Not only does sediment spread horizontality and from youngest (top) to oldest (bottom), it also continues to spread flat in all directions until it either runs out of material—pinches off—or hits a barrier. this is known as the

law of original horizontality
law of superposition
law of original lateral continuity
law of gravity and oppositional force
law of cross-cutting relationships

Law of original lateral continuity

Explanation:

The law of original lateral continuity basically explains that sediments spread out laterally and continuously until the sediments run out or depending on the limits of the basin (in other words, what catches it). As it extends outwards the layer starts to thin out. This was proposed by Nicolas Steno, who also proposed the law of superposition and original horizontally.