It all starts with an indentation on the Earth’s surface, where water flows in and gets trapped. 

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The surrounding dead and dying plants fall into the water and lower the oxygen level. Then, as a result of the microbial decay processes, and the input of acids from the atmosphere, the water becomes quite acidic.

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The increased acidity of the water means that the next dead plants that fall into this water do not fully decompose. Thus, the "pickled" plant matter collects and at some stage has a biomass production that is larger than the rate of decay. 

 

 

As these partially decomposed plants accumulate, they form layers that build up over thousands of years.

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Once the original indentation is filled with peaty soil, it officially becomes a ‘fen’.

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If the peat continues to build, the nutrient level lowers and the peat creates a dome shape, it then becomes a ‘bog’.