The law of original horizontality explains:

Law of Horizontality in Sediments ... The Law of Original Horizontality was first proposed by Danish geological pioneer Nicholas Steno in the 17th century. The law states that layers of sediment were originally deposited horizontally under the action of gravity.

The law of original horizontality explains that the deposition of sediment layer is horizontal under the gravity action.

Explanation:

The law of original horizontality was given by Nicholas Steno. This law states that the sediment layer is deposited horizontally under the influence of the action of gravity.

The law of original horizontality is important to understand the movement of tectonic plates of the earth. The folded and tilted strata has been caused by some movement and collision of earth's plate.

Thus, horizontal deposition of sediment layer has been explained by the "law of original horizontality".