Flow-oriented clasts may be "regularly-inclined", "reversely-inclined", "wave-oriented", or "vertically-oriented". An imbricated collection of clasts is a special case of isolated flow-oriented clasts. A collection of clasts can be imbricated only if the individual clasts in the collection have roughly the same size and if the shortest (c) axis of each clast is significantly less then the longer (a,b) axes of that clast. The a-b planes of clasts in a stream usually dip toward the upstream direction; they are regularly-inclined. The a-b planes of clasts on foreset beds may dip downstream more steeply than the foresets because clasts tilt into scour holes at their downstream ends; they are reversely-inclined. Sheetflow may also produce reversely-inclined clasts by erosion around clasts resting on fine-grained slopes, especially in arid climates, or by pivoting wedge- or spindle-shaped clasts resting on pavements, when friction between the pavement and the heavier ends of the clasts anchors the thicker ends of the clasts. Wave-oriented clasts are regularly-inclined with respect to incoming waves or (especially on spits) to longshore currents, and reversely-inclined with respect to swash draining from the foreshore. Very thin clasts (2c << a+b), such as shell, slate, or even oak leaves, when present in high concentrations, may be vertically-oriented by littoral currents and streams.