All of Earth's rock types fall into one of three categories: igneous, metamorphic, or sedimentary. Igneous rocks solidify from a hot, molten state. Some, like granite, cool slowly beneath the ground and produce relatively large crystals. Others, like basalt, form above the surface as molten lava rapidly cools. Metamorphic rocks are any rocks that have been transformed by heat and pressure inside Earth. Examples include slate, schist, and gneiss.

Most of the Grand Canyon's exposed rocks belong to the third category: sedimentary rock. When mineral grains are eroded from surface materials, transported by water or wind, and eventually laid down on the ground or seafloor, a loose deposit of sediments forms. Over time, these soft sediments are compacted by mounting pressure and heat from subsequent overlying layers. Squeezed of their water, the sediment grains are cemented together, or lithified, to form solid -- though brittle -- rock.

The telltale feature of sedimentary rocks is horizontal bedding, or layering, that ranges in thickness from millimeters to several meters. As a rule, deeper layers are older than those above them because they were deposited first. Because sediments form from different minerals and may be deposited on land or in water, there are many kinds of sedimentary rocks. Sand becomes sandstone, mud becomes shale, and the calcified remains of marine organisms become limestone.

Scientists learn a great deal from analyzing the progression of sedimentary layers. For example, successive limestone and shale layers indicate that sediments were first laid down in a marine environment and then in a coastal swamp environment. The presence of limestone layers in the Grand Canyon's strata suggests that shallow tropical seas periodically covered what is today a desert environment.

How can this be? Global sea level rises or falls as glaciers and ice sheets melt or freeze. When tides rise, oceans flood the land. Marine deposits settle to the seafloor and later turn into limestone. When the oceans retreat, muddy or sandy deposits typically form. These lithify into shale or sandstone.

The advance and retreat of Earth's oceans explains how ancient coral reef formations can be found in the middle of deserts, whale bones can be found in Pakistan, and, as is featured in this video segment, nautiloid impressions can be found in Arizona.

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