Site formation begins when the original inhabitants of a site occupy it. Over time, the site's artifacts are subjected to degrading processes, including weather and erosion, catastrophic destruction such as flooding and subsequent human activity, like tilling or reconstruction. As artifacts shrink in the face of degradation, natural processes tend to bury sites through deposits of natural materials, especially dust and soil, assisted by repeated layers of vegetation. This process creates layers, called strata, that -- if undisturbed by human activity -- create a timeline. The shallowest layers are the youngest, and the deeper layers the oldest. Dated layers yield information about artifacts found in those layers.
Archaeologists start with the simplest understanding of site formation by dividing processes into two basic categories: culture and nature. Cultural transformations of the site are the result of human activity. Natural transformations are the result of natural processes like weather, geology and biology. Recognizing that these processes interact significantly, archaeologists use these categories as a way to organize analysis. The shorthand in archaeology for the two categories is "C-transforms" and "N-transforms."
Cultural transformations are all the activities from the time of original occupation until the present that have been wrought by human activity. The activities include subsequent occupation of the locale, building activity, waste deposition activity, removal and addition of materials from elsewhere, cultivation and fabrication. Cultural transformations are then divided into two categories: the target culture of the original inhabitants and ancillary cultural activity that has changed the site.
Natural transformations can be cataclysmic, such as the volcano that buried Pompeii, or they can be gradual, like oxidation, erosion and biotic breakdown. Glacial movement is a gradual yet cataclysmic alteration of a site. Biotic factors that can change a site include animals carrying away or depositing things, plant roots penetrating and shattering structures, insect activity or bacterial/mycotic degradation of organic material. Extreme aridity, extreme cold, volcanic ash and some wet environments (like peat bogs) can actually contribute to the preservation of some sites.