Here's a breakdown:

* Sedimentary rocks are formed from the accumulation and cementation of sediments, which are small pieces of rock, minerals, or organic matter.

* Layers of sediment build up over time, and the weight of the overlying layers presses down on the lower layers, causing them to compact and solidify.

* The process of cementation occurs when dissolved minerals in groundwater crystallize between the sediment grains, binding them together.

Some common types of sedimentary rocks formed from layers of sediment include:

* Sandstone: Formed from cemented sand grains

* Limestone: Formed from the accumulation of marine organism shells and other calcium carbonate materials

* Shale: Formed from compacted clay and mud

* Conglomerate: Formed from cemented pebbles and gravel

* Breccia: Similar to conglomerate, but the fragments are angular rather than rounded

It's important to remember that all sedimentary rocks were once layers of sediment.