This is an interesting page on them....
http://www.ovnis.atfreeweb.com/5_crop_circles.htm
Ken Larsen, a British biologist, has shown that the way in which the stalks (wheat, rapeseedhahaha8230;) are flattened without being broken or damaged [p. 25hahaha8230;] is typical of a UHF microwave effect. Thus one can see stalks of rapeseed curving at 90 degrees, the flowers of which are still intact, although those same rapeseed stalks break easily when one attempts to bend them over by hand [p. 151]. The new position taken up by the plant becomes fixed. It continues to grow horizontally [p. 3 and 158] and breaks if one tries to return it to the upright position [p. 140].