Sedimentary rocks are called secondary rocks because they are formed from the weathering and erosion of pre-existing rocks, which are often igneous or metamorphic rocks.

Here's why:

* Primary rocks: Igneous and metamorphic rocks are considered primary rocks because they are formed directly from the Earth's mantle or from the transformation of existing rocks under high pressure and temperature.

* Secondary rocks: Sedimentary rocks form as a result of the breakdown of these primary rocks. Weathering and erosion break down the primary rocks into smaller particles called sediments. These sediments are then transported by wind, water, or ice and eventually deposited in layers. Over time, these layers become cemented together, forming sedimentary rocks.

In essence, sedimentary rocks are derived from existing rocks, hence the term "secondary."