Probably the three most critical environments of marine deposition are: the continental shelf, where the waters are repeatedly agitated by waves; the continental slope, extending downward from wave base; and the relatively flat ocean bottom.

The establishment and use of criteria for the recognition of deposits made in each of these environments might clarify many difficult problems of stratigraphy and tectonics.

Inter-stratal and intra-stratal contortions in sandy and silty beds observed in Wales and elsewhere suggest flowage while the sediments were still unconsolidated. Persistent association of such phenomena with remarkably even, thin-bedded alternations of shale with siltstone or limestone, showing aligned groove-like markings on the under sides of the beds, must be significant.

The environmental conditions which seem best to explain these phenomena are shown to be those expectable below wave base on a foreset slope. There, are combined the conditions for even bedding, for alternations of silt and mud, for gravity sliding, and for the intra-stratal crumpling by flowage of silts converted into soupy quicksand by upward-migrating water squeezed out of the compacting muds.

It is believed that the occurrence of these phenomena in association can safely be considered as strong presumptive evidence for deposition in the slope environment.