Microscopic investigations of many limestones and sandstones have revealed small-scale arched flow structures associated with spiral structures. The common property of the sediments displaying these structures is that they consisted of a groundmass containing lenticular layers of different plasticity. The structures were probably generated by slumping without initial folding of the unconsolidated sediments, in somewhat the following manner. First, the more plastic lenses were squeezed out horizontally or down the primary dip by compaction. Arched flow tructures were generated in the squeezed sediment by orientation of elongated organic or mineral fragments along semi-circular lines convex toward the direction of movement. At first, slumping pulled these beds into elongated lenses, and increased sliding coupled with friction over the underlying surface generated a rotational movement which transformed the lenses into spiral bodies. At this stage, the originally layered sediment appears already thoroughly mixed and made up of numerous interfering spiral structures. In the most plastic sediments, the process has been exaggerated and there has been disruption of the spiral structures followed by a complete dispersal of their components in the cement or the matrix.