Soils may be buried by structures and deposits on archaeological sites. Such soils are frequently preserved under ramparts, barrows and other mounds, or buried within the fill of a ditch. Turves, and the upper horizons of the soil profile which may have been cut with them, can also be preserved within mounds. The study of buried soils yields valuable information about environmental change in the area. Sometimes the marks made by ancient ploughing are preserved in the ancient profile. In addition, the pollen or snail shells which became incorporated into the soil by earthworm sorting, and are preserved by burial, may be used for environmental investigations. Soil profiles may be altered as a result of their burial and care needs to be taken when studying them.