Heavy rainfall caused minerals such as iron to be washed out or leached from the surface layers of the thin soil, in a process known as paludification. These were deposited lower down in the soil profile where they formed an impermeable layer known as an iron pan (see Figure 2). As water cannot move down through such a layer, the soil surface became waterlogged. Under these conditions the accumulation and spread of peat was made possible.

There are nice figures for explanation.

_{*Lunatic*, n.
One whose delusions are out of fashion.}