There used to be a LOT more marshes, with big chunks of what are now cities only drained in latter times - Belgravia in London was marsh, and the whole South of Manchester, like Rusholme (meaning island in a river/marsh, and Moss Side (a moss being a peat bog, either over clay or a layer of stone). Marsh gas would have been a common sight.

The wildest woods would have been the extensive marshy estuaries, the Fens in East Anglia & Levels in Somerset, the Thames Humber Severn and Mersey estuaries. These would have been places of refuge, and hunter-gathering for those with the knowledge - King Alfred hid from the Vikings in marshes, and the draining of some of the last Fens around Eely nearly 1,000 later was protested violently. When a folk song says 'The Greenwood', we should think of wet woodland like these, passable and livable only to those in the know, I think.

I'd relate the song Lord Randall to this. A Lord, going into one of the wild places, and eating the food of the poor, eels? That has to be marshes. Where people could still live beyond the reach of taxes as late as that happened anywhere. Where wild people lived, that new which plants weren't safe to eat.

Between the marsh gas, the wild people, and the outlaws, marshes would indeed have been places to feel spooked by!