and the principle of lateral continuity.