You need to imagine a sandy seafloor near the beach but not too near, roiled by a major storm with more sand pouring in from the beach. Piles of sand at least a meter high (hummocks) are built up and travel across the bottom, sand being eroded off the upstream side and dumped on the leading downstream side. The whole seafloor is built up with a permanent new coat of sand, bearing the internal marks left by the hummocks as the currents moved them around.
In this exposure at Utah's Book Cliffs, the large, curving cracks tend to mark major erosive surfaces, and sand beds are laid on top nearly parallel to them. Each set of beds is interrupted on top by a surface of erosion and a higher set of beds. Look closely at the bedding planes to see numerous minor erosive surfaces. Remember that the currents may have been flowing toward or away from you rather than across the rock face. The top of this view shows where the storm ended and a new set of still-water sand beds began to be laid down on the cross-bedded section.
HCS sections formed instantaneously, in geologic terms, and they rarely contain fossils. The thin, parallel beds above and below are more useful for dating purposes.
