Identify the principle geologists use to determine that layer that was not always tilted.

Law of Lateral Horizontality

Explanation:

According to the law of lateral horizontality, the sediments and rocks were initially deposited in a horizontal manner, which could be later deformed or it gets tilted. These changes that occur in the rocks could be due to the various processes such as the plate tectonic activity, orogenic activity, occurrence of earthquakes, folding and faulting. After tilting, the subsequent rock layers are deposited on the apex portion and eventually become flat on the surface.

This is one of the important principle that geologists often refer to.