Partially decayed and compressed vegetative matter accumulating in areas of high rainfall and/or poor drainage is termed as

Peat is formed from partially decayed vegetative matter that accumulates in waterlogged conditions like swamps and bogs. Over thousands of years, this organic material compresses but doesn't fully decompose due to lack of oxygen and acidic conditions. Marsh and swamp refer to the ecosystems themselves, not the accumulated matter, while slush refers to partially melted snow or ice.