 They commonly occur in thin mudstones interbedded with sandstones, as positive relief on the bottom of the sandstone, or as negative relief on the top of the mudstone.Subaqueous shrinkage cracks can develop on and through a surface that has been continuously covered in water.Syneresis cracks in some shales and lime mudstones may initially be preserved as small cavities, which then usually fill with silt and sand from either the overlying or underlying beds and laminae.