Unidirectional structures are best at indicating current flow in sandstone. They can occur within the bed itself, or on the surface of the bed. They are often important indicators of paleoslope and by measuring the orientation of the structure you can interpret the direction of flow. Again, a large number of unidirectional structures have been described, but the most abundant are ripple marks, imbrication, cross bedding, and flute casts.
Ripple marks are asymmetrical structures with the downstream slope steeper than the upstream. They are often the first directional structures formed when the current becomes fast enough to transport coarse silt or sand. The ripples form by frictional stress between the fluid (either air or water) and the sandy bed when the fluid moves in the same general direction. Ripples can occur on sand dunes (as seen in the image above), stream bottoms, beaches, and at all depths in the ocean. Imbrication is the stacking of platy grains with their flat surface parallel to each other at an angle to the major bedding plane. The grains lean in the direction of the current, dipping upstream (imagine dominos that have pushed over - the dominos fall in the direction of the current, and all slope 'upcurrent').
The most recognizable sedimentary structure is cross bedding. Cross bedding is produced by the avalanching of sand down the lee (downstream) slope of ripples or dunes. The cross beds dip in the direction of the current. (The ancient sand dunes that made the cross bedded sandstone at right once traveled from left to right.) Flute casts are asymmetrical structures found on the bottom of sandstone beds. They often have a tear shape and are deeper at the base of the tear and shallow at the tip. The tip of the tear points in the direction of flow. Flute casts are often associated with deep water deposits where turbidity currents are common.
Field description of sandstones and conglomerates are not complete without determining the grain size and shape, bedding, and sedimentary structures. It is important to collect as much data about the rock in the field so an accurate and detailed description of the paleoenvironment can be made. Once you return to the lab or office, it is often hard to remember the details that can be critical in making your interpretation.
