Structures that develop within the permafrost layer of the periglacial zone. In the winter, when the ground freezes and contracts, fissures are formed that penetrate into the permafrost. Water percolates in and freezes, and a wedge of ice gradually develops, continuously enlarging the fissure as it does so. When the ice melts, these fissures may be filled with sediment, forming a cast of the ice-wedge. Fossil ice-wedges of this kind are seen in many sections of sand and gravel deposits in Europe. They have been used to reconstruct the extent of the periglacial zone which developed around the Devensian and Weichselian ice-sheets. Another feature, similar in form to the ice-wedge cast, is the sand-wedge. This is a frost fissure that has been filled directly with sediment, without the formation of ice. In plan, systems of ice-wedges are arranged in a polygonal network which frequently appears as a crop mark in aerial photographs, alongside features of more archaeological interest. icon. A form of Christian painting which spread throughout Byzantium and much of the Christian world from the mid-6th century. Based on old traditions of Roman portraiture, the icons were portraits of sacred personages set into panels, always with a frontal pose and an exaggerated spiritual expression, often with large staring eyes directed at the observer. One of the earliest examples in the West is the Madonna from Santa Maria in Trastevere, Rome. Icons became objects of such devotion that in 726 the eastern Byzantine Emperor and clergy, believing that the worship of images was ftindamentally opposed to the teaching of Christ, issued a complete ban on their use. The Latin Church under successive Popes resisted this ban, and until 843 the church was split in the bitter quarrel known as the iconoclastic dispute.