The process that accounts for the formation of horizontally layered rocks is known as sedimentation. Sedimentation is the deposition of sediments, such as sand, clay, and gravel, by various agents like water, wind, ice, or gravity. Here's a more detailed explanation of the process:

1. Erosion and Transportation: Weathering processes break down rocks and minerals, producing sediments. These sediments are then transported by water (rivers, streams), wind (aeolian processes), ice (glacial processes), or gravity (mass movements like landslides).

2. Deposition: As the transporting agent loses energy or encounters obstacles, it slows down and deposits the sediments in layers. The size, shape, and composition of the sediments determine the characteristics of the layers formed.

3. Compaction: As more and more sediments are deposited, the weight of the overlying sediments compresses the lower layers, causing them to become denser and more compact. The pressure also forces out water and air from the layers.

4. Cementation: Over time, minerals dissolved in groundwater may precipitate and act as a cementing agent, binding the sediment particles together. This process is known as cementation. Common cementing agents include calcium carbonate (calcite), iron oxide, and silica.

5. Horizontal Layering: As new sediments are continually deposited and compacted, they form distinct horizontal layers. The layering represents the successive depositional events and can reflect changes in the transporting agent, sediment supply, or environmental conditions over time.

Horizontal layering is a characteristic feature of sedimentary rocks, which are formed from the consolidation and cementation of sediments. Sedimentary rocks provide valuable information about Earth's history, including ancient environments, climate conditions, geological events, and the evolution of life forms.