The problem with layers is the potential for creating a "perched water table", where water doesn't move down from fine to coarser material until the fine material has become saturated. That's why we don't put gravel in the bottom of pots, for example, because it does the opposite of what one might intuitively think and actually causes the finer material on top to hold more water. There are some pictures that illustrate this phenomenon on this link:
http://gardenprofessors.com/co...
