Here's why:

* Evaporite Minerals: When seawater evaporates, it leaves behind dissolved minerals. These minerals can crystallize and form layers of rock called evaporites. Common evaporite minerals include halite (rock salt), gypsum, and potash.

* Examples: Many sedimentary rock formations around the world contain evaporites, suggesting ancient seas once existed in those areas. The Dead Sea is a modern example of an evaporating body of water that is depositing evaporite minerals.

* Not All Deposits: However, not all deposits were formed by evaporating seas. Many other processes contribute to the formation of sedimentary rocks, such as weathering, erosion, transportation, and deposition of particles from rivers, glaciers, or wind.

Therefore, while evaporating seas do contribute to the formation of some deposits, it's not the only process responsible for all sedimentary rock formations.