Samples of siliceous shale from the Odanah Member of the Pierre Shale (Late Cretaceous, Campanian) collected from outcrops and gravel bars along the Souris River in southwestern Manitoba undergo disintegration when subjected to cyclic wetting and drying. Breakage of some fragments along curved surfaces indicates that fragmentation, believed to be related to stresses developed by ordered water in rock pores, is not bedding controlled. Breakdown of shale clasts subjected to cyclic wetting, freezing, thawing, and drying is more rapid than disintegration caused by wetting and drying only, because of the combined action of ice and adsorbed water.

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