Many, though not all, erosional structures are best seen where a coarser bed (e.g. sandstone) overlies finer grained rock such as mudstone. Sandstone may fill the depressions produced by scouring of the underlying mud, producing a mould (popularly a "cast") of the scour structure. Such structures, seen on the bottom surfaces of the courser beds, are known as "sole marks".
Images copyright © 1998, John W.F. Waldron, except where otherwise stated
Flute casts in fluvial sandstone, Mabou Group, Lismore NS. Flutes are scoop shaped depressions produced by erosion as a rapid current flows over a cohesive muddy surface. Flute casts (more accurately flute molds) are bulbous structures produced when the current fills the flute with sand.
Flute casts on base of turbidite bed. Flutes have a bulbous up-current end, and fade down-current. Current flowed toward the bottom right, parallel to the pencil.