Here's why:

* Maria: The dark, smooth plains on the Moon are called maria (Latin for "seas"). They are vast, solidified lava flows.

* Highlands: The lighter, heavily cratered areas on the Moon are called highlands. They are older, formed early in the Moon's history.

* Overlapping: In some areas, maria flows can be seen to partially cover or overlap the highlands. This indicates that the lava that formed the maria erupted after the highlands were formed.

Therefore, the maria deposits are younger than the highlands.