Fenland and Peat bog

Fenland noun - Low-lying wet land with grassy vegetation; usually is a transition zone between land and water.

Peat bog and fenland are semantically related. In some cases you can use "Peat bog" instead a noun "Fenland".

Peat bog noun - Wet spongy ground of decomposing vegetation; has poorer drainage than a swamp; soil is unfit for cultivation but can be cut and dried and used for fuel.

Fenland and peat bog are semantically related. You can use "Fenland" instead a noun phrase "Peat bog".