Periglacial Involutions in Northeastern Illinois

Certain beds of glaciofluvial sand, silt, and clay in the upper Illinois Valley are complexly deformed into variously shaped masses of silt and clay intruded into sand. Rounded forms and downward intrusions predominate. Individual structures are termed "involutions," and layers of deformed beds are called "involution layers"-equivalents of the Brodelböden and Brodelzonen of German writers. Deformation has not been observed below a depth of 12 feet and may extend within 3 or 4 feet of the surface. Single involution layers are from a few inches to 6 feet thick. These involutions are attributed to intense differential freezing and thawing and to the development and melting of masses of ground ice. It is postulated that this occurred in a surficial layer above perennially frozen ground in an area peripheral to a glacier where periglacial (arctic) conditions prevailed. The involutions are believed to have been formed when the ice front lay 30-50 miles to the northeast and are dated as late Cary (late Middle Wisconsin).