Fig. A – Many flute casts that truncate one another. Black arrow points downstream.
Fig. B – Flute casts with characteristic round heads pointing in upstream direction. Black arrow points downstream.
Pictured above are beds of sandstone displaying flute casts. Flute casts are common in channel environments (for instance river channels) where water is carrying sediment and debris (rocks, shells, sticks etc..).
As debris is carried along, it randomly grazes the mud in the channel bottom, scouring divots. The divots are typically deeper and narrower in the upstream direction, with a round head pointing upstream. Flute casts form subsequently when sand in-fills these divots and later becomes a rock, preserving casts of the divots on the bottom of the sand bed.
Flow direction indicators, such as flute casts, are one of many clues geologists use to reconstruct the history of the earth.