Flute casts are useful indicators that are common in sedimentary rocks. Here "flute" refers to the groove shape we see carved on marble columns. Moving water erodes flute marks into soft mud, but flute marks (as opposed to flute casts) are seldom preserved because the mud becomes shale, which is usually soft. A little of the gray shale is visible at the right edge of this exposure, which is at Bulgan Uul in Mongolia. After the flute marks are carved, they are filled with fresh sediment that tends to be coarser grained than the mud beneath. The resulting rock creates casts of the flute marks, hence flute casts. Flute casts are therefore a type of sole mark indicating the bottom side of a sedimentary bed, a useful thing to know where rocks are strongly tilted.
Flute marks tend to be blunt and deepest on their upcurrent end, meaning that flute casts are highest at the upstream end—three of these are visible in the upper center. They indicate that the current carving the flutes moved from left to right. Sharper, less organized marks tend to be made by dragging pebbles and are called tool marks; deeper and longer marks may be called gutter casts or even channel casts.
