Cropmarks


How are new archaeological sites found? How do archaeologists know where to dig? One method is to study the landscape looking for cropmarks - these are normally areas of a crop that either grow much better or much worse than other areas in the small field. Often the cropmarks will have a definite shape (round or rectangular) and these are clues to what may have been there hundreds of years ago. The best way to look for new cropmarks is from the air and this is known as aerial archaeology or aerial photography. Pioneered by people like Osbert Guy Stanhope Crawford in the early 20th century, new sites are contently being discovered this way.

These cropmarks (photographed by Damian Grady) form part of the prehistoric ritual site near North Stoke, Oxfordshire and were mapped as part of the Thames Valley NMP
These Cropmarks (from Cawrence, Ceredigion) show the outlines of an Iron Age defended farmstead. The outer enclosure may have been for keeping livestock while the inner enclosure was for round houses

 

Materials & Tools:

  • Small plastic/polystyrene tray (e.g. old fruit container / seed tray / ice cream tub)
  • Air-dry clay / clay
  • Small stones / gravel
  • Lego Blocks (alternate to air dry clay / stones)
  • Packet of Cress seeds
  • Clean soil / compost
  • Water spray
  • Scissors
  • Plastic sheets / Newspaper (optional)
  • Plastic gloves (optional)

Preparation:

There is little preparation (other than gathering the materials together). Provide each child / group with plastic gloves and plastic sheeting /newspaper to protect any surface they are working on and to help clear up afterwards.

Objective:

The purpose of this activity is to create the type of environment for a cropmark to show in the soil:

  • Provide each child / group with a small plastic tray
  • Using the air-dry clay / clay, create a "wall" from one side of the tray to the other, making sure that there is an empty gap between the "wall" and the two edges of the plastic tray. The "wall" will need to be almost as tall as the sides of the plastic tray (at least 2-3 cm). You will need to push the air-dry clay firmly into place, so that it sticks to the plastic tray and there are no holes in the "wall".
  • Push the small stones / gravel all over the air-dry clay "wall" (while it is still wet) to create a more life-like "wall" effect. Once the "wall" is completely covered in stones / gravel, move to the next step.
  • Fill the two gaps either side of the wall with soil / compost, up to the top level of the "wall"
  • Add a very thin final layer of soil / compost over the entire tray, so that the top of the "wall" is only just covered, but cannot be seen
  • Open the packet of cress and slowly scatter the seed evenly over the entire surface of the soil / compost (the easiest way to do this is to cut a small corner off the packet of cress, tip the packet so that it perpendicular to the tray and carefully shake the packet while moving it over the tray surface)
  • Wet the surface of the soil / compost, using the water spray
  • Place the tray in a warm and dry area that occasionally has a small amount of natural light
  • The cress will start to grow within a few days and you will notice that there will be little growth above the wall - this is known as a negative cropmark
  • Study the tray every 3 days to check on the progress and make notes

Further Activity Examples:

  • Find examples of cropmark pictures on the internet
  • Use Google Earth or Bing Maps to look for cropmarks where you live
  • Discuss cropmarks and fairy-rings / elf-circles / pixie-rings (how are they different and why do fairy-rings grow?)
  • Discuss cropmarks and so-called crop circles
  • This type of cropmark (above a wall) is normally called a negative cropmark. How would a positive cropmark work?
  • Draw a cross-section diagram of positive and negative cropmarks that explain the difference
  • What other archaeological evidence can landscapes give us ?
  • Look in the fields near you (during late summer) - can you find any real cropmarks?

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