Time to Re-examine our Assumptions about Cross Strata?

• Cross strata in quartz sandstones and in the drift typically exhibits smooth curves even when boulders are present. In most of the sandy gravel of the drift this is characteristic. It is also true of many ancient conglomerates, with a sandy matrix, and rounded pebbles, where cross strata is present. If the patterns of cross strata were due to a sedimentary process involving fast currents, the patterns would not be smooth if boulders were present because of turbulence. This is a simple fact of elementary physics. Why no turbulence effects, such as current shadows or tails of sediment accumulation on the lee side of pebbles or boulders, if fast currents were involved when the patterns were formed and the boulders and pebbles were deposited?
• The smooth, distinct laminations of many types of cross strata discredit a sedimentary origin, as in the sedimentary process where particles of varying size are deposited there is turbulence due to roughness of the bed, that results in very indistinct laminations. The patterns of cross strata formed by currents in rivers are usually irregular and not smooth like those seen in ancient sandstones or in the sands and gravels of the drift.
  
• Large scale cross strata is common in sandstones. The patterns are usually smooth, concave-upward arcs. 
  
• Preservation of patterns of cross strata even in granite and hi-grade metamorphic rocks is enigmatic for a sedimentary interpretation. The preservation of distinct patterns of cross strata is most unlikely if it has a primary sedimentary origin, due to the compression, shearing, and distortion associated with metamorphic conditions of burial. Yet the patterns of cross strata in such rocks may appear fresh, and undisturbed. Examples occur in the Lorrain Formation in Ontario, and can be observed at roadcuts along the Trans Canada Highway north of Lake Huron.
  
• The sedimentary process itself indicates that the distinct, smooth patterns of cross strata are unlikely to be sedimentary. A discussion of the physics involved in sedimentation is accompanied by this movie showing details of the process of sedimentation in a flume, and a second movie illustrating the spreading of a bedload sheet. Patterns of cross strata resembling those of sandstones in nature are not formed in flume experiments. The above movies help to show why. It is evident that the processes illustrated do not form well defined, smooth laminations like that seen in cross stratified sandstones. The formation of layers of sandy sediment in a stream is one of progressive spreading of the layer of sediment, which randomly deposits particles at its edge. There may be some grading by particle size in these sedimentary laminations. In natural cross strata, typical of sandstones, the laminations were caused by a different process, involving crystallization in successive layers. The cross-laminations were determined by the progressive nature of the crystallization front that passed through the rock. This was a pressure-related alteration.
  
• Cross-strata, like that seen in ancient sandstones, is not forming in nature today.
  
• The chemical purity of many rocks which exhibit cross strata does not support the assumption of a sedimentary environment of formation. Sandstones may be 99% pure quartz; mineralogic evidence of these sandstones points to a long history of diagenesis in their development, involving chemical dissolution and replacement of foreign minerals, and processes that would likely remove traces of original sedimentary stratification. Yet many of these rocks exhibit distinct patterns of cross strata.
  
• Formations that exhibit cross strata may contain non-sedimentary structures such as concretionary banding, balls or lenses containing material with a different texture, either more coarse or finer than the enclosing matrix, or vertical structures such as pipes or dikes. See Non-sedimentary Structures in the Drift.
• Contacts between cross stratified sandstones and underlying rocks typically exhibit sole marks formed by expansion effects as cross stratified formations spread laterally due to increased volume and porosity as they formed. Analogous marks on the bedrock below the drift caused striations.