The formation of sandstone involves two main processes – weathering/erosion and sedimentation. Sand particles originate from the weathering and erosion of pre-existing rocks. This usually occurs in source areas with lots of elevation and water flow, like mountains or highlands. Weathering breaks the rocks down into smaller particles through mechanical and chemical processes. Mechanical weathering includes processes like water erosion, ice wedging, root wedging, and abrasion. Chemical weathering decomposes the rocks using processes like hydrolysis, carbonation, and oxidation.
Once sand particles are liberated through weathering, erosion transports the particles away from the source area through actions like water, gravity, wind, or ice movement. Streams and rivers are the main transporters of sand grains. The sand particles are carried downstream until the water energy drops low enough that it can no longer support the weight of the sediment. At this point, the sand is deposited.
The deposition of sand particles leads to the accumulation of sediment in horizontal or gently-dipping layers. Over time, the weight of overlying sediments will compress the deposited sand, squeezing the grains closer together and expelling water and air from between the pores. Compaction reduces porosity and forms sedimentary layering.
Cementation is the next key process. Mineral-rich waters permeate through the sediment, depositing dissolved minerals like calcite, silica, iron oxides or clay which act as natural cement between the sand grains. Quartz is the most common cementing agent as silica from groundwater precipitates into the pores between sand grains. Cementation lithifies the loose sand sediments into solid sandstone rock.
The final steps involve uplift and erosion of the cemented sandstone. Geologic forces uplift and tilt layers of sedimentary rock, exposing them at the earth's surface. Weathering and erosion then work to sculpt the sandstone into various landforms and expose it at the surface for examination and use.
The mineralogy and texture of sandstone can provide clues to its history and origins. The dominant mineral composition, size, shape, and sorting of the sand grains all indicate the source rock and erosional conditions. The type and degree of cementation reflects the chemistry of the pore fluids during burial. All of these factors come together to form the sandstone we see today.
