The law of original horizontality explains:

The Law of Original Horizontality is a basic law in geology that was first propounded in the mid-17th century by the Danish scientist Nicholas Steno. The Law of Original Horizontality explains that sedimentary rocks were initially deposited horizontally, or nearly horizontally under the influence of gravity, and that great external forces (such as the movement and collisions of tectonic plates) is responsible for rock layers that are folded and tilted. This law provided more understanding of plate tectonics.