Loess is a unique type of silt, usually containing some clay and in some cases fine sand, which has the propeny of being easily worked, yet being so cohesive that deep cuts with venical walls may be made in it. The cohesiveness appears to be the result of its being composed of exceptionally fine particles so that molecular attraction acts in part as a bond as well as the fact that the particles are angular and tend to interlock. A third factor may be the presence of plant roots or the filling of the voids left when plant roots rotted. These voids were filled by calcite, a form of calcium carbonate, which forms natural reinforcing rods. Loess of the Midwest was derived from outwash, deposited by meltwater from glaciers during the Ice Age in environments where this meltwater-deposited material was filling up major stream valleys so rapidly that plant growth did not have time to get stanrted and thus anchor it. In addition, cold weather near the margin of the glacier inhibited plant growth. Strong winds picked up this material and deposited it in dune-like hills on the downwind side of major valleys and as a flatter blanket extending many miles downwind from the hills.