Cropmarks, also called crop marks, are variations in terrain caused by buried archaeological artifacts. Most classically, cropmarks reveal the presence of buried buildings, funeral sites, and ritual artwork. They can be easily seen from an aircraft, or a high vantage point such as a hill, platform, or building. The study of cropmarks is part of aerial archeology, a branch of archeology which focuses on looking at sites as seen from above.
The growth rate of plants is dependent on the condition of the soil below them. If the soil is loose, as might be the case in a filled grave, the plants may grow more readily than if it is hard. Buried stone buildings can cause plants to grow slowly, by wicking away water, while filled ditches may cause plants to grow more quickly, providing an ample supply of fertile soil and fresh water. When viewed from above, a buried archaeological site can manifest itself in the form of plants of different colors, plants with different growth rates, and fruits of different colors.
In a classic example of cropmarks, a farmer plants a field of barley and notes that a roughly circular patch develops where the grain grows more slowly. If an excavation is performed, that patch may prove to be a buried building or foundation. Conversely, a field might be bisected with lines of crops which grow very rapidly, suggesting the presence of buried trenches or ditches.
In addition to cropmarks, it is also possible for archaeologists to use soil marks, if they are looking at bare soil. Soil marks are patches of soil with a different color than soil in the surrounding area, illuminating patches where holes have been filled. Ancient graveyards, for example, have grave-shaped soil marks formed by the chemicals produced during decomposition, and the use of various soils for fill. Frost marks, differentiations in the heaviness of the frost in an area, can also be used like cropmarks to find ancient sites.
When cropmarks are found, archaeologists work carefully at the site to excavate it. They typically photograph the area from an aerial vantage point, and create a grid so that they can control the dig very precisely. As materials are excavated, they are sifted for smaller artifacts, and over time, the buried structure or site will be revealed, shedding further light into the lives of people who lived in antiquity. Cropmarks can also reveal more recent sites: in New England, for example, disturbances in the forest show signs of former colonial settlements. They can also point a finger at more sinister sites, such as mass graves.